Inside the Evie Ring: Sensors, Battery, App, and FDA Registration

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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.

The Evie Ring is the rare smart ring built from the ground up around women’s health rather than retrofitted with a cycle-tracking module. It’s also one of the only rings with FDA registration as a Class II medical device for pulse oximetry — a real, narrow regulatory designation that’s worth understanding before you buy. Here’s what’s inside, what it does well, and where the trade-offs are.

Sensors

Evie carries a focused sensor stack tuned for women’s-health framing rather than for athletic performance:

• Red and green LEDs: for heart rate and SpO2 measurement.
• Infrared PPG sensors: additional pulse signal for accuracy.
• Photodiodes: light receivers paired with the LEDs.
• Skin temperature sensors: overnight temperature trend for cycle tracking.
• 3D accelerometer: motion and sleep staging.

What Evie tracks: heart rate, resting heart rate, HRV, skin temperature variation, respiration rate, blood oxygen (SpO2), menstrual cycle and ovulation prediction, mood and symptom logging, and sleep quality. The cycle tracking is the centerpiece of the product, not a bolt-on feature.

Design and dimensions

• Width: 8mm
• Thickness: 3mm
• Weight: 3.2 to 3.7 grams
• Material: titanium alloy with PVD coating, plus Liquidmetal® scratch-resistant accents
• Available in multiple finishes including Rose Gold, Gold, Silver, and Black
• Sizes: 5 through 12

Evie’s design is the most visibly “ring-like” of the major smart rings — thinner edges, softer profile, finishes that read as jewelry first and tech accessory second. If you’ve passed on Oura because it looked like a piece of hardware on your hand, Evie is the one to look at.

Battery life

Evie quotes 4+ days of battery on a single charge, which puts it at the lower end of the category. The included portable charger holds enough for 10+ full ring charges — meaningful if you travel and don’t want to remember a wall plug, but the per-charge runtime trails Oura, Ultrahuman, and especially RingConn.

Water resistance — read this carefully

Here’s the spec that separates Evie from the rest of the category: up to 1m of water resistance. That covers handwashing, rain, and brief splashes. It is not rated for swimming, showering, or sustained submersion. If you wear your ring 24/7 including in the shower, that’s a real limitation. Evie’s positioning around the slimmer, more jewelry-like build comes at this cost.

FDA status

Evie is FDA registered as a Class II medical device for pulse oximetry. That clearance is for SpO2 measurement specifically — not for cycle tracking, not for contraception, not for general “health diagnosis.” The distinction matters because Evie’s marketing leans into the women’s-health framing, but the regulatory clearance covers a single sensor function. For cycle-based contraception you’d still need a separately FDA-cleared software like Natural Cycles. Evie’s own cycle features are awareness tools, not regulated birth control.

The Evie app and what it covers

The Evie app focuses on women’s health — cycle prediction, ovulation prediction, mood and symptom logging — alongside the standard sleep, heart rate, and recovery metrics. Compatibility is iOS and Android. The interface is the most thoughtfully “non-clinical” of the major ring apps; if Oura’s data-heavy dashboards feel cold to you, Evie reads warmer.

Subscription model: there isn’t one

No recurring fee. The full app feature set is included with the $269 ring purchase. That undercuts Oura significantly on year-one total cost, especially if cycle tracking is the primary reason you’re considering a ring.

Who should buy the Evie Ring

Women specifically interested in cycle tracking, ovulation prediction, and a women’s-health-first interface. People who want a ring that reads as jewelry rather than tech. People allergic to subscriptions. People who don’t shower or swim with their ring on (or are willing to take it off for that).

Who should skip it

Athletes who want serious training data — Evie isn’t pitched at that user. Anyone who wants to wear their ring in the pool or shower; the 1m water resistance is a real limit. People looking for the deepest sleep validation research; Oura still wins there. Anyone hoping the FDA registration translates to FDA-cleared contraception — it doesn’t.

The bottom line

Evie Ring is the cleanest pick if women’s-health features are your reason to buy and you don’t mind taking the ring off for showers. The hardware is honest about what it’s for: a thoughtful, jewelry-leaning device that does cycle and recovery tracking well, with a real but narrow FDA registration to back the SpO2 sensor. Just match your daily-wear expectations to the 1m water spec before you commit.

Buy the Evie Ring: Check the Evie Ring →

Order the free sizing kit before you buy — Evie’s sizing differs from standard jewelry sizing and the rings are designed to adapt to natural changes in body size, which matters more than you’d think.

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 Evie Ring FDA approved?
Evie is FDA registered as a Class II medical device for pulse oximetry. That covers the SpO2 sensor specifically. It is not FDA cleared for contraception, fertility, or any broader medical claim. For FDA-cleared cycle-based contraception, look at Natural Cycles separately.

Can I shower or swim with the Evie Ring?
No. Evie is rated to 1m of water resistance, which covers handwashing and rain but not showering, swimming, or sustained submersion. If you wear a ring 24/7 including water activities, this isn’t the right pick.

How long does the Evie Ring battery last?
4+ days per charge. The included portable charger holds enough for 10+ additional ring charges before it needs a wall outlet.

Is there a subscription fee for the Evie Ring?
No. The $269 purchase price covers the ring and all app features with no recurring fee.

What sizes does the Evie Ring come in?
US sizes 5 through 12. Order the free sizing kit; Evie’s sizing differs from standard jewelry sizing and is built to adapt to natural finger size changes.