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Smart ring

Circular Ring 2

Obsidian Black finish, US sizes 6 to 14 (or the $5 sizing kit option), charging case included

$349US price, checked Oct 24, 2026

Smart ring with on-demand ECG, a charging case and no mandatory subscription

Check price at Circular Full review →

Smart ring

Oura Ring 4

Silver finish, plus Oura Membership billed separately

$349US price, checked Oct 23, 2026

Oura's previous-generation smart ring, with the membership billed separately

Check price at Oura Full review →

Criterion	Circular Ring 2	Oura Ring 4
US price	$349Checked Oct 24, 2026 · Price details in the review	$349Checked Oct 23, 2026 · Price details in the review
Three-year ownership estimate	$349 Incomplete known subtotal. Not included: energy use.	$558.97 Incomplete known subtotal. Not included: energy use.
Choose if	You wear a US size 6 to 14 and want an on-demand ECG from a ring for $349 without a required yearly plan.	You want Oura's sleep, heart-rate and temperature-trend tracking in the previous-generation ring and don't need Ring 5's smaller size.
Main trade-off	Circular publishes no accuracy data for its blood pressure estimate and names no maker for its FDA-cleared algorithm.	You pay a yearly membership for full features, and Oura says Ring 5 is 40% smaller, with greater accuracy and battery power.
Key specifications	Battery and charging: About 8 days in power mode and about 5 days in performance mode per the spec table (the FAQ says 4 to 5). The wireless case recharges the ring in about 30 minutes. A case is included. Sensors: Single-lead ECG, optical PPG (green, red and infrared), skin temperature and a 3-axis accelerometer. Circular's science page lists the ECG at 510 Hz and the optical signal at 25 Hz. ECG and AFib: Circular says the AFib classification uses an algorithm cleared by the FDA. It names no algorithm maker or clearance number on the pages checked. You can export the trace as a PDF. Blood pressure estimate: Circular estimates blood pressure from the pulse wave timed between its ECG and optical sensors. You enter three cuff readings at setup and one every 30 days. A session takes about a minute and lands in one of six bands. Circular calls it a wellness estimate, not a diagnosis, and publishes no accuracy data. Subscription and paid features: Circular says no subscription is mandatory, and the product page says blood pressure comes included with nothing to renew. Its help center says the advanced parts of blood pressure and glucose are paid, by card or with app coins. Those parts are insights, long-term trends and PDF export. The Circular+ Guarantee add-on is $20 billed yearly. Sizes and fit: US 6 to 14, no half sizes. Sizes run about half a size above standard. A sizing kit is a $5 size option and can't be returned. Water resistance and storage: IP68, and Circular's FAQ says showers and swimming are fine. The ring stores 7 days of data and then overwrites the oldest. Circular suggests syncing every 5 to 6 days. Compatibility: iOS and Android over Bluetooth Low Energy. It syncs with Apple Health and Android Health Connect. Circular lists no minimum iOS or Android version on the pages checked. Delivery: Ships to the US with import duties and taxes prepaid (Delivered Duty Paid, per Circular's terms). Estimated delivery runs 3 to 25 days depending on method and address. Shipping and handling are included in a ring's price. US import status: Circular SAS was named in Oura's ITC case 337-TA-1398. It settled with Oura in June 2024, and the ITC ended the case against it in 2024 on that settlement. The August 2025 exclusion order covers the remaining respondents, and the cease and desist orders went only to Ultrahuman and RingConn. Oura says Circular holds a multi-year license, subject to royalties. Oura's newer ITC case, 337-TA-1468, doesn't name Circular. Next model: Circular Ring 3 comes as a Pro and a Slim. Circular says pre-orders open in January 2027 and prices come at launch. It says Ring 2 stays on sale until Ring 3 is released.	Battery life: 5 to 8 days. A charge typically takes 20 to 80 minutes, depending on battery level. Oura says battery life and longevity depend on ring size, app settings and feature configuration Sensors: Red and infrared LEDs for blood oxygen. Green and infrared LEDs for heart rate, heart rate variability and respiration rate. A digital temperature sensor and an accelerometer Membership: $5.99 a month or $69.99 a year in the US, before tax. New members get the first month free. The ring has limited functionality without it Water resistance: Up to 100 m (328 ft) Compatibility: iOS and Android Connectivity: Bluetooth Low Energy, with airplane mode support. Firmware updates come through the Oura App. Oura lists the ring as FCC-certified Weight: 3.3 to 5.2 g depending on size
Exact variant	Obsidian Black finish, US sizes 6 to 14 (or the $5 sizing kit option), charging case included	Silver finish, plus Oura Membership billed separately
Dimensions	Circular's product page lists the ring as 2.55 mm thick, 7.8 mm wide and about 4 g. Its science page gives 4.5 g and a 2.6 mm profile, so the figures differ by page. Circular doesn't state weight or inner diameter for each size.	7.90 mm wide and 2.88 mm thick.
Installation	No installation. You pair the ring with the Circular app on iOS or Android over Bluetooth Low Energy. The box holds the ring, a wireless charging case, a USB-C cable and a cleaning cloth. Circular sizes run about half a size above standard ring sizes, so a standard 9 means a Circular 10. If you don't know your size, a sizing kit costs $5 as a size option. A size exchange has to happen within 14 days of delivery.	No installation. It pairs with the Oura app on iOS or Android. It charges on the included size-specific charger with a USB-C cable. Oura offers a free sizing kit with plastic sizers before you order.
Materials	The product page says titanium with a PVD finish and a hypoallergenic inside. Circular's science page describes grade-5 titanium with a diamond-like carbon coating instead. Circular doesn't name the inner material.	The inner and outer surfaces are titanium, and the interior has no seams. Oura says minor scratches from regular wear are normal.
Warranty	One year from delivery, covering defects in materials and workmanship (two years in the EU under local law). It excludes a wrong size, normal wear and tear, cosmetic damage such as scratches, accidents and misuse. Batteries are covered only if a defect damaged the product. Circular's help center says the battery can't be replaced and offers a discount code toward a new ring if it weakens. The optional Circular+ Guarantee costs $20 billed yearly, and the product page describes it only as extra protection.	A 1-year protection plan is included with purchase. The product page offers upgrades to 2 years for $45 or 3 years for $60.
Returns	14 days from receipt, for any reason, if the ring is unused and in its original condition. You email support with a photo and your order number to start the return. For a change-of-mind return you pay shipping back, and Circular keeps a fixed $25 of shipping and handling from the refund. Import taxes aren't refunded. Sizing kits can't be returned. A size or color exchange has its own 14-day window, and you pay the shipping.	Bought directly from Oura's online store, the ring can be returned within 30 days of receiving the order, if it's fully functional and undamaged and comes back with the charger and USB-C cable. Ring sets can't be partly returned, exchanges are only for a different size of the same ring, and each item can be returned or exchanged once. Refunds go to the original payment method, but original shipping and handling aren't refunded. Oura's return FAQ says to cancel the membership when you return a ring, and the product page says canceling doesn't entitle you to a full or pro-rated refund of fees you've paid. Warranty replacements can't be returned for a refund, and rings bought from other retailers follow that retailer's policy.
Unresolved questions	Which company makes the FDA-cleared AFib algorithm, and its 510(k) number Independent accuracy data for Ring 2's ECG, sleep, blood oxygen and blood pressure, since Circular's own validation work is still in review What the Circular+ Guarantee covers and what a Circular+ subscription costs Whether the advanced parts of blood pressure are paid for every buyer, since the pages disagree Whether Oura's license names Ring 2, how long it lasts and what the royalty is Battery lifespan, out-of-warranty replacement cost and charger power draw Inner diameter for each size and the minimum iOS and Android versions The ring's FCC equipment authorization, which the pages checked don't mention How long Circular will keep selling and supporting Ring 2 after Ring 3 launches	Who pays return shipping, which Oura's return FAQ doesn't say How long Oura will keep selling and supporting Ring 4 now that Ring 5 is on sale How accurately Ring 4 itself stages sleep against polysomnography Battery lifespan, out-of-warranty replacement cost and charging power draw The FDA regulatory status of Ring 4 features

These comparisons use published specifications, not head-to-head tests. Prices and terms are dated. Unknown costs are not zero. Check the exact variant before you buy.