Quick Verdict
The Circular Ring Slim is the most aesthetically refined smart ring in this comparison. French-designed, available in multiple jewellery-grade finishes, with a profile so slim it genuinely looks like a fashion ring rather than a health tracker. The health data is good but not quite at Oura levels. No subscription, 45-day cookie for affiliates. Best for buyers who value aesthetics alongside function.
Pros
- Most stylish design — looks like jewellery
- No subscription fee
- Ultra-slim 2.8g profile
- Multiple premium finish options
- Good sleep and health tracking
- European design quality
- iOS and Android support
Cons
- 4-day battery (short)
- Only ATM5 water rating (50m)
- Less data depth than Oura
- Smaller company, less app maturity
- More expensive (€299) for the data quality
Overview
Circular is a French health technology company that has taken a fundamentally different approach to smart ring design: beauty first, data second. Where Oura, Samsung, and Ultrahuman have focused on maximising sensor arrays in functional rings, Circular asked the question: what if a smart ring looked like something you'd actually want to wear?
The Ring Slim is the result — an ultra-thin band available in gold, silver, and rose gold finishes that genuinely reads as jewellery at a glance. The sensor package is comprehensive enough to track all major health metrics, and the Circular app provides solid daily insights without requiring a subscription.
Key Specifications
| Price | €299 / ~$320 (no subscription) |
| Subscription | None |
| Battery Life | 4 days |
| Water Resistance | 50m / ATM5 |
| Material | Stainless steel with gold/silver/rose gold plating |
| Weight | 2.8g |
| Sensors | PPG optical, accelerometer, temperature, SpO2 |
| Compatibility | iOS and Android |
| Sizes | 49–68mm inner diameter (EU ring sizes 8–27 approx) |
| Colours | Silver, Gold, Rose Gold, Black |
Design — The Standout Feature
The Circular Ring Slim's design is its raison d'être. At 2.8mm wide with a polished metal exterior, it is visually indistinguishable from a plain metal band ring at any distance. It works as a health tracker AND as everyday jewellery in a way no other ring in this review achieves. For users who have avoided smart rings because they look too "techy" or medical, this is the answer.
The gold and rose gold finishes in particular are convincing jewellery lookalikes. The ring stacks comfortably with other rings for a layered jewellery look — something thicker rings cannot achieve.
Health Tracking Performance
The Circular Ring Slim tracks sleep stages, continuous heart rate, HRV, SpO2, and skin temperature — the full standard sensor suite for a premium smart ring. In accuracy testing, the sleep staging is good but less refined than Oura Ring 4's algorithm, particularly for identifying REM sleep. Heart rate in resting conditions is accurate and consistent.
The Circular app provides daily Kira Score (overall health summary), sleep quality score, and 30-day trend charts. The interface is elegant and user-friendly. Third-party integrations with Apple Health and Google Fit are included.
Water Resistance — A Note
The Ring Slim's 50m/ATM5 rating is lower than most competitors (Oura, Samsung, Ultrahuman, RingConn all offer 100m/ATM10). ATM5 is still suitable for swimming and showering, but not for deep-water diving. This is a minor limitation for most users but worth noting for those who spend significant time in water.
Who Should Buy the Circular Ring Slim?
Fashion-conscious buyers who want health tracking that doesn't advertise itself. Women (and men) who want a ring they can wear to formal occasions alongside casual life. Those who prioritise aesthetics alongside function. If maximum data accuracy is the priority, the Oura Ring 4 or Ultrahuman Ring AIR will serve you better. If you want the most beautiful smart ring available, the Circular Ring Slim wins without question.