Kids' mattress size guide: what size mattress should you buy for your child?

Find the right mattress to keep your little ones comfortable, with our guide to kids' mattress sizes in the UK and USA

Young boy lying in bed
(Image credit: Alex Green from Pexels)

There's little that's more important to your child's development than making sure they get a good night's sleep. Finding the right mattress is a big part of that. Here on T3, we have dedicated guides to the best toddler mattress, the best kids' mattress and of course a general best mattress guide that includes UK singles / US Twins and above. 

But all of that's no use if you're not quite sure what you're looking for – and below adult beds, it can be quite difficult to know what size and style of bed is suitable for what age. Below, you'll find information on the standard sizes of mattresses for cot beds, crib beds, toddler beds and your child's first 'proper bed', as well as the rough ages they correlate to. 

Note: these are pretty uniform throughout the industry, but there are variations, and there's no legal requirement to conform to these exact sizes. So if you're buying a child's mattress separately from an existing bed, do get out the tape measure and check it will fit first.

1. What size is a crib mattress, aka cot mattress?

The best mattress for a newborn is one specifically designed for babies: what's known as a cot mattress in the UK and a crib mattress in the USA. This should be the correct size for your baby's cot/crib, so it fits perfectly and there are no dangerous or uncomfortable gaps. 

In the USA, the standard size of a crib mattress is 27.5 x 52 inches (70 x 132cm). 

In the UK there are two standard sizes for cot mattresses, namely:

  • Standard UK cot bed mattress size: 70cm x 140cm (27.5 x 55 inches) 
  • Continental cot size: 60cm x 120cm (24 x 48 inches)

2. What size is a toddler mattress in the UK and USA?

The best mattress for toddlers is, as you might expect, one that fits a toddler bed. 

Although there's no hard and fast rule, babies are usually moved from a cot bed to a toddler bed at around 18-36 months. This is a transitional bed between a cot and a full bed, although many parents decide to skip this stage and move their child straight to the latter.

In both the UK and USA, mattresses for toddler beds are the same size as those for cot beds. In other words, they are 27.5 x 52 inches (70 x 132cm) in the USA, and 70 x 140cm (27.5 x 55 inches) in the UK.  

In theory, then, it's fine to use the same mattress as they had in their cot, and you don't need to buy a new one. In practice, of course, there may be wear and tear on the old mattress that means it's a good time to replace it anyway. 

3. What size is a child's first 'proper bed' in the UK and USA?

Silentnight Imagine Mirapocket 600 Mattress

The Silentnight Imagine Mirapocket 600 Mattress is available in UK Single, Small Double and Double sizes

(Image credit: Silentnight)

Once your child has outgrown a toddler bed, the next step is to move them to a regular bed. On the whole, the best kids' mattress is one that's at least 10cm / 4 inches longer than the height of the child sleeping on it.

Depending on their size and age, for children in the UK you'll generally want one of the following bed sizes (and mattress dimensions to match):

  • Small Single: 75 x 120cm
  • Standard Single: 90 x 190cm
  • European Single 90 x 200cm
  • Small Double: 120 x 190cm

In the USA, meanwhile, you'll be looking at the following standard sizes for children. These are all adult sizes, with the exception of the Junior category, which is specifically designed as an intermediary between a toddler bed and a full bed.

  • Junior: 27.5 x 63 inches (70 x 160cm)
  • Twin/Single: 38.5 x 74.5 inches (98 x 189cm)
  • Twin XL: 38.5 x 79.5 inches (98 x 202cm)
  • Full/Double: 53.5 x 74.5 inches (136 x 189cm)

Of course, there's no limit to how big your child's bed can or should be, beyond your budget and the size of their room. So there's no reason in theory you can't buy them an even bigger bed than those listed. If you wish to do just that, then check out our general guide to mattress sizes for adults.