Ring size converter and on-screen sizer
Convert ring sizes between US, UK, EU/ISO and Japanese systems, plus an on-screen sizer you calibrate against a bank card and a printable strip.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
Half sizes are fine.
Your screen is showing about 3.8 pixels per millimetre.
Change the size above and the circle resizes. Measure a ring that already fits the right finger — it is far more reliable than measuring the finger itself.
Check the print before you trust it: lay a bank card on the paper next to the circles. If the card is not 85.6 mm wide, the scale is wrong and so are the circles.
| US | UK | EU / ISO | JP | Diameter (mm) | Circumference (mm) |
|---|---|---|---|---|---|
| 3 | F | 44 | 4 | 14.1 | 44.2 |
| 3.5 | G | 45.5 | 5 | 14.5 | 45.5 |
| 4 | H | 46.8 | 7 | 14.9 | 46.8 |
| 4.5 | I | 48 | 8 | 15.3 | 48 |
| 5 | J 1/2 | 49.3 | 9 | 15.7 | 49.3 |
| 5.5 | K 1/2 | 50.6 | 10 | 16.1 | 50.6 |
| 6 | L 1/2 | 51.9 | 12 | 16.5 | 51.9 |
| 6.5 | M 1/2 | 53.1 | 13 | 16.9 | 53.1 |
| 7 | N 1/2 | 54.4 | 14 | 17.3 | 54.4 |
| 7.5 | O 1/2 | 55.7 | 15 | 17.7 | 55.7 |
| 8 | P 1/2 | 57 | 16 | 18.1 | 57 |
| 8.5 | Q 1/2 | 58.3 | 17 | 18.5 | 58.3 |
| 9 | R 1/2 | 59.5 | 18 | 18.9 | 59.5 |
| 9.5 | S 1/2 | 60.8 | 19 | 19.4 | 60.8 |
| 10 | T 1/2 | 62.1 | 20 | 19.8 | 62.1 |
| 10.5 | U 1/2 | 63.4 | 22 | 20.2 | 63.4 |
| 11 | V 1/2 | 64.6 | 23 | 20.6 | 64.6 |
| 11.5 | W 1/2 | 65.9 | 24 | 21 | 65.9 |
| 12 | Y | 67.2 | 25 | 21.4 | 67.2 |
| 12.5 | Z | 68.5 | 26 | 21.8 | 68.5 |
| 13 | Z+1 | 69.7 | 27 | 22.2 | 69.7 |
Every ring size is one number in disguise
Unlike shoes and clothes, ring sizing has a genuine physical basis that all the systems agree on: the inside diameter of the band. Everything else is a labelling scheme laid over it.
- US and Canada — a numeric scale where each full size adds 0.8128 mm of diameter (about 2.55 mm of circumference).
- UK, Ireland, Australia — letters, roughly two per US size, starting at A.
- EU / ISO 8653 — the inside circumference in millimetres. A US 7 is 54.4 mm, so it is a European 54.
- Japan and China — their own numeric scale, running from about 1 to 27.
Because circumference is diameter times π, converting between the diameter and circumference columns is exact. Converting between the national labels is a lookup, which is what the chart on this page provides.
The two reliable ways to measure at home
Measure a ring you already own
This is by far the most accurate home method, because you are measuring a finished object rather than a soft, variable finger. Calibrate the on-screen sizer against a bank card — every ID-1 payment card is exactly 85.60 mm wide, which lets the page work out how many pixels your display uses per millimetre — then lay the ring over the circle. The inside edge of the band should just touch the line.
Print the strip
The printable sizer below the tool draws its circles in real millimetres. Print at exactly 100% scale — turn off "fit to page" and "shrink oversized pages" — then check the print by laying a bank card next to it. If the card is not 85.6 mm wide on the paper, the scale is wrong and so are the circles.
Why string does not work
Wrapping string or paper around a finger and measuring it is the method everyone tries and it is reliably a half to a full size out. String stretches, paper compresses, and neither has to pass over the knuckle. A ring has to clear the knuckle and then sit comfortably on a narrower part of the finger, which is a two-constraint problem that a circumference measurement of the base of the finger cannot capture.
Fingers change size
| Factor | Effect |
|---|---|
| Temperature | Up to half a size smaller when cold, larger when hot |
| Time of day | Largest in the late afternoon and evening |
| Salt, alcohol, heat | Temporary swelling, often a quarter to half a size |
| Exercise | Hands swell during and after; measure at rest |
| Pregnancy | Often a full size or more, usually reversing afterwards |
| Band width | A band over 6 mm wide fits tighter; size up a quarter to a half |
Measure two or three times on different days at a normal temperature and take the middle reading. If you are between sizes, size up: a ring that will not come off is a much worse problem than one that is slightly loose, and loose rings can be sized down or fitted with beads.
Buying a surprise
Borrow a ring she already wears on the relevant finger and measure that, noting which finger it is worn on — the ring finger and the middle finger are typically a size apart. Failing that, US 6–6.5 and US 9–10 are the most common women's and men's sizes respectively, and most jewellers will resize once for free within a set period. Ask before you buy.
All of this runs in your browser
The chart, the calibration and the printable strip are computed in the page. Nothing about your measurement, your calibration or what you are shopping for is sent anywhere, because there is no server behind this page to send it to.
Questions
How do I measure a ring size at home?
Measure a ring that already fits the right finger. Lay it over the on-screen circle after calibrating the screen with a bank card, or print the sizer at 100% scale and match it there. Measuring the finger with string is much less reliable.
How does the screen calibration work?
Every ID-1 payment card is exactly 85.60 mm wide. You drag the on-screen box until it matches a real card, which tells the page how many pixels your display uses per millimetre — after that, the ring circle is drawn life size.
What is the EU ring size?
The ISO 8653 size, which is simply the inside circumference of the band in millimetres. A US 7 is 54.4 mm, so it is a European 54 or 54.4.
How do UK letters map to US numbers?
Roughly two letters per US size: UK N is close to US 6.75, UK L 1/2 is US 6, and UK T 1/2 is US 10. The full table is on this page, with the millimetre figures that both are derived from.
When should I size up?
Fingers swell in heat and late in the day and shrink in cold, by as much as half a size. Wide bands fit tighter than narrow ones at the same nominal size. For a permanent ring, measure warm and consider a quarter size up for a band over 6 mm wide.