Ring Size Converter
Type any ring size, an inside diameter or circumference, or the length of a string wrapped round your finger, and get your size in every system with a true-size ring to check.
Quarter and half sizes are fine: 6.5, 6½ or 6 1/2.
If your knuckle is bigger, we size halfway so the ring slides on but does not spin.
Wide bands feel tighter: we add half a US size.
True size on your screen
Lay your ring on the screen: the inside edge should match the dashed circle. Screens differ, so check the 1 cm ruler with a real ruler, or calibrate in Advanced.
Hold any bank or ID card against the screen and move the slider until the bar is exactly as long as the card's edge. On phones, use the short edge.
Ring size chart: US, UK, EU, Japan, diameter and circumference
| US & Canada | UK & AU | EU / ISO | Japan | Diameter | Circumference |
|---|---|---|---|---|---|
| 3 | F½ | 44 | 4 | 14.07 mm | 44.2 mm |
| 3½ | G½ | 45 | 5 | 14.47 mm | 45.5 mm |
| 4 | H½ | 47 | 7 | 14.88 mm | 46.8 mm |
| 4½ | I½ | 48 | 8 | 15.29 mm | 48.0 mm |
| 5 | J½ | 49 | 9 | 15.69 mm | 49.3 mm |
| 5½ | K½ | 51 | 11 | 16.10 mm | 50.6 mm |
| 6 | L½ | 52 | 12 | 16.51 mm | 51.9 mm |
| 6½ | M½ | 53 | 13 | 16.91 mm | 53.1 mm |
| 7 | N½ | 54 | 14 | 17.32 mm | 54.4 mm |
| 7½ | O½ | 56 | 16 | 17.73 mm | 55.7 mm |
| 8 | P½ | 57 | 17 | 18.13 mm | 57.0 mm |
| 8½ | Q½ | 58 | 18 | 18.54 mm | 58.2 mm |
| 9 | R½ | 60 | 20 | 18.95 mm | 59.5 mm |
| 9½ | S½ | 61 | 21 | 19.35 mm | 60.8 mm |
| 10 | T½ | 62 | 22 | 19.76 mm | 62.1 mm |
| 10½ | U½ | 63 | 23 | 20.16 mm | 63.3 mm |
| 11 | V½ | 65 | 25 | 20.57 mm | 64.6 mm |
| 11½ | W½ | 66 | 26 | 20.98 mm | 65.9 mm |
| 12 | X½ | 67 | 27 | 21.38 mm | 67.2 mm |
| 12½ | Z | 68 | 28 | 21.79 mm | 68.5 mm |
| 13 | Z+1 | 70 | 30 | 22.20 mm | 69.7 mm |
Worked out from the formulas below. Jewellers' charts can differ by about half a size.
How your ring size is worked out
Every system measures the same thing: the inside of the ring.
- 1
Start from your size
11.63 + 0.8128 × 7
= 17.32 mm across - 2
Across and around
17.32 × π
= 54.41 mm around - 3
US: take off 11.63
5.69 ÷ 0.8128
= 7.00 → US 7 - 4
UK: steps after A
16.91 ÷ 1.25
= 13.53 → UK N½
Read more: the formula, a worked example, the systems and how to measure
Every ring size system measures the same thing: the inside of the ring. Some use the distance across (the inside diameter), some the distance around (the inside circumference), and some count steps on a scale. So the converter turns whatever you type into the inside circumference in millimetres, then works out each system from that.
The formula
circumference = diameter × π (π ≈ 3.14159)
US diameter (mm) = 11.63 + 0.8128 × US size
US size = (diameter − 11.63) ÷ 0.8128
UK letter steps after A = (circumference − 37.5) ÷ 1.25
EU / ISO size = circumference in mm
Japan, China, Italy, Spain, Swiss ≈ circumference − 40
string method: circumference = string length
with a bigger knuckle: (finger + knuckle) ÷ 2
wide band: add ½ a US size
US sizes go up 0.8128 mm across (2.55 mm around) per whole size, and quarter sizes are common. UK letters go up 1.25 mm around per letter, with half sizes in between, so one US size is about two UK letters. After Z come Z+1, Z+2 and so on. Nearest sizes are rounded to the quarter (US), half letter (UK) or whole number (EU and Japan).
Worked example
A US 7 ring: diameter = 11.63 + 0.8128 × 7 = 11.63 + 5.6896 = 17.3196 mm (17.32 mm, or 0.68 in).
Circumference = 17.3196 × π = 54.41 mm (2.14 in). EU / ISO size: 54.
UK: (54.41 − 37.5) ÷ 1.25 = 16.91 ÷ 1.25 = 13.53 steps after A. The nearest half step is 13.5, and the 13th letter after A is N, so UK N½.
Japan: 54.41 − 40 = 14.41, so Japan 14. Italy, Spain and Switzerland also come out at 14.
The string method: a string that is 55 mm round your finger and 59 mm round your knuckle gives (55 + 59) ÷ 2 = 57 mm. 57 ÷ π = 18.14 mm across, and (18.14 − 11.63) ÷ 0.8128 = 8.01, so a US 8.
How to measure at home
- Use a ring that fits. Measure straight across the inside with a ruler in mm: that is the inside diameter. Or lay it on the true-size drawing above.
- Use a string or paper strip. Wrap it snugly round the base of the finger, mark the overlap, and measure. Thread stretches, so paper or floss is better than wool.
- Add a little for knuckles. If the ring has to pass a bigger knuckle, measure that too and size about halfway.
- Measure at the end of the day. Fingers are smaller when cold and bigger when warm or after salty food. Measure a few times and use the middle.
- Between two sizes? Pick the larger one, especially for wide bands; a ring that is a little loose can be fitted with a sizing bead, but one that is too tight cannot go on.
The systems
| Where | Scale | Step |
|---|---|---|
| US, Canada, Mexico | Numbers 0–16 from 11.63 mm across | 0.8128 mm across per size |
| UK, Ireland, Australia, New Zealand | Letters A–Z, then Z+1… from 37.5 mm around | 1.25 mm around per letter |
| France, Germany, most of Europe (ISO 8653) | Circumference in mm | 1 mm |
| Japan and China | Numbers ≈ circumference − 40 | about 1 mm |
| Italy, Spain, Switzerland | Numbers ≈ circumference − 40 | about 1 mm |
| India (approximate) | Numbers close to the Japanese scale | about 1 mm |
Charts differ slightly between jewellers and brands, often by half a size, and Indian and Chinese sizes vary most. Treat the results as a guide and confirm with a ring sizer or a jeweller before buying an expensive ring. A bank card is 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Checked 13 October 2026.
Common questions
How do I convert US ring sizes to UK sizes?
Turn the US size into the inside diameter (11.63 mm + 0.8128 mm per size), multiply by π to get the circumference, then count 1.25 mm steps from UK size A at 37.5 mm. A US 7 is 17.32 mm across and 54.41 mm around, which is UK N½. A US 6 is about L½ and a US 8 about P½.
What is the most common ring size?
For women it is often said to be around US 6 to 7 (UK L½ to N½, EU 52 to 54), and for men around US 9 to 10 (UK R½ to T½, EU 60 to 62). Fingers vary a lot, so measure rather than guess.
How do I measure my ring size with a string?
Wrap a thin string or a strip of paper snugly round the base of your finger, mark where it overlaps, and measure that length with a ruler in mm. That length is the circumference: type it in with "String around my finger". If your knuckle is bigger, measure it too; a size halfway between the two slides over the knuckle but does not spin.
Should I size up for a wide band?
Yes, usually by about half a US size for bands 6 mm wide or more, because a wide band touches more of your finger and feels tighter. Turn on "Wide band" in Advanced to add it.
Are ring size charts the same everywhere?
Nearly, but not exactly. Each country uses a standard (ISO 8653 in Europe, a letter scale in the UK and Australia, a number scale in the US and Japan), and jewellers round between them a little differently, often by half a size. Use this converter as a guide and confirm with a ring sizer or a jeweller before buying.
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