Circular Ring 2 vs Oura: optional coins or a required membership

The verdict

Coins vs membership: what each charges

Oura is the clearest case of a required fee in smart rings. The Oura Membership costs $5.99 a month or $69.99 a year in the US. Without it, Oura says you'll “only be able to see your three daily Oura scores (Readiness, Activity, and Sleep), ring battery, basic profile information, app settings, and the Explore content” ( Oura Support). There's no lifetime plan.

Circular promises “no forced subscription” and says “every core feature is included with the ring” (Circular). There is a catch, reported by Tech Advisor: some future features will need “Circular Coins,” a virtual currency you can buy. We couldn't find which features will cost coins or what coins cost, so treat it as an unknown: nothing you pay for today, but not a promise that everything new stays free.

The details of Oura's plan are in what the Oura membership costs, and which smart rings require a subscription sets out every brand's policy, optional extras included.

ECG and AFib: what Circular lists, and what it means

This is Circular's headline difference. Its product page lists single-lead ECG and AFib detection. Oura's Ring 5 page lists heart rate, HRV, blood oxygen, temperature trends, Symptom Radar, Cardiovascular Age, Blood Pressure Signals and Cycle Insights, and no ECG.

So the ECG feature is a real difference on the spec sheet, but not a medical one you can lean on. We found no study of how well the Circular Ring 2's ECG or AFib detection performs. More on ring blood pressure features in can a smart ring measure blood pressure?

What the evidence says

This is Oura's side of the ledger, and it's a long one. Dial et al. (Physiological Reports, 2025) tested Oura Gen 3 and Gen 4 against a Polar H10 chest strap across 536 nights in 13 adults; the Oura rings had the highest agreement of the wearables tested, with Gen 4 resting heart rate error of 1.94% and HRV error of 5.96% (Dial et al.). For sleep, Svensson et al. (Sleep Medicine, 2024) compared Oura Gen3 with multi-night ambulatory polysomnography in 96 participants and reported overall accuracy of about 91.7%, and a 2025 meta-analysis by Khan et al. of six studies (388 people) found no statistically significant differences from polysomnography or actigraphy on total sleep time, sleep efficiency or time in light, deep and REM sleep.

For the Circular Ring 2, we found no published validation at all. One trap to avoid: Herberger et al. also tested a ring called “Circul,” which is a different product, not Circular's ring. Its results say nothing about the Circular Ring 2. Tech Advisor's Circular reviewer, for what it's worth, wrote: “I've not tested a smart ring that I really think does a good job of tracking heart rate during exercise.” Our smart ring accuracy guide and sleep tracking accuracy guide cover every study.

Sizing

The two brands size differently, and neither matches standard ring sizes.

• Circular Ring 2: US sizes 6-14, “running approximately half a size larger than standard ring sizing.” Circular sells a physical kit of replica rings separately and offers a printable sizing guide. Its help page says sizing kits are consumables and not refundable.
• Oura Ring 5: sizes 6-13, whole sizes only, which Oura says “differ slightly from standard US ring sizes.” The sizing kit is free on Oura's store and sold on Amazon with a matching credit toward the ring. Oura recommends the index finger.

So a Circular size doesn't tell you an Oura size, or the reverse, and no brand says its sizes match another's. Use each brand's own sizer on the finger you'll wear it on. See smart ring sizing compared and our sizing guide.

Battery and charging

Circular claims “8 days (power mode) or 5 days (performance mode).” Its pages disagree on charge time: the product page says 30 minutes to full, while its shop page says about 60. Circular doesn't state a charging case. Oura claims 6-9 days for Ring 5 “depending on ring size and usage,” and DC Rainmaker got 6-7 in his review; there's no charging case either. On claims alone, it's close, and it depends which Circular mode you run. Our smart ring battery life guide puts every claim side by side.

Where to buy, and the fine print

• Circular Ring 2 is sold only by Circular. We found no Amazon.com listing for it. An Amazon search does show an unrelated “Circul Smart Ring” from a different brand, so don't mix them up. The button on the Circular card is a plain link to Circular's store; we don't earn from it.
• Returns: Circular offers 14-day returns (“no questions asked” on its shop page); its help page says a “fully functional and undamaged” ring can be returned within 14 days of receipt for a full refund. Oura Ring 5 is listed on Amazon.com, so Amazon's return process applies there.
• Warranty: Circular gives one year in the US (two in the EU). Oura gives one year from purchase, with batteries excluded as consumables. See smart ring warranty and returns.
• Phones: Circular's homepage lists iOS 15 and later and Android 9 and later. Oura supports iOS and Android.
• HSA/FSA: both brands say their rings are eligible. Plan rules vary; see smart ring HSA/FSA eligibility.
• Legal status: Circular has held a license from Oura since June 25, 2024, described by Oura as “a multi-year license to continue selling Circular smart rings and associated app, subject to royalty payments” (Oura).

Side by side

On our ring rubric (methodology), the Circular Ring 2 scores 6.0 and the Oura Ring 5 scores 5.8. They get there from opposite ends: Circular scores well on cost of ownership and data ownership and near the bottom on evidence; Oura is the reverse. The RingConn Gen 2 scores 8.2 because it does well on both.

Mistakes to avoid

• Buying the wrong ring on Amazon. A search for Circular on Amazon.com turns up an unrelated “Circul” ring. The Circular Ring 2 is sold by Circular's own store.
• Ordering your jewelry size. Circular runs about half a size large, and Oura's whole sizes differ from standard US sizes too. Both brands tell you to use their sizer, and Circular's kit isn't refundable, so order the kit for the ring you'll actually buy.
• Forgetting Oura's included month ends. New Oura members get one month of membership with the ring. After that the fee starts, and without it you're down to three daily scores.
• Assuming “no forced subscription” means nothing will ever cost extra. Circular Coins are optional today, but some future features will use them, and we couldn't find what they'll cost.
• Buying for the ECG alone. Circular says the ring is not a diagnostic device. If a heart rhythm question is the real reason you're shopping, start with a clinician.
• Missing the return window. Circular's window is 14 days from receipt, and the ring must be fully functional and undamaged. Decide quickly whether it fits.

Who should buy which

• Circular Ring 2: you want no required fee and ECG and AFib detection on the feature list, and you're comfortable buying direct and sizing carefully. Read our Circular Ring 2 review.
• Oura Ring 5: you want the brand with the most published evidence and accept the membership as the price of it. Read our Oura Ring 5 review, and if you're choosing between Oura generations, Oura Ring 4 vs 5.
• RingConn Gen 2: you want no fee, a chest-strap comparison of its own (a preprint) and an Amazon-sold ring. See Oura vs RingConn or our RingConn Gen 2 review.

Every ring in one table: the smart ring comparison chart. More matchups: the compare hub.