Inside the Circular Ring 2: ECG, AFib Detection, Dual-Mode Battery, and No Subscription

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This article is for general information only and is not medical advice. Readers should consult a qualified healthcare provider for diagnosis or treatment.

Circular Ring 2 is the only mainstream smart ring with a built-in clinical-level ECG and AFib detection — and that single fact is the reason it deserves a closer look than the spec sheet alone suggests. The brand also markets aggressively around 140+ biosignals and an AI coach. Some of that is hype; some of it is real. Here’s the spec breakdown, what’s genuinely differentiated, and what to know before you buy.

Sensors

Circular Ring 2 carries the most ambitious sensor stack in the category:

• ECG (electrocardiogram): on-demand single-lead heart rhythm monitoring with AFib detection. Circular markets this as clinical-grade — the only ring currently shipping with this capability.
• PPG sensors: 24/7 heart rate and HRV.
• SpO2: medical-grade pulse oximetry during sleep and rest.
• Skin temperature: continuous temperature tracking for cycle and fever detection.
• 3-axis accelerometer: activity classification, sleep staging, automatic workout detection.
• Breathing rate: derived from sleep-period sensor data.

Circular’s marketing claim of “140+ biosignals across 13+ health features” is a reframing of the same physical sensors processed through different software lenses. The ECG is the genuine hardware differentiator. Glucose-trend and blood-pressure features are listed as “coming soon” — treat those as roadmap, not shipped.

Design and dimensions

• Width: 7.8mm
• Thickness: 2.55mm
• Weight: 3 grams
• Material: titanium body, hypoallergenic interior
• Sizes: US 6 through 14 (no half sizes)
• Multiple finishes including matte black and gold options

2.55mm thickness puts Ring 2 among the thinner rings — slimmer than Oura‘s 2.88mm, slightly thicker than RingConn‘s 2mm. Three grams is light. The build reads premium and matches the marketing claims.

Battery life

Circular publishes two battery modes:

• Power Mode: up to 8 days per charge, with reduced sampling frequency on some sensors
• Performance Mode: 5 days, with full continuous sampling including 24/7 HR/SpO2/temperature

Charging is wireless and quick — Circular quotes about 30 minutes for a meaningful top-up. The dual-mode approach is unusual and useful: you can stretch battery for travel without losing the ring entirely, or run full-fidelity at home.

Water resistance

Fully waterproof — safe for swimming, showering, and daily wear. Circular doesn’t publish a depth rating as prominently as competitors, but the marketing language covers normal water exposure.

The Circular app and “Kira” AI coach

The Circular app gives you sleep, recovery, activity, ECG readings, AFib alerts, and a feature called Kira — an in-app AI coach that turns your data into plain-English suggestions. AI coaching in wearables is generally a marketing layer rather than a clinical tool; treat Kira as a friendlier-to-read summary, not a doctor. The core data and trend views are solid and on par with the rest of the category.

iOS and Android compatibility, with Apple Health and Google Health Connect integration.

Subscription model: there isn’t one

No recurring fee — Circular’s primary marketing wedge against Oura. The $349 sticker buys the ring, the app, and the full feature set. No daily-score gating, no premium-features paywall.

Note: Circular offers HSA/FSA eligibility on the U.S. site. If you have year-end health-spending funds to use, that’s a meaningful tax win.

Who should buy the Circular Ring 2

People who specifically want ECG and AFib detection in a ring form factor — that’s not available anywhere else in the category. Quantified-self users who appreciate the dual-mode battery flexibility. Buyers who want the no-subscription savings versus Oura. HSA/FSA holders who can use pre-tax dollars.

Who should skip it

People who want a long brand track record and proven post-sale support — Circular has accumulated a non-trivial set of Trustpilot and Kickstarter complaints around delivery delays and hardware quality on the original first-generation product. Ring 2 is a different product than the originals, but the company’s customer-service history is part of the buying decision and worth weighing. People expecting the “coming soon” features (glucose trends, blood pressure) at launch should wait until those actually ship before buying. People who want the deepest sleep research validation; that’s Oura’s territory.

The bottom line

Circular Ring 2’s ECG and AFib detection are genuinely differentiated and the only reason in the category to choose this ring over more established alternatives. The dual-mode battery, the no-subscription pricing, and the HSA/FSA eligibility all add real value. The customer-service history is the asterisk to keep in mind. If the ECG capability is what’s drawing you in, this is the ring that has it.

Buy the Circular Ring 2: [AFFILIATE LINK PENDING: Circular — Awin] — until that’s live, the brand’s product page is at shop.circular.xyz.

If you’re considering this primarily for AFib monitoring, talk to your cardiologist first. A consumer ECG is a screening prompt, not a diagnostic substitute for clinical evaluation.

What to read next

• Best Smart Rings of 2026: The Complete Buyer’s Guide — the full 2026 lineup, ranked.
• Oura Ring 4 vs Ultrahuman Ring PRO — the flagship head-to-head most buyers care about.
• Best Smart Rings with No Subscription — the rings that skip monthly fees entirely.

Frequently Asked Questions

Is the Circular Ring 2 ECG actually FDA cleared?
Circular markets the ECG as “clinical-level” and “medical-grade.” Specific FDA clearance status varies by region and feature; check Circular’s current regulatory documentation and your country’s regulator before relying on the ECG for any medical decision. As with all consumer wearables, an ECG reading is a screening tool, not a diagnosis.

Is there a subscription fee for the Circular Ring 2?
No. The $349 purchase price covers the ring and full app feature set with no recurring fee.

How long does the Circular Ring 2 battery last?
Up to 8 days in Power Mode (reduced sampling) or 5 days in Performance Mode (full 24/7 sensor sampling). Wireless charging takes about 30 minutes for a meaningful top-up.

Can I detect atrial fibrillation with the Circular Ring 2?
The ring includes on-demand ECG with AFib detection. As with all consumer ECG devices, treat the alert as a prompt to see a cardiologist for a clinical 12-lead ECG, not as a diagnosis. Smart-ring ECGs are single-lead screening tools.

Is the Circular Ring 2 HSA/FSA eligible?
Yes in the U.S. Confirm with your specific plan administrator before purchase.