Circular Smart Ring Slim review

The Circular Smart Ring Slim aims to prove that Oura doesn't have to be your only smart ring option.

This is Circular's second ring after it launched the Circular Ring Pro back in 2022 with its interchangeable shell design. That raised over $600,000 across two crowdfunding platforms to bring it to life. 

Unlike Oura, it doesn't require an additional subscription to get the very most out of it and claims to be the thinnest and lightest ring that can track metrics like sleep, heart rate, steps, and more.

We had the original Circular Ring in for testing and had some problems both with the hardware and software that meant we couldn't even complete our testing. So have things got better with the Slim?

Here's our comprehensive take on what it's like to live with the Circular Smart Ring Slim.

Our reviews: Top smart rings tested

Price and competition

The Circular Smart Ring Slim costs $289/£225, which means it's significantly cheaper than the Circular Smart Ring Pro ($396/£311).  

They key differences are design, materials, promised battery life, and storage capacity – but has all the same sensors included.

If you compare that to similar smart rings that promise similar levels of tracking and features, the Oura Ring Gen 3 is $299 plus a $6.99 monthly subscription.

The RingConn Smart Ring costs $279/£220 and doesn't require a subscription while the Ultrahuman Ring Air is also subscription-free and is pricier at $349.

So it's roughly in line with the RingConn in terms of pricing and one of the cheaper rings to pick up.

Design and comfort

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Circular says the Slim is the slimmest and lightest smart ring you can wear and while that may well be the case, it doesn't make up for a look that feels a bit cheap.

We've been wearing it at the same time as the Oura Ring Gen 3 and the RingConn Smart Ring, both of which feel higher quality than what Circular is serving up.

It's not helped by the decision to drop a big white logo on the top of the ring and only offer it in a single black color option.

That quickly became a mix of black and silver as the ring has scratched more easily than most smart rings we've tested.

It doesn't follow the Ring Pro in letting you add in shells on top of that ring to give you more looks to play with.

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In terms of being the thinnest and lightest ring, it measures 2.2mm thick and weighs 2g. The Circular Smart Ring Pro is 2.75mm thick and weighs 4g.

The Oura Ring Gen 3 is 2.55mm thick and weighs 4-6g depending on ring size. While it might be slim, the ring does, in general, feel a bit bigger when you put it next to other smart rings. 

It's not a fully round ring either, with a flatter edge where the sensors are situated. That's also the place where you'll find the charging ports and it rests on top of USB-C-style charger that plugs directly into a USB-C port.

The small nature of the charger makes it easier to clip onto your keys if you want to carry it around. If you want to charge it in a laptop on a table, the ring gets knocked out of place. 

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Like other rings, it requires a sizing kit to make sure you enjoy the best level of tracking accuracy. The ring itself is comfortable to wear and that overall lightness makes it generally easy to forget about. We did find that the ring can rotate a little more than other rings, but the fit in general was good.

Another disappointing aspect here is the level of waterproofing you get here. While other rings are suitable for submerging in water, the Slim like the Pro is only splash resistant, which essentially covers washing your hands.

Health and fitness tracking

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The Slim packs in the same sensors as the Pro and rival rings like Oura, Ultrahuman, and RingConn. That means optical sensors, an infrared one, surface temperature, and motion sensors to detect movement.

And it does the same things as its bigger brother. It wants to be your activity and sleep tracker, pay closer attention to stressful periods of your day, and also offer additional insights through its Kira+ assistant.

Before any of that is useful though you need to complete a calibration period of 14 days. Once that's up, you should get more reliable data for aspects like heart rate monitoring and sleep and provide better data for that Kira+ assistant.

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Outside of continuous monitoring there is the ability to take live measurements of your heart rate, SpO2 levels, heart rate variability and show the ratio between your resting heart rate and maximum heart rate.

This measurement requires clenching your fist with the ring on and it'll indicate whether data accuracy is going to be good or poor depending on that positioning. Looking at heart rate readings, the accuracy felt pretty good on the whole and offered similar readings to Garmin's solid real-time heart rate monitoring.

As a way to track your steps and general moving about and staying active will track your steps but will also capture activity intensity over the day whilst wearing the ring.

It also dishes out cardio points, which is a bit like Amazfit's PAI Health Scores to show the days where you've worked that heart the hardest.

Something the ring won't do is specifically track exercise. There's no GPS support from the app and no app integrations with fitness apps. Step counts really are the main thing here, and we found those step totals on the high side compared to Oura and Garmin's.

We never expect them to all match up exactly, but the Circular's ring's data felt like an outlier. Generally it was around 1,000 steps off those two trackers.

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For sleep, Circular will score your sleep quality and also record sleep duration. It additionally offers data around heart rate, heart rate variability, breathing rate, temperature variation and SpO2 levels.

Core sleep data wasn't impeccable on all nights we took it to sleep. On some nights sleep duration and stages were largely in line with Oura. Metrics like temperature variation were similar to Oura, though heart rate did feel high at times.

On some nights it suggested we had no deep sleep and also didn't pick up we'd quite clearly woken up during the night. It meant that some sleep quality scores felt artificially high.

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There's a few ways that Circular aims to put some action behind that data.

The most notable one is its Kira+ assistant, which offers daily recommendations based on your personal data.  

Those recommendations appear in a long stream of text, which feels like a chore to read through in its entirety. If you do read carefully, some of the advice offered among that stream of text can offer some useful, albeit not groundbreaking tips. 

When our breathing rate was elevated, it suggested doing some deep breathing before bed.

That's kind of useful. You really have to pick through to find useful stuff here.

Circular will also fire notifications to your phone that will tell you about best periods to drink coffee, work out or to consider winding down for bed. It's something we've seen through Ultrahuman's ring that again just aims to help add structure to your day with some sensible suggestions.

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Lastly there's Energy Points, which is essentially a lot like Garmin's Body Battery Energy monitor, Training Readiness metric or Polar's Boost (part of Sleep insights). It's all about recovery and understanding your recovery needs. 

It takes HRV information, daily steps, respiratory rates and a host of indicators to better understand whether you're getting the balance of recovery, activity, sleep and everything in between right.

The Energy Point scores never seemed to drop below 80, which seemed to be a good thing and meant the balance of those things was good on most days. When you get into the more detailed breakdown of what makes up those Energy Point things start to feel overwhelming.

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Circular does want to keep your stress in check, using heart rate variability to display stress levels and even predict when you're going to feel more stressed. So when we exercised in the morning, it predicted we'd be stressed an hour or so after. So, nothing groundbreaking here.

It does offer some of the more involved guided breathing support, which does make use of the onboard vibration motor to prompt you when to breathe in and out during the small collection of exercises available.

You can also build your own breathing exercises once you get bored of the three that are included.

The app and extra features

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The best way to describe the Circular companion app is that it feels busy. The best apps are the ones you can dip in and out of and get what you need from them in a quick fashion. We're not sure that can be said of this app.

There's a lot going on in the main screen with a stream of data widgets showing of information like steps and sleep. You also have Circles, which are essentially silos of different tracking elements and features. It lets you go deeper into sleep or your Energy scores. This is also the same place you'll find the Kira+ assistant insights and jump into your calendar. 

Circular retains the same non-tracking features found on the Pro, which means you can use this ring to set up and use timers and alarms to wake you up in the morning. When the time is up or you need to wake up it'll send a vibration to the ring at which point you can tap twice on the top of the ring to stop the alarm. You can choose from different style vibrations, there's a snooze mode and crucially control the intensity of those vibrations too. They're nice extras that give the Circular Ring something different from other smart rings.

Battery life

The battery life you'll enjoy on here entirely depends on what tracking mode you decide to use it in.

Opt for Circular's Performance battery mode, which gets you the best accuracy and the richest sleep stats and we found it lasts two days at most. Choose the Eco mode and you'll sacrifice sleep stage data and the best heart rate tracking accuracy and get three to four days based on our testing. Circular says you should get up to five days, but that will entirely depend on what features you're using on a regular basis. 

In that performance mode, the battery drop-off overnight was well over 20% to give you an idea of what you're playing with here. It will last just 2-3 days.

I've spoken about the charging setup, which has its good and bad points. I like the portability of it, but don't like that its design doesn't always work well in all charging scenarios. You can buy an optional extender cable to improve things, but it feels like maybe that should've been added in here.

Circular Ring Slim Specs