Smart ring sizing guide: how to size a smart ring with the brand's own kit

Why your jewelry size is the wrong starting point

A smart ring isn't a band of metal. It's a shell with sensors on the inside surface, and each maker shapes that inside differently. So a “size 9” means whatever that brand says it means. Here's what each brand says, in its own words:

• Oura: “Oura uses only whole sizes (no half sizes) and they differ slightly from standard US ring sizes, so it's important to test with our sizers even if you know your usual ring size.” And: “The sizing for each Oura Ring model differs slightly between generations” ( Oura Support). Ring 5's “redesigned interior fits differently than Oura Ring 4.”
• RingConn: “RingConn Gen 3 ring sizes differ from other RingConn rings, standard rings and other smart ring brands” (RingConn). For Gen 2 Air: “RingConn ring sizes differ from standard rings.” Gen 2 and Gen 2 Air share sizes.
• Samsung: “Galaxy Ring's size is slightly different from that of regular rings” ( Samsung).
• Ultrahuman: its sizing “differs from other smart rings” (from its Ring AIR FAQ).
• Circular: US sizes 6-14, “running approximately half a size larger than standard ring sizing” (Circular).

Notice what nobody says. No brand claims its sizes match another brand's, and two of them (Oura and RingConn) say sizes differ even between their own models. That's why the question “is RingConn sizing the same as Oura?” has no official answer. We go through it pair by pair in smart ring sizing compared.

Getting it wrong costs you twice. A ring that's loose moves around and can read worse. A ring that's tight is miserable to sleep in, and sleep is the main job. And since Oura and Ultrahuman sell whole sizes only, you can't always split the difference.

Every brand's size range

Size ranges and sizing kits for every ring we cover, each linked to the maker's page. A dash means the brand doesn't publish it on the page we cite.

The widest ranges belong to Oura Ring 4 (4-15), Galaxy Ring (5-15) and RingConn Gen 3 (6-15). Oura Ring 5 is narrower at 6-13. If you wear a very small or very large size, check the range before anything else; our guide to smart rings in large and small sizes covers it in detail.

How to size a smart ring, step by step

1. Pick the ring first. Brand and model, including the generation. Sizes differ between brands, and Oura and RingConn say they differ between their own models too. A sizer for the wrong ring is no help. If you haven't chosen yet, start with our best smart rings without a subscription.
2. Get that brand's sizing kit.
   • Oura: free on Oura's store. The Ring 5 kit is also sold on Amazon, and Android Police reported it comes with a matching Amazon credit toward the ring (Android Police).
   • RingConn: complimentary with Gen 2 and Gen 3, free with Gen 2 Air. For Gen 3, the ring ships after you confirm your size.
   • Samsung: a complimentary sizing kit; Amazon listings are titled “Size First w/Sizing Kit.”
   • Ultrahuman: free with the Ring PRO on Ultrahuman's store, and Ultrahuman says it's yours to keep, with no return required. The Ring PRO sizing kit (sizes 5-14) is also on Amazon, though the ring isn't.
   • Circular: a physical replica-ring kit for a small extra charge, or a printable sizing guide. Circular says sizing kits are “consumables that are not refundable” (Circular).
3. Choose your finger. Oura recommends the index finger for the best accuracy. Pick the finger before you size, because every finger is a different size. More below.
4. Find the size that fits, then wear it. Try the sizes near your best guess and pick the one that slides over the knuckle with a little resistance and sits comfortably at the base of the finger. Then wear that sizer through a full day and a night, including sleep.
5. Check it under real conditions. Wear it during whatever you'll wear the ring for: sleep, workouts, a warm afternoon, a cold morning. It should stay comfortable when your finger is at its biggest and not slide off when it's at its smallest.
6. If you're between sizes, ask the brand. Oura and Ultrahuman don't do half sizes, and we didn't find a published tie-break rule on the brand pages we cite. The maker's support team knows its own sizing best.
7. Order the ring in the size the sizer gave you. Not your jewelry size, and not a size from another brand's kit. Keep the sizer until the ring arrives and fits.

Why wear the sizer for a full day

None of the brand pages we cite sets a minimum time to wear the sizer, so this is our advice rather than a maker's rule. Fingers aren't the same size all day. They change with the time of day, the temperature and what you're doing. A sizer that feels right at your desk in the afternoon can feel different at 3 a.m. or after a workout.

A smart ring is worn around the clock, and its most important job, sleep tracking, happens at night. So the fit that matters most is the one you'll have in bed. Sizing over a full day and a night catches the problems a five-minute try-on misses. If you have the time, a couple of days is better than one.

Which finger to size

Oura recommends the index finger for best accuracy (Oura Support). We didn't find an equivalent published recommendation from the other brands on the pages we cite. What matters for sizing is simple: size the finger you'll actually wear the ring on, because a size that fits your index finger won't fit your ring finger. The trade-offs between fingers are covered in which finger to wear a smart ring on.

What to look for in a sizing kit

• It's made by the ring's maker, for the exact model you'll buy. A generic ring sizer measures jewelry sizes, which is the thing that doesn't work here.
• It covers the full size range of the ring, so you can try the sizes on either side of your guess.
• It's free, or credited back. Most brands give theirs away, and the Oura kit on Amazon was reported to come with a matching credit.
• You can keep it long enough to wear it overnight. Ultrahuman says its kit is yours to keep.

More on the two rings in our RingConn Gen 3 review and Samsung Galaxy Ring review. Buying for someone else? Read the next section before you order anything.

Sizing a ring for someone else

You can't guess someone's smart ring size, and borrowing one of their jewelry rings won't help for all the reasons above. There are two clean ways around it. Give the sizing kit first and the ring once they've worn the sizer, or buy a ring that doesn't ship until the size is confirmed, which is how RingConn handles Gen 3. Ultrahuman says its kit is yours to keep and share, so one kit can serve a household, and the Oura kit on Amazon was reported to come with a matching credit toward the ring, which makes it a sensible first present. Our smart ring gift guide builds a whole gift around this.

If the size turns out wrong

Act quickly, because return windows are short. Circular offers a 14-day, no-questions return on the ring, though not on the sizing kit. Rings bought on Amazon go through Amazon's return process, and rings bought from a brand's own store follow that brand's terms. Every window and condition we found is in our smart ring warranty and returns guide.

Don't keep wearing a ring that's too tight to sleep in and hope it settles. A ring you take off at night can't track your sleep, which is most of the reason to own one. The fix is a different size, and the return clock is already running. If a ring that used to fit suddenly feels tight and your finger hasn't changed, stop wearing it and read our battery swelling safety guide.

Sizing mistakes to avoid

• Ordering your jewelry size. Oura, RingConn, Samsung and Circular all say their sizes differ from standard ring sizes, and Ultrahuman says its sizing differs from other smart rings.
• Using another brand's kit. RingConn says Gen 3 sizes differ from other smart ring brands, and no brand claims to match another.
• Reusing an old size after an upgrade. Oura says sizing differs between generations, and Ring 5 fits differently than Ring 4. RingConn says Gen 3 differs from its other rings. Gen 2 and Gen 2 Air are the exception: RingConn says their sizes are the same.
• Sizing in five minutes. Wear the sizer through a day and a night before you decide.
• Forgetting Circular runs large. Circular says Ring 2 runs about half a size larger than standard sizing. Its sizer accounts for that; your jewelry size doesn't.
• Ignoring battery. Smaller sizes can mean shorter battery life: Samsung rates Galaxy Ring sizes below 12 at up to 6 days rather than 7. See our smart ring battery life guide.

Sizing, battery, water and fit guides all live on the battery and sizing hub, and every ring's size range sits next to its fee and battery in the smart ring comparison chart.