Smart rings · Oura
Oura ring size chart
Oura numbers its sizes on the US scale, so an Oura 8 is 18.1 mm across the inside — but a smart ring does not fit like a wedding band. It is wide, it is rigid, and the sensors on the inside only work if they stay in contact with the skin. That is why Oura ships a sizing kit and why this page is mostly about the things a chart cannot tell you.
- ≈8 mm
- Band width
- +½–1
- Sizes for width
- 24 h
- Kit wear time
- Up
- When between sizes
Why a chart is not enough for a smart ring
An ordinary wedding band is 2 to 4 mm wide and sits loosely against the finger. An Oura ring is roughly 8 mm wide, which is squarely in wide-band territory: it presses against more of the finger and therefore wears tighter at the same nominal size. The band width allowance — half a size from about 5 mm, a full size from about 9 mm — applies here as much as it does to a chunky gold band.
The second difference is what the ring is for. The sensors sit on the inside of the band and read through the skin. A ring that rotates or lifts away loses contact and returns worse data, so a smart ring wants to be snugger than a decorative one — but a ring that is genuinely tight restricts a finger that swells overnight, which is exactly when it is doing most of its measuring.
That narrow window between "loose enough to be comfortable for 24 hours" and "snug enough to keep contact" is why Oura sends a plastic sizing kit and asks you to wear a size for a full day before ordering. Nothing measured in a single moment, on this site or anywhere else, replaces that.
The table
Oura sizes in millimetres
Oura uses the US numbering, in whole sizes. Use this to work out roughly where you will land, then confirm with the sizing kit before ordering.
| US / CA | Ø mm | Circ. mm | UK / AU | EU / ISO | India | Japan |
|---|---|---|---|---|---|---|
| 4 | 14.9 | 46.8 | H | 47 | 7 | #6 |
| 5 | 15.7 | 49.3 | J | 49 | 9 | #9 |
| 6 | 16.5 | 51.9 | L | 52 | 12 | #11 |
| 7 | 17.3 | 54.4 | N | 54 | 14 | #14 |
| 8 | 18.1 | 57.0 | P | 57 | 17 | #16 |
| 9 | 18.9 | 59.5 | R | 60 | 19 | #18 |
| 10 | 19.8 | 62.1 | T | 62 | 22 | #21 |
| 11 | 20.6 | 64.6 | V | 65 | 25 | #23 |
| 12 | 21.4 | 67.2 | X | 67 | 27 | #25 |
| 13 | 22.2 | 69.7 | Z | 70 | 30 | #28 |
Derived from ISO 8653 and labelled with the US numbering Oura uses. The exact range offered changes between generations, so check the current list before ordering — and note that these are nominal sizes, before the width allowance below.
Fitting a smart ring
What actually decides the size
- Wear the kit overnight
- Fingers are largest in the evening and after exercise, and can swell again during sleep. A size that is comfortable at 9 a.m. and tight at 2 a.m. is the wrong size.
- Index or middle finger
- Oura reads best from a finger with a flat, even base. The index finger is the common recommendation; the little finger and the thumb are usually poor choices for sensor contact.
- It should not spin freely
- A decorative ring may rotate harmlessly. A smart ring that spins moves the sensors off the artery it is reading and quietly degrades the data.
- Between two? Go up
- A loose sensor gives bad data. A ring you cannot get on gives none at all, and cannot be resized — smart rings have electronics inside the band.
- Both hands differ
- The dominant hand is usually slightly larger. Size for the hand you will actually wear it on, not the one that was easier to measure.
- It cannot be resized
- There is no plain metal to cut. The only remedy for a wrong size is an exchange, which is why the kit exists.
Using a home measurement alongside the kit
A home measurement still earns its place: it tells you which sizes in the kit to try first, and it gives you a number to sanity-check against. Measure a ring you already wear on the finger you intend to use, on its inside diameter, with the calibrated sizer. If that ring is a thin band, add half a size to a full size for the smart ring’s width before you start trying kit sizes.
If the ring you measured is itself wide, no allowance is needed — you already measured a wide-band fit. This is the one case where measuring the band matters as much as measuring the ring, which is why the sizer can measure band width on the same calibrated stage.
Where a home measurement should not be trusted is as a substitute for the 24-hour wear test. The failure it cannot catch is nocturnal swelling, and that is the failure that makes people return smart rings.
Ordering from elsewhere
US numbering, wherever you buy it
Smart rings are sold on the US scale everywhere. If you only know your size in another system, convert it here first, then apply the width allowance.
Do not know it? Measure a ring on your calibrated screen — it is the only number here that is not a conversion of another number.
Indian numeric
14
A number from 1 to about 30. Indian jewellers frequently print the UK letter beside it, because both are in daily use.
India. Indian jewellers quote a numeric scale of their own, roughly 1 to 30. Tanishq, Malabar Gold, Kalyan and GIVA all use it, and many print the UK letter beside it. Online-first brands often list US sizes as well, so check which scale a listing means before ordering. Full India ring size chart.
United States. The US numeric scale, sold mostly in half sizes. Quarter sizes exist and a jeweller can cut them, but they are rarely stocked. Full USA ring size chart.
United Kingdom. Letters, in half-letter steps. British hallmarked rings are usually stamped with the letter, so an inherited ring often tells you its own size if you look inside the band. Full UK ring size chart.
Europe. France, Germany, the Netherlands and Scandinavia quote the ISO 8653 number, which is the inside circumference in millimetres. Italy, Spain and Switzerland quote the same measurement minus 40, so a 54 becomes a 14. Full Europe ring size chart.
Japan. The Japanese numeric scale runs #1 to about #27 for the sizes people actually order. Mikimoto, Tasaki and 4°C all use it. Japanese sizes are usually whole numbers only. Full Japan ring size chart.
China. Mainland Chinese listings usually give 圈号 — the inside circumference in millimetres, the same number as EU/ISO. Hong Kong uses a separate numbered scale, and marketplace listings mix both with US sizes. Order by inside diameter in millimetres when a listing does not say which scale it means.
Australia. Australia and New Zealand use the UK letter scale unchanged, so a UK N is an Australian N.
Canada. Identical to the United States. A US 7 is a Canadian 7, with no conversion in between.
Singapore. Singapore retail is mixed: international brands quote US numbers, goldsmiths in the Chinese tradition quote Hong Kong numbers, and British-founded chains quote letters. Ask for the inside diameter in millimetres and you sidestep the whole problem.
Malaysia. As in Singapore, US numbers dominate modern retail while traditional goldsmiths use Hong Kong numbers. Confirm the scale, or the diameter, before you order.
United Arab Emirates. Gulf retail quotes US sizes most often. British-founded chains use letters, and the many Indian gold houses in Dubai use the Indian scale, so the same finger can be quoted three ways along one street.
South Africa. South Africa follows the UK letter scale. Online stores that ship from the United States will quote numbers instead.
Brazil. Brazilian sizes are the inside circumference in millimetres minus 40 — the same arithmetic Switzerland uses. A 54 mm circumference is a Brazilian 14.
Mexico. Mexican jewellers use a local numeric scale that is not the US one, and the published conversions for it disagree with each other. We will not print a number we cannot stand behind, so order by inside diameter in millimetres, or ask the retailer for the diameter of the size they are offering. Large chains and online stores generally quote US sizes.
Questions
Smart ring sizing, answered
Is Oura ring size the same as normal ring size?
The numbering is the same US scale, but the fit is not. The band is about 8 mm wide, so it wears tighter than a thin ring of the same number — expect to go half a size to a full size up from your usual band size.
Can I skip the sizing kit?
You can, and it is the most common reason people end up exchanging. A measurement catches your finger at one moment; the kit catches it over a full day and night, including the swelling that happens while you sleep.
Which finger should I wear it on?
The index finger is the usual recommendation, with the middle and ring fingers as alternatives. Sensor contact is better on a finger with a flat, even base, and worse on the little finger or thumb.
What if I am between two sizes?
Take the larger. A loose ring gives degraded data; a ring that will not go on gives none, and a smart ring cannot be resized because there are electronics in the band.
Does the ring need to be tight to work?
Snug, not tight. The sensors need steady skin contact, which means it should not spin freely — but a ring that leaves a mark or throbs overnight is too small and will be uncomfortable exactly when it is measuring you.
What size is an 18 mm Oura ring?
An 18.1 mm inside diameter is a US 8 on the nominal scale. Before ordering, add the width allowance and confirm with the kit.
Start from a measured diameter, then wear the kit
The sizer measures a ring you own on a screen calibrated against a bank card, and measures the band width on the same stage — which is exactly the allowance a smart ring needs.