Ring Size Converter
Convert ring sizes between US, UK, EU (ISO) and Japanese systems, work out your size from a ring's inside diameter or your finger's circumference, or print a true-scale ring sizer.
Enter what you know
On-screen ring sizer (calibrate with a credit card)
1. Hold a bank card (85.6 mm wide, an ISO ID-1 card) against the screen and drag the slider until the bar is exactly as wide as the card.
2. Lay a ring flat on the screen and resize the circle until it matches the ring's inner edge.
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Ring size chart
Click a row to select it.Values are approximate. Ring sizing standards differ slightly between jewelers and countries, and wide bands fit tighter, so confirm with your jeweler if you can.
Printable ring sizer preview
Print at 100% / Actual size (turn off "Fit to page"). Then check that the calibration ruler measures exactly 100 mm with a real ruler before using it.
How to use the Ring Size Converter
- Choose how you want to enter the size: a size in a known system, an inside diameter, or a circumference.
- Enter the value and press Convert to see the size in every system.
- No ring to measure? Press Print ring sizer, print at 100% scale and check the calibration ruler with a real ruler.
- Use the paper strip: cut it out, wrap it around your finger and read the size where the end meets the scale, or lay a ring on the circles.
- Between sizes? Pick the larger size, especially for wide bands.
Ring size formulas
- US and Canada: inside diameter (mm) = 11.63 + 0.8128 × size. Each whole size adds 0.032 in (0.8128 mm) of diameter, or about 2.55 mm of circumference.
- EU / ISO 8653: the size is the inside circumference in millimeters, so EU 54 has a 54 mm circumference. Circumference = π × diameter.
- UK, Ireland and Australia: letters A to Z (then Z+1, Z+2…). Size A has a circumference of about 37.5 mm and every letter adds about 1.25 mm, with half sizes in between.
- Japan, China and Switzerland: the Japanese number is about the circumference minus 40 mm, so size 14 ≈ 54 mm. Swiss sizes use the same idea.
Worked example: a ring with an 18.1 mm inside diameter has a circumference of π × 18.1 ≈ 56.9 mm (EU 57). Its US size is (18.1 − 11.63) ÷ 0.8128 ≈ 7.96, so it is a US 8.
Common ring sizes at a glance
| US | UK / AU | EU | Japan | Diameter (mm) | Circumference (mm) |
|---|---|---|---|---|---|
| 4 | H½ | 47 | 7 | 14.88 | 46.8 |
| 5 | J½ | 49 | 9 | 15.69 | 49.3 |
| 6 | L½ | 52 | 12 | 16.51 | 51.9 |
| 7 | N½ | 54 | 14 | 17.32 | 54.4 |
| 8 | P½ | 57 | 17 | 18.13 | 57.0 |
| 9 | R½ | 60 | 20 | 18.95 | 59.5 |
| 10 | T½ | 62 | 22 | 19.76 | 62.1 |
| 11 | V½ | 65 | 25 | 20.57 | 64.6 |
| 12 | X½ | 67 | 27 | 21.38 | 67.2 |
The average women's ring size in the US is around 6 to 7, and the average men's size is around 9 to 10.
Tips for measuring your ring size at home
- Measure at the end of the day and at room temperature. Fingers are smaller in the morning and in the cold.
- With a string or paper strip, wrap it snugly but not tightly, and make sure it can slide over the knuckle.
- To size an existing ring, measure the inside diameter across the widest point, not including the metal.
- Wide bands (6 mm or more) feel tighter: go up a quarter to half size.
- If your knuckles are larger than the base of your finger, choose a size that fits over the knuckle.
Frequently asked questions
How is ring size measured?
Ring sizes are based on the inside of the ring. US and Canadian sizes go up by 0.8128 mm (0.032 in) of inside diameter per whole size, starting from 11.63 mm at size 0. EU (ISO 8653) sizes are simply the inside circumference in millimeters.
What is a US size 7 ring in mm?
A US 7 ring has an inside diameter of about 17.3 mm and an inside circumference of about 54.4 mm. That is about UK N½, EU 54 and Japanese size 14.
How do I convert a UK ring size to a US size?
UK and Australian sizes use letters. Each full letter adds about 1.25 mm of circumference, so about two letters equal one US size. For example UK J½ is about US 5, UK N½ is about US 7 and UK R½ is about US 9.
How do Japanese ring sizes work?
A Japanese ring size number is the inside circumference in millimeters minus 40. Size 9 is about 49 mm and size 14 is about 54 mm, so each size is 1 mm of circumference.
Is a printable ring sizer accurate?
It is accurate if it is printed at 100% (actual size) with no fit-to-page scaling. Always measure the calibration ruler on the printout with a real ruler before you use it. A professional jeweler's sizer is still the most reliable.
What if I am between two ring sizes?
Choose the larger size. Fingers swell in the heat and during the day, and wide bands (over about 6 mm) fit tighter, so many jewelers suggest going up a quarter or half size for them.