Smart Ring Fit Advisor
A cautious, model-specific starting point for Oura Gen3 / Ring 4, Samsung Galaxy Ring, RingConn Gen 2, Ultrahuman Ring AIR, and Evie — from your usual jewellery size, finger, wear time, and between-size status. It never replaces the official sizing kit.
Jewellery size in, kit size to test out
Get your cautious starting size
Answer four questions. The result names one kit size to test first, why, and the exact 24-hour checks that confirm it.
Smart-ring fit advisor
Pick your model and usual jewellery size — get a cautious starting size plus the 24-hour kit test that confirms it. This never replaces the official sizing kit.
Guidance only — not a substitute for the official sizing kit. Smart rings cannot be resized, so confirm every suggestion with a 24-hour kit test and the manufacturer guide linked in your result.
Cross-brand size ranges at a glance
All five brands overlap on nominal US numbers but differ in width, interior geometry, and range. Never transfer a size between brands without that brand's kit.
| Model | Whole sizes | Band width | Interior / shape | Between sizes |
|---|---|---|---|---|
| ModelOura Ring Gen3 | Whole sizes6–13 | Band width7.9 mm | Interior / shapeRound band with raised interior sensor domes. | Between sizesSize up, then 24-h kit test |
| ModelOura Ring 4 | Whole sizes6–13 | Band width7.9 mm | Interior / shapeRound band with recessed/flush sensors (smoother than Gen3 domes). | Between sizesSize up, then 24-h kit test |
| ModelSamsung Galaxy Ring | Whole sizes5–13 | Band width7 mm | Interior / shapeConcave titanium exterior; 3 raised interior sensor nodes. | Between sizesSize up, then 24-h kit test |
| ModelRingConn Gen 2 | Whole sizes6–14 | Band width6.8 mm | Interior / shapeSquircle (softly squared) exterior, circular interior; dual palm-side sensors. | Between sizesSize up, then 24-h kit test |
| ModelUltrahuman Ring AIR | Whole sizes5–14 | Band width8.1 mm | Interior / shapeWide flat band with raised interior PPG nodes. | Between sizesSize up, then 24-h kit test |
| ModelEvie Ring | Whole sizes5–12 | Band width8 mm | Interior / shapeSplit-shank open-gap band; sensors rest on the palm side. | Between sizesSize up, then 24-h kit test |
Oura Ring Gen3
Sizes 6–13 · 7.9 mm wide · 2.55 mm thick
Gen3 uses Oura sizes 6–13. Oura warns its sizes differ from standard jewellery sizing — use the Gen3 guide and kit, not a transferred jewellery size.
Fit bias: Snug sensor contact; dome bumps feel slightly tighter than a flat band.
Oura Ring 4
Sizes 6–13 · 7.9 mm wide · 2.55 mm thick
Ring 4 has its own model-specific sizing guide and kit. Oura sizes run 6–13 (check the Ring 4 guide for your region) and differ from jewellery sizing.
Fit bias: Snug but slightly smoother interior than Gen3; still size up when between sizes.
Samsung Galaxy Ring
Sizes 5–13 · 7 mm wide · 2.6 mm thick
Sold in 9 whole sizes, 5–13. Three raised PPG/temperature sensor bumps make it fit slightly snugger than a flat band.
Fit bias: Slightly snug due to sensor bumps; half sizes must size up.
RingConn Gen 2
Sizes 6–14 · 6.8 mm wide · 2.0 mm thick
Gen 2 uses its own 9-piece kit (sizes 6–14). Do not reuse a Gen 1 kit: Gen 2 is thinner (2.0 mm) and narrower (6.8 mm) with a squircle anti-spin profile.
Fit bias: Structured anti-spin feel; squared corners need comfortable knuckle clearance.
Ultrahuman Ring AIR
Sizes 5–14 · 8.1 mm wide · 2.45–2.8 mm thick
Ring AIR comes in whole sizes 5–14. At 8.1 mm wide it is the widest major smart ring, so it feels roughly half a size tighter than a slim jewellery band.
Fit bias: Very snug due to 8.1 mm contact area; always size up when between sizes.
Evie Ring
Sizes 5–12 · 8 mm wide · 3.0 mm thick
Evie comes in whole sizes 5–12 only (no 13/14). The split-shank open gap flexes ~0.5–1.0 mm, which helps with knuckles and cyclical swelling but does not replace kit testing.
Fit bias: Open gap eases knuckle passage; test both neighbours when between sizes.
Why smart-ring sizes differ from jewellery sizes
The four physical reasons manufacturers warn against transferring a jewellery size.
Raised interior sensors
Oura domes, Galaxy Ring's three PPG/temperature nodes, RingConn's dual palm-side sensors, Ultrahuman's PPG nodules, and Evie's optical array all protrude into the finger cavity. They need steady skin contact, which reads as snugger than a flat comfort-fit band at the same nominal number.
Wide-band grip
Smart bands run 6.8 mm (RingConn Gen 2) to 8.1 mm (Ultrahuman AIR) wide, versus 2–4 mm for many jewellery bands. A wide band contacts more skin and traps more moisture, so it grips firmly — the main reason half sizes size up instead of down.
24/7 swelling window
Fingers change up to half a size between afternoon and morning, plus heat, workouts, salt, and hormonal cycles. A jewellery ring worn for evenings out can be tighter than a tracker worn through sleep. The kit test must include one full night — morning fit decides.
No half sizes, no resizing
None of these models offers half sizes, and none can be cut or stretched afterwards — batteries and sensors are sealed inside. Ordering the "nearest" size without a kit test is how most exchanges happen. When borderline, test the larger kit size first.
Generation notes: use the kit that matches your exact model
Same brand, different hardware — the kit is not interchangeable.
- Oura Gen3 vs Ring 4: Oura publishes separate guides per generation with different interior smoothness (domed Gen3 vs smoother Ring 4). Pick your generation in the tool and use that generation's kit and official Oura sizing guide.
- RingConn Gen 1 vs Gen 2: Gen 2 is thinner (2.0 mm), narrower (6.8 mm), and squircle-shaped for anti-rotation. A Gen 1 kit does not replicate that feel — use the Gen 2 sizing pageand its 9-piece kit (sizes 6–14).
- Ultrahuman, Galaxy, Evie: each ships its own kit geometry (8.1 mm wide AIR, concave Galaxy Ring with 3 sensor bumps, open-gap Evie that flexes ~0.5–1.0 mm). A kit ring from one brand never validates another brand's size.
Finger and wear-time rules the tool applies
Two answers that change the caution level, not just the number.
Test the exact finger
Index is the default recommendation for sensor signal across Oura, Galaxy, RingConn, and Evie; middle and ring fingers are supported alternatives. Fingers commonly differ by half to a full size, so a ring-finger jewellery size never transfers to an index finger. If you switch fingers, restart the kit test on the new finger.
Assume overnight wear
Even for daytime-only plans, decide from an overnight test: sleep swelling is the tightest the ring will feel. The passing kit ring slides over the knuckle with mild resistance, survives a fist and grip check without pinching, and moves your skin slightly when twisted instead of spinning freely.
The universal 24-hour kit protocol
Same steps for every brand — only the kit pieces differ.
- Order the official kit for your exact model and generation before ordering the ring.
- Put the suggested size on the exact finger, sensors facing the palm side.
- Wear it 24 continuous hours: sleep, typing, handwashing, light exercise.
- Morning check: no numbness, tingling, deep marks, or struggle over the knuckle.
- Grip check: make a fist, hold a handle, carry a bag — secure without pinching.
- Twist check: skin moves with the ring; free spinning means too loose for tracking.
- Borderline? Size up, wear another 24 hours, and confirm in the official guide.
Model charts and official guides
Per-model millimetre tables on this site, plus the manufacturer page to check before ordering.
- Oura Ring Gen3 size chart
Sizes 6–13. Official guide: Oura — How to Choose the Right Oura Ring Size
- Oura Ring 4 size chart
Sizes 6–13. Official guide: Oura — How to Choose the Right Oura Ring Size (Ring 4 / Gen3 guides)
- Samsung Galaxy Ring size chart
Sizes 5–13. Official guide: Samsung Support — Galaxy Ring sizing kit guide
- RingConn Gen 2 size chart
Sizes 6–14. Official guide: RingConn — How to find the perfect size
- Ultrahuman Ring AIR size chart
Sizes 5–14. Official guide: Ultrahuman — Ring AIR sizing guide
- Evie Ring size chart
Sizes 5–12. Official guide: Movano Health — Evie Ring sizing guide
This advisor gives starting sizes only. Millimetre anchors shown in results come from this site's verified standard scale for orientation — always confirm against the brand's own model-specific chart and kit. Trademarks (Oura, Samsung Galaxy Ring, RingConn, Ultrahuman, Evie) belong to their owners; this independent guide is not affiliated with or endorsed by them.
Frequently asked questions
Can I just order my usual jewellery size in a smart ring?
No — treat your jewellery size only as a starting hint. Smart rings are 6.8–8.1 mm wide with raised interior sensors, so the same nominal size feels tighter than a slim jewellery band. Manufacturers explicitly warn their sizes differ from standard jewellery sizing. Use the advisor to pick which kit size to test first, then wear that kit ring for 24 hours before ordering.
What should I do if I am between two smart-ring sizes?
Size up to the larger whole size and test that kit ring first. All five brands covered here (Oura, Galaxy Ring, RingConn Gen 2, Ultrahuman AIR, Evie) sell whole sizes only, and every manufacturer recommends sizing up when between sizes. A slightly larger ring still tracks; a too-tight ring constricts overnight swelling and usually gets returned. If the larger kit ring spins freely, step back down one size and retest for a full day.
Does my Oura, RingConn, or other size transfer to another brand?
Never assume it transfers. Nominal US numbers overlap (e.g. size 8 exists in most brands), but interior sensor bumps, band widths (6.8 mm RingConn vs 8.1 mm Ultrahuman), and shapes (squircle vs concave vs open-gap) change the effective fit. Test each brand with its own official kit — including the correct generation, such as Oura Ring 4 vs Gen3 or RingConn Gen 2 vs Gen 1.
Which finger should I test the sizing kit on?
Test the exact finger you will wear every day — sizes typically differ by half to a full size between fingers. Most brands recommend the index finger for the strongest sensor signal, with the middle and ring fingers as supported alternatives. Thumb and pinky placement is discouraged because sensor contact is unstable. Do not transfer a ring-finger jewellery size to an index finger without retesting.
Why does the advisor insist on a 24-hour kit test including sleep?
Fingers swell up to half a size overnight and with heat, exercise, salt, and hormonal cycles. A ring that feels perfect for five afternoon minutes can feel tight at 6 AM. The universal manufacturer protocol is: wear the plastic sample for at least 24 continuous hours (sleep, typing, handwashing, grip checks), with sensors facing the palm. Morning comfort decides, not afternoon comfort.
Can a jeweller resize a smart ring if I pick wrong?
No. Every smart ring here seals a battery, circuitry, and optical sensors inside the band — cutting, stretching, or heating destroys the electronics and voids the warranty. That is why the advisor is deliberately cautious and always ends at the kit test: getting the size right before ordering is the only fix.
Trademark disclaimer
Oura, Samsung Galaxy Ring, RingConn, Ultrahuman, and Evie are trademarks of their respective owners. RingSizeConvert.com is an independent sizing reference and is not affiliated with, endorsed by, or sponsored by any smart-ring manufacturer. Sizing guidance is educational only.