A ring that fits keeps its place without pinching. Ring sizes are really a measure of the length of the inside of the ring: its inner circumference or diameter in millimetres. The size labels you see in shops, such as US 7, UK N or ISO 54, are only different ways of writing that length. This calculator takes the measurement of your finger, or of a ring that already fits, and gives your size in the five common systems.
How to use the ring size calculator
- Choose what you measured: the circumference of your finger, or the inner diameter of a ring that fits you.
- Measure. For the finger, wrap a strip of paper or thread around the base of the finger, mark the overlap and measure it with a ruler. For a ring, measure straight across the inside from edge to edge.
- Enter the number in millimetres, centimetres or inches.
- Read your sizes. The US, UK, ISO, Japanese and Swiss sizes appear together with the inner circumference and diameter of that size.
Ring size formulas
US size = (circumference − 36.5) ÷ 2.55
UK letter: C = 40 mm, then 1.25 mm per letter
Japan (JCS) size = (diameter − 13) × 3 + 1, diameter = circumference ÷ π
Italy / Spain / Switzerland = ISO − 40
The ISO figure comes from ISO 8653, which defines ring sizes as the inner circumference in millimetres, and the US and UK rules from the Wikipedia ring size article (retrieved 2026-10-15). The Japanese rule follows the JCS scale as described by Moocota and Luft: size 1 is 13 mm across and each size adds one third of a millimetre. Results are rounded to the nearest quarter size (US), half letter (UK) and whole number (others).
| US size | Inner circumference (mm) | Inner diameter (mm) | ISO / EU | UK | Japan | Italy / Spain / Switzerland |
|---|---|---|---|---|---|---|
| 3 | 44.1 | 14.1 | 44 | F 1/2 | 4 | 4 |
| 4 | 46.7 | 14.9 | 47 | H 1/2 | 7 | 7 |
| 5 | 49.2 | 15.7 | 49 | J 1/2 | 9 | 9 |
| 6 | 51.8 | 16.5 | 52 | L 1/2 | 11 | 12 |
| 7 | 54.3 | 17.3 | 54 | N 1/2 | 14 | 14 |
| 8 | 56.9 | 18.1 | 57 | P 1/2 | 16 | 17 |
| 9 | 59.5 | 18.9 | 59 | R 1/2 | 19 | 19 |
| 10 | 62.0 | 19.7 | 62 | T 1/2 | 21 | 22 |
| 11 | 64.5 | 20.5 | 65 | V 1/2 | 24 | 25 |
| 12 | 67.1 | 21.4 | 67 | X 1/2 | 26 | 27 |
| 13 | 69.7 | 22.2 | 70 | Z 1/2 | 29 | 30 |
The table above was calculated from the formulas. Tiffany’s published size guide lists, for example, US 7 at 54.4 mm and US 10 at 62.1 mm, which agrees with the formula to within 0.1 mm.
Japanese sizes
| Japan (JCS) | Inner diameter (mm) | Inner circumference (mm) | US |
|---|---|---|---|
| 5 | 14.33 | 45.03 | 3.25 |
| 7 | 15.00 | 47.12 | 4.25 |
| 9 | 15.67 | 49.22 | 5 |
| 11 | 16.33 | 51.31 | 5.75 |
| 13 | 17.00 | 53.41 | 6.75 |
| 15 | 17.67 | 55.50 | 7.5 |
| 17 | 18.33 | 57.60 | 8.25 |
| 19 | 19.00 | 59.69 | 9 |
| 21 | 19.67 | 61.78 | 10 |
| 23 | 20.33 | 63.88 | 10.75 |
| 25 | 21.00 | 65.97 | 11.5 |
Worked example
A finger measures 54.4 mm around.
- US: (54.4 − 36.5) ÷ 2.55 = 7.02, so size 7.0.
- ISO / EU: 54; Italy, Spain and Switzerland 14.
- Diameter: 54.4 ÷ π = 17.3 mm.
- Japan: (17.32 − 13) × 3 + 1 = 13.96, so size 14.
- UK: 14.4 mm above C divided by 1.25 gives 11.5 letters, so N 1/2.
Getting an accurate measurement
- Time of day: measure in the afternoon or evening, when fingers are warm. Cold fingers can be a full size smaller.
- Knuckle: the ring must pass over the knuckle, so if the knuckle is larger than the base, pick the size between the two.
- Band width: wide bands feel tighter; many jewellers advise going up a quarter to half size.
- Repeat: measure three times, on different days.
The Ring Size Converter (US/UK/EU/JP) translates a size you already know. For other body measurements try the bracelet-size-calculator and Glove Size Calculator. This page is general guidance and does not replace a jeweller’s ring gauge.
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify published charts.
- Wikipedia — Ring size (ISO 8653, US, UK and other systems) — checked 2026-10-15
- Tiffany & Co. — ring size guide (US size, diameter, circumference) — checked 2026-10-15
- Moocota — Japanese ring size chart (号数, diameter, circumference) — checked 2026-10-15
- Luft — ring size converter (JCS scale) — checked 2026-10-15
Frequently asked questions
How do I measure my ring size at home?
Wrap a thin strip of paper or a piece of string around the base of the finger you will wear the ring on, mark where it overlaps, and measure the length in millimetres. That is the finger circumference. Measure at the end of the day, when fingers are warmest and largest.
What if I am between two ring sizes?
Jewellers generally suggest the larger size if you are between two, especially for wide bands and for knuckles bigger than the base of the finger. The calculator rounds to the nearest quarter size, so check the next size up if the ring has a wide band.
What is the formula for US ring size?
Wikipedia gives the inside circumference in millimetres as 2.55 × size + 36.5, and the inner diameter as 0.8128 × size + 11.63. A US 7 is therefore 54.35 mm around and 17.3 mm across.
How do UK and Japanese sizes work?
UK sizes are letters in steps of 1.25 mm of circumference, with C at 40 mm. Japanese sizes start with size 1 at 13 mm diameter and add one third of a millimetre of diameter per size. See the Ring Size Converter (US/UK/EU/JP) to translate between systems.
Why do charts from jewellers give slightly different numbers?
Brands round their charts differently and UK letters were once defined by diameter, so the same size can differ by 0.2 to 0.6 mm between charts. Use the chart of the shop you buy from when it publishes one.