Best mattress UK 2021: the leading memory foam, pocket sprung and hybrid mattresses

We've ranked the best mattresses from all the top brands so you won't lose sleep finding your perfect bedding

best mattress
(Image credit: Future)

Need to know what the UK's best mattress is in 2021? Here's where you can find out. Buying a mattress has completely changed over the last decade; it used to mean traipsing to an out-of-town store and self-consciously testing out all the models you liked the look of, but now you can simply order online in the knowledge that if you don't like it, you can easily send it back. But to save you from that inconvenient scenario, we've put together this list, informed by our own extensive research, covering all the different options for various sleep styles, preferences and budgets, so that you can order with confidence.

Shopping online is safe and easy, with many online-only companies delivering your mattress, rolled and boxed, to your door, and even offering to set up your mattress in your room of choice for you, and get rid of the old one for a small fee. It'll also give you a better chance of finding the right model for you – generous free trial periods mean you can get the feel of the mattress much better than you'd be able to in five minutes in a shop.

If you've spent any time browsing mattress brands online you'll know that the only thing they love more than mattresses is selling you their mattresses at a sweet discount; basically you should never have to pay the RRP for any mattress, ever. The price widget with each entry on our list will automatically pull in the cheapest price for each model, but if price is paramount, you should check out our cheap UK mattress deals page for a regularly updated roundup of the best offers we've found. 

But it's worth having some idea what you're looking for first, so let's kick off our ranking of the UK's best mattresses. 

The best mattress you can buy today

Our number one best mattress is the luxurious Simba Hybrid Pro. The proud winner of the T3 Award 2021 for best mattress, it delivers sumptuous comfort and strong cooling tech. It's not cheap though, so if you're on a tighter budget we'd recommend the Emma Original. A double T3 Award-winner (in 2019 and 2020), this memory foam mattress is also extremely comfortable and great value to boot. If neither of those take your fancy, we have plenty of other options for you to explore too.

How often should you change your mattress? More often than you might think: recommends upgrading every 6-8 years under normal conditions. It might be an investment, but it's one worth making if you can afford it. Studies have found that newer mattresses can significantly improve your sleep quality and reduce aches and pains. And even if you clean your mattress regularly, sweat, dead skin cells and dust mites build up to frankly horrifying levels fairly quickly.

If you're unsure about sizing, our mattress size guide will help you out (with info on the sheets and bedding you need, too). These are adult mattresses – we have separate guides to the best kids mattress and the best toddler mattress. 

Best mattress UK 2021, ranked

Simba Hybrid Pro, our best mattress pick, on white backgroundT3 Awards 2021 winner badge

(Image credit: Simba)

1. Simba Hybrid Pro

This high-end hybrid is the best mattress overall – and a T3 Award Winner

Best for: Side and back sleepers
Sizes: Single, Double, King, Super King
Depth: 28cm
Turn: No (rotate)
Filling: Springs and foams
Comfort: Medium
Trial : 200 nights
Guarantee: 10 years
Reasons to buy
+Good temperature regulation+Top layer delivers cloud-like softness+Excellent support
Reasons to avoid
-Top cover not removable

The Simba Hybrid Pro is one of Simba's high-end offerings, and delivers exceptional comfort. It's on the pricey side, but for our money the best premium mattress around (in fact, we've given it the T3 Award for Best Mattress 2021). The seven specially engineered layers include a dreamily soft top layer, a natural wool layer to help regulate temperature, and two layers of springs. The top layer of softness gives you something to sink into as you lay down, but just a little, before you hit the supportive layers underneath, which deliver a reassuring level of firmness. 

These firmer lower layers also make it easy enough to move around on, and the motion isolation and edge support is good too. Many memory foam mattresses can trap heat, but our reviewer was very impressed with the temperature regulation on the Hybrid Pro. The only down-side of the design, really, is the lack of removable cover. For more info head to our full Simba Hybrid Pro review.

The Simba Hybrid Pro is rated 6.5 out of 10 on the firmness scale, which means it's at the softer end of medium. If you prefer something firmer, check out the Eve Premium Hybrid at #4 instead, or take a look at the Simba Hybrid Luxe – the brand's most advanced model, which is pricey but hugely comfortable and supportive.

Now, all that advanced tech does cost, and the Simba Hybrid Pro is at the pricier end of the market. Keep an eye on our guide to the best Simba mattress deals to find the latest discounts on it, though. And if you like the sound of a Simba but can't afford this one, we also highly rate the cheaper, less advanced Simba Hybrid Original

Emma Original mattress on white backgroundT3 Awards 2020

(Image credit: Amazon)

2. Emma Original mattress

The best value mattress you can buy now

Best for: All sleepers
Sizes: UK, 5 (single-super king); EU, 3 (single-king); US, 6 (twin-king)
Depth: 25cm
Turn: No
Filling: Memory foam
Comfort: Medium-firm
Trial: 200 nights
Guarantee: 10 years
RRP: Single: £429, Double: £649, King: £699, Super king: £799
Reasons to buy
+Exceptionally comfy and loads of awards+Incredible value+Suitable for lots of sleeping styles+Removable and machine washable cover
Reasons to avoid
-Sleeps quite warm

On balance of features to price, the Emma Original mattress is hard to beat. Comfortable, versatile, supportive in all the right places and amazing value, it earned a full five stars in our Emma mattress review. It won the T3 Best Mattress award in both 2019 and 2020 (although in 2021 the Simba Hybrid Pro just squeaked the crown). 

The design of this medium-firm mattress is simple but effective (it's also at the top of our best memory foam mattress ranking). It consists of three layers of different foams: Airgocell foam on top (to mould gently to your body without trapping heat), visco-elastic memory foam in the middle (for pressure relief and support), and a firm HRX foam foundation. The cover features a temperature-regulating top and a breathable non-slip bottom as well as handles for rotating (you don't have to flip it), and it's removable and machine-washable, too.

Thanks to a 2019 price drop, this mattress is now one one of the cheapest of the big-name brands, and what's more, if you keep your eyes peeled, there are regular Emma discount codes and deals to bring the price even lower. In short, you're looking at outrageous value for money here. 

Like many of the mattresses in this guide, the Emma Original vacuum packed, rolled, and delivered in a conveniently sized box, ready to be unfurled. Expansion takes around two hours. If you're worried about buying a mattress you haven't tried, there's a 100-day, love-it-or-return-it trial for peace of mind, plus a healthy 10-year guarantee. This brand scores reassuringly well on customer service, too: it currently boasts an excellent 4.6 out of 5 stars from over 15,000 reviews on consumer review site Trustpilot.

Brooke + Wilde Elite mattress

(Image credit: Brooke + Wilde)

3. Brook + Wilde Elite

An excellent luxury hybrid mattress

Best for: All sleepers
Sizes: 4 (single-super king)
Depth: 28cm
Turn: No
Filling: Foam and pocket springs
Comfort: Soft, medium, or firm
Trial: 100 nights
Guarantee: 10 years
RRP: Single: £899, Double: £1,199, King: £1,299, Super king: £1,399
Reasons to buy
+Choose your firmness+Luxurious feel
Reasons to avoid
-Expensive at full price-Longer delivery times

Ever found yourself getting excited about a new mattress only to discover that its firmness level's wrong for you? That's not a problem with the Brooke + Wilde Elite; this luxurious premium mattress comes with a choice of three firmness levels, from soft to firm, so you can pick the rating that's just right.

It features eight layers designed to deliver absolute comfort and support, including two layers of pocket springs separated by a layer of specialist support foam to aid spinal alignment and show your shoulders some love. 

Each Brook + Wilde Elite mattress is made to order, which means it'll take longer than other mattresses to arrive – expect to wait around three weeks – but when it turns up Brooke + Wilde will set it up in the room of your choice. There's a 100 night comfort trial in which you can decide whether you got the firmness level right; if not Brook + Wilde will exchange it for a different firmness level or a different mattress free of charge, or simply give you a refund if you prefer.

Going by sticker price, this mattress is far from cheap, but the good news is there's almost always an offer to take advantage of – head to our Brook + Wilde mattress deals page for the best current prices. Get all the details in our Brook + Wilde Elite mattress review.

Nectar Mattress with white top and grey sides

(Image credit: Nectar Sleep)

4. Nectar Memory Foam mattress

Another excellent mattress, with a super-long trial and forever warranty

Best for: Most sleepers
Sizes: Single - super king
Depth: 25cm
Turn: No
Filling: foam/memory foam
Comfort: medium/firm
Trial: 365 nights
Guarantee: Forever
RRP: Single: £599, Double: £699, King: £799, Super king: £899
Reasons to buy
+365 day trial period+'Forever' warranty+Super supportive
Reasons to avoid
-Too firm for some

100-night trials are pretty common among the established challenger mattress brands, but Nectar goes the extra mile with not only a 365-night trial, giving you a whole year to make your mind up, but also a lifetime warranty for your new mattress. Nectar's also a climate-neutral company that offsets its emissions through various global schemes, including an Amazon forest protection programme.

The Nectar mattress feels comfortable from the minute it comes out of its minimal packaging, and offers a medium-to-firm support level that suits any type of bed and all kinds of sleepers. There's a bit of give in the top layer, which neatly moulds around the body, while a complex supporting base layer adds the required firmness for a peaceful sleep. The adaptive cooling cover top layer is supposed to draw heat away from the body, and while we're not sure it actually does this, it certainly feels cooler than many memory foam options. 

Price-wise, Nectar sits in the middle of the market, although it's around £50 pricier than the Emma (for a full comparison, head to our Emma Original vs Nectar mattress showdown). This brand promises 'the best available price' at all times when you buy direct, and runs regular Nectar discount codes and deals to help you save (often, these deals mean free sleep accessories such as the Nectar mattress protector – one of the best mattress protectors around – to keep your mattress box-fresh).

Delivery is super smooth and stress-free. Amazingly, even the enormous Nectar Super King model comes packaged in a very manageable box. We experienced some off-gassing when our review sample was first delivered, but this soon settled down. The Nectar is a great and comfortable mattress, and the 365-day money-back trial proves just how confident this company is that it has created one of the best mattresses you can buy.

Eve premium hybrid mattress, white with yellow piping

(Image credit: Eve Sleep)

5. Eve Premium Hybrid

Another outstanding premium hybrid mattress

Best for: Everyone
Sizes: UK - 3 (double - super king)
Filling: springs, foam, memory foam
Depth: 28cm
Turn: no
Comfort: medium-firm
Trial: 200 nights
Guarantee: 10 years
RRP: Single: N/A, Double: £899, King: £1,049, Super king: £1,199
Reasons to buy
+Exceptionally supportive and comfortable+Dreamy cushioning+Great temperature regulation
Reasons to avoid
-Motion transfer could be improved-No Single size

Another excellent mattress for your consideration, the Eve Premium Hybrid was Highly Commended in the 2020 T3 Awards. This bed-in-a-box mattress is incredibly supportive and extremely comfortable to sleep on. The luxurious, 28cm deep hybrid design includes five layers of soothing foam alongside 1,500 full-sized pocket springs to deliver extra support, improved airflow and pressure relief. It's on the firmer end of the scale, but dreamy cushioning keeps things extremely comfortable. In our Eve Premium Hybrid Review we were especially impressed with the temperature regulation, too. 

This is a premium mattress, which means it's top quality and advanced in design and features. It also means the price tag is significantly bigger than more basic models in our list, including the excellent value Emma at #1 (although bear in mind too that there are regular Eve mattress discounts to take this price down). Head to our  for more info. 

If you have the budget and value all the extra features – the more so if you tend to overheat at night – the Eve Premium Hybrid could well be the best mattress for you. And if you can't quite stretch to the Premium Hybrid, check out our Eve Original mattress review for Eve's cheaper option. Or if you're on a really tight budget, Eve makes one of the best mattress toppers around. 

Head to our Eve Premium Hybrid vs Simba Hybrid Pro showdown to see how it compares to a comparable model from a rival brand. 

Otty Pure hybrid mattress with bamboo and charcoal

(Image credit: Otty)

6. OTTY Pure Bamboo and Charcoal mattress

Superbly supportive with an eco twist

Sizes: Single - Super King; EU sizes
Depth: 25cm
Filling: Springs, bamboo memory foam with charcoal
Firmness: 6.5 (we'd rate it 7.5 though)
Trial: 100 nights
Guarantee: 10 years
RRP: Single £549.99, Double £749.99, King £849.99, Super King £949.99
Reasons to buy
+Pioneering, sustainable foam+Very supportive+Feels luxurious
Reasons to avoid
-Edge support could be better

The OTTY Pure mattress pushes the boat out with some pioneering materials. In place of regular memory foam you've got a foam made from bamboo infused with charcoal. These materials are temperature regulating, antifungal and antibacterial, hypoallergenic, and also sustainable, making this mattress good for you as well as for the planet. It also has a layer of generous 16cm springs to add bounce and promote airflow further. 

In our OTTY Pure review we found this mattress superbly supportive, with decent temperature regulation and a luxurious feel. OTTY says it's a 6.5 on the firmness scale but we think it feels firmer than that – perhaps a 7.5. The surface feels plush and soft when you press on it with your hand, but lying down there's great overall support, including underneath your hips. You'll also sink in very slightly, but this is a mattress you sleep 'on' rather than 'in'. 

You do pay a premium for the bamboo and charcoal, so if it's out of your price range consider the OTTY Hybrid Original, which we were also impressed with. Or hold out for an Otty discount code or deal to take the price down.   

Sealy 1400 Nostromo mattress

7. Sealy 1400 Nostromo / Pocket Teramo 1400

The best pocket sprung mattresses, bar none

Best for: Side and back sleepers; allergies
Sizes: Single - super king
Depth: 30cm
Turn: No
Filling: pocket spring/latex
Comfort: medium
Guarantee: 5 years
RRP: Single: £920.40, Double: £1,216.80, King, £1,372.80, Super king: £1653.60
Reasons to buy
+Fantastic support and comfort+Great for allergy sufferers+Cool even in summer+Very durable
Reasons to avoid
-Not the best for heavier people

If you haven't really been paying attention to the world of mattresses and all these brands are new to you, here's a name that feels reassuringly familiar. Sealy's been around for a long time, and as an old-school manufacturer it knows how to do pocket springs, as the Sealy Nostromo – also known as the Pocket Teramo 1400 – amply demonstrates. It's an outstanding pocket sprung mattress, with seven zones, 1,400 pocket springs, tencel fibres, a latex comfort layer and a luxurious 29cm depth. It’s also a lot more durable than most other mattresses, and it’s both hypoallergenic and pleasantly cool, even in hot weather. 

Suitable for all sleepers – back and side sleepers in particular – this mattress works well with most body types, although if you’re a taller or heavier-than-average back sleeper you might prefer the Eve Original mattress above, which gives exceptional back support. Which? magazine made it a best buy, it's been awarded Allergy UK's seal of approval, and there’s a five-year warranty for peace of mind. You can generally find it on Amazon with a decent discount on the RRP too (although right now Mattress Online has slashed the price in half) so it’s worth buying online rather than in a shop. 

Silentnight Eco Comfort Pocket 1200 mattress

8. Silentnight Eco Comfort Pocket 1200

A cheaper pocket sprung mattress

Best for: Most sleepers
Sizes: Single - super king
Depth: 29cm
Turn: Yes
Filling: Pocket springs
Comfort: medium/firm
Trial: 60 nights
Guarantee: 5 years
RRP: Single: £369, Double: £489, King: £569
Reasons to buy
+Good all-rounder+Different comfort zones+Very supportive 
Reasons to avoid
-Requires turning-On the firm side of medium

Silentnight's Eco Comfort Pocket 1200 is a cheaper pocket sprung mattress than the Sealy 1400 Nostromo, and one with a smart solution to the whole firmness problem. Rather than deliver the same firmness across the whole mattress, it’s designed with different zones to ensure that the lumbar area of your back gets more support than your shoulders. 

That makes it particularly good for anyone whose back needs something firm but who’d rather not wake up with pins and needles everywhere else, and it also means your partner’s movements won’t impact your side of the bed. There are 1,200 pocket springs and a hypo-allergenic cover, and the 29cm depth is pretty luxurious for a mattress in this price range. There’s a five-year warranty but remember to turn it frequently: pocket sprung mattresses need regular turning to keep their shape.

REM-Fit 500 Ortho mattress

(Image credit: REM-Fit)

9. REM-Fit 500 Ortho mattress

The best orthopaedic mattress for back and joint pain

Best for: back and joint pain; couples
Sizes: Single - super king
Depth: 25cm
Flip: No
Filling: pocket spring/foam
Comfort: firm
Guarantee: 15 years
RRP: Single: £649, Double: £799, King, £849, Super king: £949
Reasons to buy
+ Orthopaedic support + Comfortable cushioning + Good temperature regulation 
Reasons to avoid
- May be too firm for some 

If you tend to wake up with back and joint pain, the REM-Fit 500 Ortho Mattress is a premium orthopaedic mattress you're looking for. It’s a hybrid mattress with 2,000 orthopaedic pocket springs combined with five layers of cushioning, supportive foam, and it's seriously firm. We rated it an 8 out of 10 for firmness (with 10 being the firmest). As one of the firmer mattresses in this guide, it took us a few nights to get used to, but once we did, we were blown away by just how comfortable it is. 

The REM-Fit 500 Ortho is amazingly supportive, thanks to a Mediflex foam layer that moulds to the shape of your body and correctly aligns your spine, and those orthopedic springs providing provide upper and lower back support. Luxury memory foam also provides just the right amount of cushioning to pressure points like your shoulders, hips and knees. 

Thanks to a cool gel-infused memory foam layer, you won’t get too hot sleeping on the REM-Fit 500 Ortho, either. We it worked really well, even during a heatwave. While the REM-Fit 500 Ortho isn't the cheapest mattresses around, we think that if you suffer from aches and pains at night it's one of the best mattresses you can choose. It’s also a great option for couples thanks to its low motion transfer, and has great edge support (so you can snuggle right up to the edges without them buckling). 

If you’re not sure whether a firm mattress is right for you, give the 100-night trial a go, or check out our REM-Fit 500 Ortho mattress review for more details.

Dormeo Memory Plus mattress

10. Dormeo Memory Plus

A very comfortable no-flip mattress

Best for: Allergies
Sizes: Single - super king
Depth: 17cm
Turn: No
Filling: foam/memory foam
Comfort: medium
Trial: 60 nights
Guarantee: 20 years
RRP: Single: £479, Double: £629, King: £779.99
Reasons to buy
+Extremely cheap – great value+Supportive and durable
Reasons to avoid
-Quite thin-No handles to rotate

The Dormeo Memory Plus is a fantastic memory foam mattress that you can always find for an exceedingly cheap price; you'd have to be having a very unlucky day to ever pay anything near the RRP. It's a medium-firm mattress made with 3cm of memory foam on top of breathable Ecocell 3D foam to give you plenty of support, and it's also anti-allergenic, with a cover that's infused with anti-bacterial, antimicrobial and anti-dust mite protection.

If you like a nice thick mattress then the Dormeo Memory Plus might not be for you. Its svelte 17cm thickness makes it a lot thinner than most mattresses, and it might not be suitable for slatted bases or heavier, but it with an average user rating of 4.7/5 on the Dormeo website it's definitely doing something right. Its thinness means you'll probably need to rotate it a bit more often than other memory foam mattresses, but as long as you look after it properly its 20-year guarantee will help you sleep soundly.

Happy Beds Ortho Royale mattress

(Image credit: Happy Beds)

11. Happy Beds Ortho Royale

The best sprung mattress if you're on a budget

Best for: Value for money
Sizes: Small single - Super King
Depth: 25cm
Turn: Yes
Filling: Open coil springs
Comfort: Medium-firm (7.5)
Trial: None
Guarantee: 5 years
RRP: Single: £199.99; Double: £274.99; King: £314.99
Reasons to buy
+Supportive surface and plenty of bounce+Low price point (and regular deals)+Great airflow and won't trap heat
Reasons to avoid
-Requires flipping every 6 weeks-No trial period and only 5 year guarantee

There's nothing particularly advanced about the Happy Beds Ortho Royale. It has none of the fancy features you'll find in most of the other mattresses in this guide, but if you want a great value sprung mattress then it's hard to beat. It doesn't even have pocket springs; instead it has an open coil system that allows air to flow through, with vents for stale air to escape from, resulting in a lovely cool mattress that won't trap heat like memory foam can.

Happy Beds classes this as a medium-firm mattress, ranking it around 7.5. Some reviewers found the Ortho Royale too firm, although for others this extra support is a plus point. Note that as this is a spring mattress, you'll sleep 'on top' of the surface, rather than sinking into it as you would with memory foam. Firmness will also depend on body weight, and the Happy Beds site has a handy tool that lets you input your weight to show you how firm the mattress will feel to you.

There's a hand-tufted damask cover, and satin pads provide a comfy sleep surface. The real star of the show here is the price though: even at RRP it's very cheap, and regular offers make things even more appealing. Note that you're sacrificing a trial period in order to get that cheap price point, and the guarantee is shorter than the industry standard too. But overall we think this mattress offers absolutely excellent value for money.

IKEA Morgedal mattress

12. IKEA Morgedal

Incredibly cheap but not actually at all bad

Best for: Small budgets
Sizes: Single - king
Depth: 18cm
Turn: Yes
Filling: memory foam
Comfort: medium
Trial: 90 nights
Guarantee: 25 years
RRP: Single: £125, Double: £165, King: £195
Reasons to buy
Reasons to avoid
-It's basic-Quite thin

Ikea has a bit of a reputation for its weird-sized beds, but don't let that put you off looking at its mattresses; in addition to European sizes, the Ikea Morgedal memory foam mattress comes in UK single, double and king size too. It’s a thick foam mattress in a polyester wrapping, and it’s best suited to a slatted bed base or mattress base. Don’t jump on it straight away: it’s delivered rolled up and needs three to four days to return to suitable flatness. 

In true Ikea style there's nothing particularly cutting-edge about the Morgedal. It's basically a fat chunk of foam that's comfortable and fairly immune to the movements of your partner. The foam can get a little damp if you tend to sweat a lot; if you do, a thick mattress protector is a good idea. There's a 90-day trial period, and it's won a gong from Which?, so it's still got some strong plus points, despite its distinct lack of bells, whistles and 'challenger brand' credentials. If you're on a really tight budget or need something to put in the spare room, this is the cheapest mattress you'll find. 

Which is the best type of mattress?

There are a few different mattress types on the market right now. As you start your hunt for the best mattress, there are certain terms you're going to come across a lot – the difference isn't complex, but it's helpful to understand what each term means before you get shopping. The three main mattress types you're going to come across are memory foam, pocket sprung or hybrid. For more info, head to our memory foam vs spring mattress comparison. 

Memory foam
This type of mattress has exploded in popularity over recent years. Memory foam is a responsive material that reacts to your temperature and weight to mould gently around your body as you lie on it. Memory foam mattresses offer great support and comfort, and are perhaps the most popular mattress type right now, with most of the big brands offering their own takes on this material. 

One potential down-side is that memory foam traps body heat in a way you don't get so much with springs. So in the winter you'll have a lovely cosy bed, but in the summer you might find yourself getting a little warmer than usual – which can be an issue if you sleep hot. Many brands have introduced cooling top layers to counteract this. You can read more in our memory foam mattress buying guide.

Pocket sprung
Pocket sprung is a more traditional mattress type. These mattresses include thousands of small springs inside their own fabric pockets. Although they will adapt to your body shape, they're not as supportive as memory foam, and may provide less movement isolation (problematic if you've got a restless bedmate). They offer a bouncier surface to sleep on – you won't sink into a pocket sprung mattress the same way as you would a memory foam one.

The term 'hybrid' technically could refer to any mattress that's made up of different materials, but it typically means a mattress that combines layers of pocket springs and memory foam. This makeup offers more bounce than memory foam-only mattresses, but with the added support and comfort of a memory foam upper.

There are a few more terms you might come across in your search. These mattress types are a little rarer, but still worth knowing about.

  • Latex: A breathable, springy material that's exceptionally durable and resilient. Can be expensive. 
  • Continuous and coil: Made from a single looped wire, or from single springs fixed with one wire. These are cheaper and less likely to give a good night’s sleep.
  • Bed in a box: This refers to a delivery method, in which mattresses are compressed, rolled or folded into a box for easy home delivery. They're usually foam, or a memory foam and spring combination. 

One more bit of advice before you start shopping: don't put too much emphasis on mattress type. Personal factors like your height, weight, body shape, sleeping style and temperature are far more likely to affect how comfortable you find a specific mattress. All our picks recommended in our best mattress guide above have been designed to deliver outstanding support and comfort for the majority of people – regardless of their material. 

How to choose the best mattress for you

Before buying a mattress, make sure it’s the right size. Your mattress – like your bed – should be 10cm longer than the tallest person sleeping in it. It should also be wide enough for you and a partner to put your hands under your head without your elbows touching (or without touching the edge of the mattress if you’re not sharing). 

Next, do the hand test: lying down on your back, push your hand under the small of your back. If you can’t, the mattress is too soft; if the gap is very large, it’s too hard. And if you have a partner, make sure they try it, too: what feels fine on your own may be like a bouncy castle with someone else on board. 

Lastly, also check whether a mattress needs turning. Many don't these days, but if you’re considering one that does, make sure it has handles.