Circular Ring 2 Review: The First Smart Ring With FDA-Cleared ECG -- But Is It Ready?

The smart ring category has been a two-horse race between Oura and Samsung. Now a French-designed challenger is making a bold claim: Circular Ring 2 is the first smart ring with FDA-cleared ECG and AFib detection, and it does it all without requiring a subscription.

After six weeks of daily wear, we can say this: Circular Ring 2 is the most ambitious smart ring ever built. It packs more health sensors into a ring form factor than anyone thought possible. But ambition and execution are not the same thing, and Circular Ring 2 has some rough edges that keep it from dethroning the competition outright.

What Makes Circular Ring 2 Different

Every smart ring on the market tracks sleep, heart rate, and activity. What separates Circular Ring 2 from the pack is a feature set that reads more like a medical-grade wristband than a piece of jewelry.

The headline feature is FDA-cleared single-lead ECG. To take a reading, you touch the outer electrode on the ring with a finger from your opposite hand, creating a circuit across your chest. The ring records a 30-second ECG trace and analyzes it for signs of atrial fibrillation. This is the same basic principle used by the Apple Watch ECG -- but compressed into a titanium band that weighs under 5 grams.

Beyond ECG, Circular Ring 2 includes continuous SpO2 monitoring, skin temperature sensing, HRV tracking, sleep stage detection (light, deep, REM), respiratory rate estimation, and an AI-powered health assistant called Kira that synthesizes your data into daily recommendations.

ECG on a Ring: How It Actually Works

The physics of ECG recording require measuring the electrical potential difference between two points on the body. Traditional 12-lead ECGs use electrodes placed across the chest and limbs. Smartwatches simplified this to a single-lead approach: one electrode on the wrist (back crystal), one on the opposite finger (digital crown).

Circular Ring 2 uses the same single-lead methodology, but the electrode placement is different. The ring sits on one finger, with the inner electrode contacting the skin of the ring finger and the outer electrode touched by a finger from the opposite hand. This creates a lead I equivalent -- measuring electrical activity between the left and right arms.

The Technical Challenge

Fitting ECG electrodes into a ring is significantly harder than fitting them into a watch. The surface area is smaller, skin contact is less consistent, and motion artifact from finger movement is more pronounced. Circular addressed this with:

• Dry titanium electrodes with a specialized surface coating to reduce impedance
• Advanced signal processing algorithms that filter out motion noise
• A mandatory stillness period -- you must hold your opposite finger on the ring and stay still for 30 seconds
• Quality scoring -- the app rejects readings that don't meet signal quality thresholds

In our testing, approximately 1 in 5 ECG attempts failed the quality check and needed to be retried. This improved as we learned the optimal finger positioning, but it remains less reliable than the Apple Watch ECG, where the digital crown provides a larger, more stable contact surface.

AFib Detection Accuracy

Circular's FDA submission data showed a sensitivity of 97.2% (correctly identifying AFib when present) and a specificity of 99.1% (correctly identifying normal sinus rhythm). These numbers are comparable to Apple Watch's reported sensitivity of 98.3% and specificity of 99.6%.

However, FDA clearance data is generated under controlled conditions. Real-world accuracy is typically lower due to motion artifact, poor electrode contact, and user error. In our testing, the ring successfully flagged a simulated AFib pattern in our test scenarios, but we could not evaluate real-world AFib detection accuracy without actual AFib episodes.

The Subscription-Free Model

This is where Circular Ring 2 makes its strongest argument. While Oura Ring 4 locks most of its useful features behind a $5.99/month Oura Membership ($72/year), Circular Ring 2 includes all 13+ core features at no recurring cost.

That includes sleep staging, HRV analysis, SpO2 trends, skin temperature tracking, activity tracking, ECG recording, AFib detection, respiratory rate, the Kira AI assistant, readiness scores, and guided breathing exercises.

Circular has stated that it plans to monetize through premium AI features and optional coaching programs in the future, but the core health tracking features will remain free indefinitely. This is a significant competitive advantage, especially as subscription fatigue grows among wearable users.

Sleep Tracking: Good, Not Great

Sleep is where smart rings excel due to their unobtrusive form factor, and Circular Ring 2 delivers solid sleep data -- with some caveats.

The ring detects four sleep stages: awake, light, deep, and REM. In our testing against a simultaneous Dreem 3S EEG headband (our reference standard), Circular Ring 2 achieved approximately 78% epoch-by-epoch agreement for sleep staging. For context, Oura Ring 4 achieves approximately 82% agreement in similar comparisons, and the Apple Watch Series 11 hits about 75%.

Where Circular falls short is sleep onset detection. The ring occasionally misidentified quiet reading in bed as light sleep, inflating total sleep time by 15-25 minutes on several nights. This is a common issue with wrist-free wearables that rely heavily on heart rate and motion data without the accelerometer fidelity of a wristband.

HRV and Recovery Metrics

Circular Ring 2 measures HRV using rMSSD (root mean square of successive differences), sampled during the first stable period of deep sleep. This methodology aligns with current best practices and produces consistent, comparable data.

The ring's Energy Score -- its equivalent of Oura's Readiness Score -- combines HRV, resting heart rate, sleep quality, skin temperature deviation, and SpO2 data into a 0-100 composite score. In our experience, the Energy Score tracked logically with perceived recovery: lower after poor sleep or heavy training, higher after rest days.

The App: Where Things Get Rough

Circular's hardware ambition is impressive. The companion app, unfortunately, does not match it.

During our six-week testing period, we experienced data sync failures approximately twice per week, where the ring would not upload overnight data until force-closing and reopening the app. On three occasions, an entire night of sleep data was lost entirely -- the ring recorded it, but the app failed to process the transfer.

The app's UI is clean and modern but suffers from inconsistent navigation. Health metrics are spread across multiple tabs without clear hierarchy, and the historical data views lack the polished charts and trend analysis that Oura and Apple Health provide.

Firmware updates during our review period did improve stability, and Circular has acknowledged app quality as a top priority for their engineering team. But as of April 2026, the app experience is the single biggest reason we cannot give Circular Ring 2 an unreserved recommendation.

Circular Ring 2 vs Oura Ring 4 vs Samsung Galaxy Ring

The smart ring market now has three serious contenders. Here is how they compare across every major dimension.

Who Should Buy Circular Ring 2?

Buy it if:

• You want ECG and AFib detection in a ring form factor -- no other ring offers this
• You refuse to pay subscription fees for health data you generate
• You prioritize comprehensive health metrics (SpO2, skin temp, ECG) over app polish
• You are comfortable with a product that is still maturing through software updates

Skip it if:

• You need a reliable, polished app experience today -- Oura is significantly ahead
• You are deeply invested in the Apple or Samsung ecosystem and want seamless integration
• Sleep tracking accuracy is your primary concern -- Oura has the edge
• You want a large community and extensive third-party app support

The Kira AI Assistant

Circular's AI health assistant, Kira, analyzes your biometric trends and delivers daily insights. In practice, Kira works best when it has at least two weeks of data to establish baselines.

After the initial learning period, Kira provided useful observations like correlating late caffeine consumption (logged manually) with reduced deep sleep, and identifying that our HRV consistently dropped on days following high-intensity training without adequate protein intake.

However, many of Kira's recommendations felt templated rather than truly personalized. Suggestions like "try to maintain a consistent sleep schedule" and "stay hydrated throughout the day" don't require AI analysis. The system shows promise but needs more sophisticated personalization to compete with WHOOP Coach 2.0 or Apple's Vitals AI.

Rating

Circular Ring 2 earns a 7 out of 10. The hardware is genuinely innovative -- putting FDA-cleared ECG into a smart ring is a technical achievement that deserves recognition. The subscription-free model is consumer-friendly and forward-thinking. But the app reliability issues, occasional ECG reading failures, and still-maturing AI features prevent it from claiming the top spot.

If Circular can ship consistent app updates and resolve the data sync problems, this score could easily climb to an 8 or higher. The foundation is strong. The software just needs to catch up with the hardware.

Technical Specifications

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