Circular Ring 2

Overview

The Circular Ring 2 is the second-generation smart ring from Circular, a Paris-based quantified-self startup. It launched in 2024 with a positioning unique in the consumer smart-ring market: on-demand ECG plus FDA-cleared AFib detection — features no other smart ring delivers. Combined with the broadest claimed biomarker set (140+ tracked metrics), the Ring 2 targets the medical-quantified-self segment that wants ring-form-factor convenience plus clinical-credibility features.

In the 2026 smart-ring landscape, Circular Ring 2 sits in a niche of its own. Where Oura optimizes sleep/readiness, Whoop optimizes recovery, Samsung optimizes ecosystem integration, and Ultrahuman optimizes metabolic health, Circular optimizes breadth and clinical-credibility features. The trade-off: companion app maturity is well behind the established competitors, and the smaller brand means less mature support infrastructure.

What We Measured

Note: This review is based on Circular’s published specifications, FDA-clearance documentation for the AFib detection feature, and aggregated user reports. Hands-on instrument testing with Polar H10 + Kubios HRV (Bland–Altman analysis on 7-day continuous wear) is pending. Verdict will be updated upon completion.

The ECG and AFib detection feature

The Circular Ring 2’s standout feature is on-demand ECG with FDA-cleared atrial fibrillation detection. The user initiates an ECG reading, the ring records single-lead ECG via the finger contact, and the algorithm classifies the rhythm. This is the same pathway Apple Watch uses — ECG + AFib classification — applied for the first time to a ring form factor.

The FDA clearance for AFib detection is the editorially significant fact: it’s a real, verifiable regulatory statement that the device’s AFib classification has met FDA’s substantial-equivalence standard. No other smart ring carries this clearance.

Important context: AFib screening (what the Ring 2 does) is not equivalent to AFib diagnosis (what a clinical ECG / Holter monitor does). The Ring 2 is a screening tool that flags suspected AFib for follow-up clinical evaluation. The clearance language matches that scope.

The 140+ biomarker claim

Circular markets a “140+ tracked metrics” specification. Independent verification suggests this counts include both raw metrics (HR, SpO2, temperature) and derived metrics (recovery scores, stress indices, calculated readiness). The unique-measured-metric count is smaller — comparable to Oura’s metric depth — but the breadth claim is technically accurate per Circular’s counting methodology. Buyers should match expectations to the underlying definition.

Sensor suite

Beyond ECG: SpO2, HRV, skin temperature, stress, sleep stages, activity, steps, distance, calorie tracking. The ring covers the consumer smart-ring metric set comprehensively.

Build quality

All-titanium body, four color finishes (gold, silver, black, rose). ~7+ day battery life — competitive with Oura, longer than Helio Ring or Luna Ring Gen 2. Build quality reports in user feedback are positive; the app, less so.

App maturity — the central trade-off

User reports consistently describe the Circular companion app as buggy with sync errors, unfinished features, and inconsistent data presentation. Trustpilot 3.4 (600+ reviews) and app-store ratings (3.9 iOS, 3.7 Android) reflect this. The hardware appears to outpace the software.

For users who can tolerate occasional app friction, the underlying data quality is strong. For users prioritizing software polish (Oura’s primary advantage), Circular trails materially.

3-Year Cost of Ownership

Component	Cost
Circular Ring 2 (current pricing range)	$379–$549
Subscription	$0 (none)
3-year total — flagship pricing	~$379–$549

Compare: Oura Ring 4 ($565), Ultrahuman Ring Pro ($349–$479), RingConn Gen 2 ($299), Samsung Galaxy Ring ($399).

Circular Ring 2 sits at the higher end of subscription-free smart rings. The premium reflects the ECG/AFib feature uniqueness; whether that value matches the price is the buyer-side question.

Regulatory Status

General Wellness Device for the underlying smart-ring features. FDA-cleared specifically for AFib detection — that single feature carries its own 510(k) clearance, separate from the ring’s general-wellness positioning.

This is parallel to how the Apple Watch operates: the watch is general-wellness, but specific features (ECG, AFib detection, fall detection) carry individual clearances. The Circular Ring 2 follows the same pattern, applied to the ring form factor.

When Circular Ring 2 Makes Sense — And When It Doesn’t

Strong fit:

• You specifically want ECG and AFib screening in a ring form factor. No other smart ring delivers this.
• You’re tracking a broad biomarker set and value metric breadth over algorithm maturity.
• You’ll tolerate companion-app friction in exchange for the unique feature set.

Weaker fit:

• You want the most polished software experience — Oura is still the answer.
• You’re cost-sensitive on smart rings — RingConn Gen 2 ($299) delivers core HRV/sleep/recovery without the ECG premium.
• You don’t need ECG / AFib — Oura, Samsung, or Ultrahuman are stronger general-purpose picks.

Verdict: Conditional

The Circular Ring 2 earns a conditional verdict on the strength of its uniquely-FDA-cleared AFib detection feature and broadest-in-category metric breadth — balanced against the well-documented companion-app maturity issues and premium pricing relative to subscription-free alternatives.

For buyers who specifically want ECG and AFib screening in a smart ring (a genuinely niche but real use case), the Circular Ring 2 is the only legitimate option. For general-purpose subscription-free smart-ring buyers without a specific AFib concern, RingConn Gen 2 or Samsung Galaxy Ring deliver better software polish at lower cost.

We’ll re-evaluate after independent HRV-accuracy validation. If the Ring 2’s HRV measurement also outperforms competitors, the verdict tightens toward recommended for the broader medical-quantified-self segment.

Changelog

• 2026-05-05: Initial review published based on Circular specifications, FDA-clearance documentation for AFib detection, and aggregated user-report data. Hands-on Polar H10 + Kubios HRV validation pending.