💍Circular SLIM — progress, but is it enough? Comprehensive review

It’s been over a year since I published a review of the Circular Ring smart ring, which is now de facto called the Circular PRO:

💍Circular SLIM — progress, but is it enough? Comprehensive review

It’s been over a year since I published a review of the Circular Ring smart ring, which is now de facto called the Circular PRO:

Circular ring — full review
Some time ago, I published an article on the topic of a paper comparison between Circular Ring and Oura Ring 3. Now I…

The 🇫🇷 company ‍Circular, SAS believes that it has finally solved all the problems of the broad spectrum (manufacturing, waterproofing, failure rates, refunds…) and has now come up with a savior that will get them out of their misery — Circular SLIM, the expectations of which I have commented on below:

💍Circular SLIM — Resurrection?
The French brand Circular has a very tarnished reputation. Just browse the forums on reddit and you’ll find all sorts…

At the same time, they didn’t get rid of the original version of the ring, so they now offer two rings, which is the basic paper comparison:

First attempt failed

I received the Circular Slim twice. The first time everything looked normal, the ring arrived, I activated it, paired it and used it for a night. On the second day, the Slim was disconnected from the phone app and I could not get it paired again.

I have tried everything. I’ve contacted customer support via the web, via email (especially here, it’s been a real nightmare) and most recently via reddit. To cut a long story short, I sent the ring back at Circular’s expense and waited for a new one.

Supposedly my problem was completely isolated, but I was unable to find out exactly what was causing it.

I usually divide my reviews into hardware (ring and accessories) and software (mobile apps). I will do the same here. The Circular Slim is physically a new ring, and the Circular app has undergone some changes since my last review.


Circular claims that Slim is the world’s thinnest ring. Is it really?

Dimensions and weight

Until now, the Ultrahuman AIR (2.56 mm) was considered the thinnest ring. The Circular Slim can indeed theoretically boast 2.2 mm, I measured 2.21 mm for my size 10. However, it has its limitations. The ring’s thickness varies around its circumference.. I measured 4.49 mm at the bottom, where the charging pins are.

In terms of weight, the Ultrahuman Air still wins with 2.85g. The Circular Slim weighs in at 3.03g.

Logo and plasticity

I criticized the old version of the PRO for its “plasticity” and not very workshop-like ring. There has definitely been progress here. They haven’t got rid of the plastic completely, but they have added aluminium and carbon fibre to the finish and the ring is definitely more aesthetically pleasing.

One slight disappointment for the community was the white logo on the top of the ring. I mean, in the render video it looked like the logo was embossed. However, the reality is different.

Circular have even said that they will address this logo in the next batch. What exactly that means we can only guess at the moment.


In its marketing materials, Circular gives a battery life of 5 days. However, they no longer state that this figure refers to Eco Data mode. Eco mode, unlike Performance mode, does not provide data for sleep phases and also has a lower heart rate detection.

I mainly tested it in performance mode, and the battery life realistically lasts for 2 days.

The charger

The charger is the same as the PRO version. In other words, a small USB-C pendant with magnetic pins. Unlike, for example, the Oura ring, charging is by contact.

Great progress has been made in minimizing the size of the ring’s charging pins. In the original version, the pins protruded quite a bit from the body of the ring, which was particularly noticeable on the finger in the morning.

Haptic control

To be honest, the lack of a button is a bit of a disappointment. However, I am glad that there is at least one way of interacting with the ring. This involves tapping the top of the ring. For example, you can deactivate the (vibrating) alarm clock.


Vibration sets the Circular apart from the competition and can be used primarily for a discreet alarm, timer and abnormal HR and SP02 alerts. Notifications from a mobile phone, such as an incoming message, are not yet possible with the Circular.

The vibration can also be triggered manually from the app, which can be useful for searching or dancing…


Water resistance

The Slim’s predecessor, the Circular Pro, had various water resistance issues — read more here. The Circular Slim has an IPX8 rating, which means it can be submerged up to 1.5 meters for a maximum of 30 minutes.


Rather for the sake of order, which won’t come as a surprise to regular readers of this blog, I’ll mention the classic sensors. These are a 3-axis accelerometer, a temperature sensor and a PPG sensor for measuring blood pulse and related metrics.

📲 Software

The latest tested version of Circular is 16.0.0

The navigation

I think it will be very illustrative to show the current state of the application in the form of this picture.

It’s been over a year since my last full review, so there have been some changes. For example, the (circle ⭕) Activity Analysis tab has been renamed Energy Analysis, and Circle Stress Levels and Vital Alerts have been added. Well, let’s go through them one by one.

The basic screen is divided into these 3 parts (from top to bottom)

  1. My Circles — customisable tabs (i.e. they can be hidden) with all the main areas (Alarm, Sleep, Energy, Guided Breathing, Live Measurement, Stress Level, Vital Alerts).
  2. Quick Access Menu — where 3 of these options can be displayed:
  • “Data mode” to toggle between Performance/Eco modes,
    Calendar for day selection — the calendar seems a little unfortunate to me. Personally, I prefer the ability to switch left/right between days (as Oura or Ultrahuman do, for example).
  • Timer — a timer with a vibrating ring on top,
    Alarm clock,
  • Airplane mode — activating the airplane mode.

3. Feed:

  • Dashboard — where you can find basic metrics for the day or last night: steps, cardio points, HRV, RHR, respiration rate, SpO2, temperature var. etc.
  • “Kira+” is an assistant that, on request, gives you a “personalised” verbal recommendation based on the measured data. Kira+ works with Open AI. At the moment, it basically works by adding up your scores and writing a recommendation, but it’s pretty general in nature.

The Kira+ is one of the fundamental differences to other smart rings. It’s a shame you can’t continue the conversation with Kira and ask for more insights.

Let’s take a look at the individual circles.

⭕ Sleep analysis

The sleep section provides some fairly basic information that probably won’t surprise you. This is primarily the sleep quality score and then the individual attributes, which include sleep time, efficiency, sleep stages, etc. See more in the images below.

The accuracy of determining sleep stages is somewhat weaker. Sometimes the hypnogram is downright dubious and poorly segmented. For validation and demonstration, I chose one night and compared it to the leader, the Oura ring, and also to the FRENZ Brainband headband (which cascaded through the study with 87.8% agreement against the PSG gold standard).

⭕Alarm clock

The vibrating alarm is one of the Circular Ring’s most important features. None of the other Oura, Ultrahuman Air and Ringconn rings have this feature. It is an alarm that wakes you up by vibrating the ring. With the ‘Smart Alarm’ setting it should wake you in a light sleep phase and you can set the ‘spread’, that is the time window in which the vibration should wake you.

I criticised the accuracy of the sleep stage detection above, but ironically the Smart Alarm feature was quite good. For the most part, it kept me vibrating somewhere between light sleep and wakefulness.

At the same time, the Circle also gives you a wake-up score, which tells you whether you woke up at the right stage of sleep.

More recently, the “ideal bedtime” feature has been released, which is a vibrating notification that is activated as a reminder that it is a good time to go to bed based on the set alarm.

⭕Energy Analysis

The Energy Analysis ring is dominated by the summary score, of course, and then the whole tab is divided into three parts:

  • Activity metrics — steps, distance, calories, cardio points, V02 Max and maximum HR of the day. Personally, I’m not entirely convinced by the cardio points, which are not explained in this section. At the same time I have doubts about the V02 Max calculation, the value fluctuates very drastically from day to day.
  • Energy score details — these are actually the contributors to the overall score, you can click on them to find out what they mean and how they are calculated.
  • Activity graphs — graphs with the metrics I mentioned in Activity metrics, i.e. steps, distance etc. in weekly view. I don’t find these graphs very clear.

Circular does not currently allow you to measure a separate exercise, which would then be stored as an isolated activity. Therefore, the Energy Score ring provides more detailed information about your movement throughout the day.

⭕Guided Breathing

Guided Breathing is a guide to breathing exercises that uses both visual (i.e. within the app) and vibrational instructions. The accompanying instructions are actually quite a subtle guide to when to inhale and exhale. Ten Circle includes 3 breathing exercises and the user can choose their own.

⭕ Live measurement

Live measurement is used to measure instantaneous HR, HRV and SPO2 values. The ring indicates that a resting state is required for good accuracy.

⭕ Stress levels

Stress levels is a relatively new feature. Circular, like Oura, uses HR & HRV along with temperature and movement to determine stress levels. It rates this on a scale of 0–5, with these states:

  • 0–2.5 — RELAXED state
  • 2.5–3.5 — ANXIOUS
  • 3.5 — OVERWHELMED state

Circular also tries to predict the stress level for the next few hours based on previously measured data.

At the same time, it displays tags (notes) that can be added to the calendar.

For more information on this feature, click here.

⭕ Vital Alerts

Vital Alerts is another rather unique feature in the field of smart rings that benefits from the presence of vibration functionality.

It allows you to set HR and SP02 limits for when the ring should vibrate and alert the user when the set limits are exceeded. Personally, I would probably like to see an additional setting that takes movement into account. For example, whether to alert me to a higher HR when I am moving or when I am at rest. Now the alert is sent regardless of whether you are moving or not.

⭕ Medication reminder

Medication reminder 💊 is a new feature that was added to the app during the writing of this review. It’s actually a variation of an alarm clock that can remind you to take your medication at pre-set intervals. Again, you can set the type of selection to differentiate between different reminders. Oh, and if you forget what’s alerting what, you can check the schedule in the app.


Thanks to the Oura ring, we often discuss the subscription model. Circular doesn’t have one, you can use the app without any additional charges.

However, a rather subtle footnote has recently appeared in the Customise Circles section — namely that Stress, Meds and Vital alerts are free for a limited time only.

This means that these 3 features, which are based on the principle of vibrating notifications, will probably be premium features in the future and will have to be paid for. If that happens, it will be a very strange move by Circular…

One feature that users have been clamouring for is the ability to receive discreet call and message notifications from their mobile phone. Circular has been promising this for more than a year. Unfortunately, still nothing.

I’m really spoiled by the Oura ring and the daily Readiness recommendations. They are short and to the point. Circular is betting on AI in this area with the name Kira+. However, so far these recommendations have tended to be very general in nature, with AI sauce around them.

Circular was part of my recent common cold prediction test:

Will the Smart Ring detect an impending common cold? 🤒
Can an Oura Ring, Ultrahuman or Circular predict and warn of impending illness?

In terms of connectivity, the Circular is friends with both iOS Health and Google Fit, allowing you to record these metrics:


In conclusion, I will be very cautious in my evaluation. My experience with the Circular Slim ring has been thoroughly good. However, judging by the predominantly angry community on Reddit, I fear that the company’s approach to me (as a reviewer) was not entirely standard.

I have had the opportunity to assess the progress and evolution from Circular PRO (one of the earlier versions) to Circular Slim, and the progress is noticeable. The ring has undergone a decent slimming down, but it must be added that this is at the expense of battery life. Additionally, the craftsmanship has improved, and the charging pins have been reduced in size, making the Circular Slim definitely more comfortable to wear than my previous version.

The information about the thinnest smart ring is only half true; the ring is thinnest at its thinnest part. Personally, I do not particularly like the white logo on the top side of the ring, but on the other hand, it is a clear visual identifier of the correct position of the ring.

If we were to compile a list of Circular app functionalities, it would be very long. Perhaps longer than Oura’s. Unfortunately, the app’s clarity is not that good. I particularly miss the ability to swipe between days and clearer trends and graphs.

Here I also present an overview of the most notable pros & cons, I believe I have mentioned most of them in the article.

If I were to recommend the ring to someone, it would especially be to those who are looking for discreet notifications that can be used for a wide range of reasons — reminders to take medication, smart alarm, notifications for breaking vital limits that you set, etc.

If you are interested in the ring, my link will give you an 8% discount:

💍CIRCULAR SLIM DISCOUNT 8 % — https://fitnesator.link/circular

💍 OURA 3 –40–100 EUR DISCOUNT — https://fitnesator.link/oura
💍 CIRCULAR RING SLIM 8% DISCOUNT — https://fitnesator.link/circular
💍 RINGCONN — https://fitnesator.link/ringconn
💍 ULTRAHUMAN AIR DISCONT CODE — 10 %— https://fitnesator.link/ultrahuman