Circular Ring Slim review: comfortable design, good app but needs serious improvements

The Circular Ring Slim has a lot of promise but it’s not quite there yet

Circular Ring Slim review
(Image credit: Beth Girdler-Maslen / T3)
T3 Verdict

Dubbed the ‘world’s thinnest smart ring’, the Circular Ring Slim lives up to these expectations with its lightweight and comfortable design. It uses a combination of sensors to measure and collect health and wellness data, and the accompanying app offers a wealth of knowledge about your wellbeing. However, the features and functionalities of the Circular Ring Slim gets lost, as the battery life is terrible and the app has too many syncing and loading issues. It needs some serious improvements to compete with the likes of Oura or Ultrahuman.

Reasons to buy
  • +

    Collects health and wellness data, including sleep, stress and steps

  • +

    Gives helpful tips via the app and AI wellness assistant, especially for sleep

  • +

    Lightweight and comfortable design

Reasons to avoid
  • -

    Battery life is terrible, even on eco mode

  • -

    App has some data loading and syncing issues

  • -

    Doesn’t always connect to the app when you’re wearing it

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The best smart rings are the latest health and wellness tracking craze, that are set to give the best smartwatches a serious run for their money. This new type of wearable is more comfortable and less intrusive to wear, and has no screen or buttons, allowing it to sit quietly on your finger and collect valuable and accurate data.

A new addition to the smart ring market is the Circular Ring Slim. Launched in late 2023, the Circular Ring Slim was described as the world’s thinnest smart ring, thanks to its slim and light design. The second smart ring from Circular, the Circular Ring Slim was updated from the original with new fitness and wellness features, including an AI wellness assistant called Kira+.

As a self-proclaimed wellness expert and someone who doesn’t enjoy wearing watches, I was excited to try the Circular Ring Slim to see all the many health, fitness and wellness statistics and recommendations it gave me. After a month or so of testing, the Circular Ring Slim leaves a lot to be desired… here’s my full review.

Circular Ring Slim review: price and availability

The Circular Ring Slim with vibrations costs £225 / €264 and is available to buy at Circular. It can be bought without vibrations, and shoppers can also buy an extra long charger for easier charging.

Circular Ring Slim review

(Image credit: Beth Girdler-Maslen / T3)

Circular Ring Slim review: unboxing and set-up

The Circular Ring Slim arrived in a petite box with the ring, USB-C charger and an extra long charging cable. To set it up, I charged the Circular Ring Slim for most of the day before downloading the Circular app.

Through the app, I set up the Circular Ring Slim by choosing the finger I wanted to wear it on before providing details about myself, including my name, gender, weight and height. The app was easy to use and it had tutorials that showed you how to use the different sections and introduced the Kira+ wellness assistant.

Once the app was set up and the Circular Ring Slim was charged, I had to wear the ring for 14 days straight. This was called the ‘calibration period’ and it was for the app to understand my patterns, ask questions about my habits and collect data to support Kira+.

Circular Ring Slim review

(Image credit: Beth Girdler-Maslen / T3)

Circular Ring Slim review: design and features

As the thinnest smart ring to date, the Circular Ring Slim is designed to be worn constantly, including during sleep. Available in black, the Circular Ring Slim weighs 2 grams and measures 2.2mm in thickness. The outside of the ring has a small white Circular logo and the inside houses the sensors and charging port.

The Circular Ring Slim uses advanced PPG sensors, a surface temperature sensor and a three-axis accelerometer count. When taking certain measurements, the Circular Ring Slim lights up with three different LED lights. When in use, the Circular Ring Slim monitors and tracks all your health and wellness, including your step count, energy levels, sleep, heart rate, blood oxygen levels and more.

All your information can be viewed in the Circular app and as the ring learns more about you, it’ll give you suggestions on how to improve. A standout feature of the Circular Ring Slim is the introduction of Kira+, Circular’s AIR-powered health and wellness assistant. Kira+ uses generative AI language models to analyse data, ask you questions and make personalised suggestions.

The Circular Ring Slim claims to have six days of battery life and 20 days of on-board memory and haptic navigation. It also uses vibrations (if you buy it with vibrations) which can be used for alarms, reminders and breathing exercises.

Circular Ring Slim review

(Image credit: Beth Girdler-Maslen / T3)

Circular Ring Slim review: app

The Circular app is free to download and the app and Kira+ doesn’t require any monthly subscription fees, although you may have to pay for some ‘circles’ like the medication reminder. The app has a concise, colourful and attractive layout, which is easy to use and understand.

At the top of the app, you can flick between your circles that are sectioned into: alarm clock, sleep analysis, energy analysis, guided breathing, live measurement, stress levels and vital alerts. When you click each circle, you’re shown a score, activity duration and statistics on your performance.

On the main homepage underneath the circles, you can choose which ‘data mode’ the Circular Ring Slim is on, and create timers or look in your calendar. Underneath this is where you can see your step count, cardio pints and live metrics, and it’s also where you can access Kira+.

Circular Ring Slim review

(Image credit: Beth Girdler-Maslen / T3)

Kira+ lives in the Circular Ring Slim app and during its calibration process, it asks many questions and records your answers. After 14 days, Kira+ began giving insights into my daily life like when I’m in my caffeine or workout zone, and when I should start to wake up or go to sleep. I enjoyed the addition of Kira+ to the app, although I found the calibration questions could have been more in depth.

The Circular app is my favourite part of the Circular Ring Slim experience, although it’s not without its faults. During testing, it took a while to sync to the ring and had a few loading and connection issues, so oftentimes it took ages to show my data or didn’t collect it properly.

Circular Ring Slim review

(Image credit: Beth Girdler-Maslen / T3)

Circular Ring Slim review: performance

The main reason I was excited to try the Circular Ring Slim is because I don’t like wearing a watch, but I didn’t want to miss out on the tracking features that smartwatches offer. Considering it’s the thinnest smart ring, I did expect the Circular Ring Slim to be a little slimmer but it’s surprisingly comfortable and I hardly noticed it while I wore it, even during sleep.

As mentioned already, I really liked using the Circular app and enjoyed seeing what data it collected and the extra information it gave me to improve my daily habits. The sleep, energy and stress level circles gave me great insight into all these areas, and the sleep analysis was so in depth, going into REM sleep, circadian rhythm and the total time I was asleep. I also liked being able to quickly see my step count, and earning cardio points for movement and workouts felt really encouraging.

Circular Ring Slim review

(Image credit: Beth Girdler-Maslen / T3)

Having said all that, the Circular Ring Slim had connectivity problems and didn't always alert you to that fact. For example, I woke up with the Circular Ring Slim on, checked it had synced to the app, wore it all morning and went to check my statistics, only to see that the ring hadn’t properly connected and I didn’t have any data to show for my morning.

The biggest negative and what really lets the Circular Ring Slim down is its battery life. It claims to offer six days of battery life but I could only get two out of it at the most. It didn’t last long at all, even when on eco mode and with the vibrations turned off. The real disappointment for me was when I’d charged the Circular Ring Slim for most of the day, wore it for a few hours and found that it had dropped to 14% by the evening on the same day.

Circular Ring Slim review

(Image credit: Beth Girdler-Maslen / T3)

Circular Ring Slim review: verdict

The Circular Ring Slim is a good concept but it needs some serious improvements before it takes on the other smart rings on the market. My main positives of the Circular Ring Slim was its thorough sleep tracking data, fitness and wellness features, and comfortable design. While Kira+ could be a bit more in depth, it shows real promise and I think a lot of people will enjoy using the wellness assistant, like I did.

The Circular app was my favourite thing about the Circular Ring Slim but it had too many issues for me to really appreciate it. The battery life seriously lets it down but I think if it solves that problem, the Circular Ring Slim could be a really amazing smart ring, especially for wellness fans.

Circular Ring Slim review: alternatives to consider

The best smart ring T3 has tried (so far) is the Oura Ring Gen 3. It’s lightweight with a good build quality, impressive health features and can even detect illnesses. It does have a membership free and a high price, but it’s a good alternative to consider.

If you like the sound of the Circular Ring Slim’s sleep features, the Ultrahuman Ring Air is our favourite smart ring for sleep tracking. It comes with tons of recommendations and the app offers fitness and mindfulness exercises with no subscription fee.

Bethan Girdler-Maslen
Home Editor

Beth is Home Editor for T3, looking after style, living and wellness. From the comfiest mattresses to what strange things you can cook in an air fryer, Beth covers sleep, yoga, smart home, coffee machines, grooming tools, fragrances, gardening and much more. If it's something that goes in your house, chances are Beth knows about it and has the latest reviews and recommendations! She's also in the know about the latest deals and discount codes from top brands and retailers.


Having always been passionate about writing, she’s written for websites, newspapers and magazines on a variety of topics, from jewellery and culture, to food and telecoms. You can find her work across numerous sites, including Wedding Ideas Magazine, Health & Wellbeing, The Bristol Post, Fashion & Style Directory, TechRadar, CreativeBloq and more. In her spare time, Beth enjoys running, reading, baking and attempting craft projects that will probably end in disaster!