Converter · Millimetres
MM to ring size
You have a measurement in millimetres and need a size. First make sure which millimetre you have: the diameter is the straight line across the inside of the ring, the circumference is the distance around it, and they differ by a factor of π. Mixing them up is the single most common sizing error, and it puts you about three sizes out.
Instant converter
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- US / CA
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- UK / AU
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- EU / ISO
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- India
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- Japan
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- Circumference
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No calibration needed — this is pure conversion. Measure now
The table
Millimetres to ring size
Inside diameter in half-millimetre steps, with every scale beside it. If your figure is a circumference, use the second column to find your row instead of the first.
| Ø mm | Circ. mm | US / CA | UK / AU | EU / ISO | India | Japan |
|---|---|---|---|---|---|---|
| 13.5 | 42.4 | 2¼ | E | 42 | 3 | #3 |
| 14.0 | 44.0 | 3 | F | 44 | 4 | #4 |
| 14.5 | 45.6 | 3½ | G | 46 | 5 | #5 |
| 15.0 | 47.1 | 4¼ | I | 47 | 8 | #8 |
| 15.5 | 48.7 | 4¾ | J | 49 | 9 | #9 |
| 16.0 | 50.3 | 5½ | K | 50 | 11 | #10 |
| 16.5 | 51.8 | 6 | L | 52 | 12 | #11 |
| 17.0 | 53.4 | 6½ | M | 53 | 13 | #13 |
| 17.5 | 55.0 | 7¼ | O | 55 | 15 | #15 |
| 18.0 | 56.5 | 7¾ | P | 57 | 17 | #16 |
| 18.5 | 58.1 | 8½ | Q | 58 | 18 | #17 |
| 19.0 | 59.7 | 9 | R | 60 | 19 | #18 |
| 19.5 | 61.3 | 9¾ | T | 61 | 22 | #21 |
| 20.0 | 62.8 | 10¼ | U | 63 | 23 | #22 |
| 20.5 | 64.4 | 11 | V | 64 | 25 | #23 |
| 21.0 | 66.0 | 11½ | W | 66 | 26 | #24 |
| 21.5 | 67.5 | 12¼ | Y | 68 | 28 | #27 |
| 22.0 | 69.1 | 12¾ | Z | 69 | 30 | #28 |
| 22.5 | 70.7 | 13¼ | Z+1 | 71 | 31 | #29 |
Derived from ISO 8653. The EU column and the circumference column carry the same figure, because an EU size is the circumference in whole millimetres.
The arithmetic, if you would rather do it yourself
Circumference from diameter: multiply by π. A 17.3 mm inside diameter is 17.3 × 3.1416 = 54.4 mm around.
US size from circumference: subtract 36.537 and divide by 2.5535. So 54.4 mm gives (54.4 − 36.537) ÷ 2.5535 = 7.0, a US 7. Going the other way, multiply the size by 2.5535 and add 36.537.
That linear relationship is the whole US scale. It is also why precision matters more than it looks: one millimetre of diameter is more than a full US size, so a measurement that is 1 mm out is a ring that does not fit.
Before you trust the number
Where millimetre measurements go wrong
- Diameter or circumference?
- A figure in the low teens is a diameter. A figure in the forties to seventies is a circumference. If yours is around 20, check twice — that is diameter territory, but a small EU size looks similar.
- Inside, not outside
- Measure the inside of the band. On a chunky ring the outside diameter can be two millimetres larger, which is more than two US sizes.
- The ruler has to be right
- A screen drawing a "17.3 mm" circle without calibrating is guessing — a CSS pixel has no fixed physical size. Ours calibrates first, to about ±0.15 mm.
In your market
The same millimetre, wherever you are ordering
A diameter converts into any national scale. Pick the market you are buying from and read the local number.
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
Millimetres and ring sizes
What ring size is 17 mm?
A 17.0 mm inside diameter is a US 6½, a UK M½ to N, and an EU 53 — 53.4 mm of inside circumference.
What ring size is 18 mm?
Close to a US 7¾. An 18.1 mm inside diameter is a US 8 exactly, at 56.9 mm of circumference.
How many mm is a size 7 ring?
17.3 mm across the inside, 54.4 mm around it.
Is ring size measured in diameter or circumference?
Both are used. The US, UK, Indian and Japanese scales are indexes; the EU/ISO scale is the circumference itself in millimetres. Jewellers usually measure diameter, because a mandrel reads across.
How precise does the measurement need to be?
Half a millimetre of diameter to land on the right half size. That is achievable at home with a caliper or a calibrated screen, and not achievable with a ruler held against a curved band.
No measurement yet?
Calibrate your screen against a bank card and read the diameter off a ring you already own.