Ring Size Calculator: Convert mm & Inches to US, UK, EU Sizes
Ordering a ring online, planning a surprise proposal, or converting a size from a European jeweller? Ring sizes look simple until you realise the US, UK, EU, French and Japanese systems all number the same finger differently. This ring size calculator removes the guesswork: measure once, in millimetres or inches, and get every major size at once.
You can start from either measurement. If you do not own a ring yet, measure the circumference of your finger with a strip of paper. If you already have a ring that fits the right finger, measure its inner diameter instead. The calculator converts between the two using pi, then matches your circumference to the standard US size bands.
- Ring size calculator
- How to measure your ring size accurately
- Common ring sizing mistakes
- US, UK, EU, French and Japanese sizes explained
- Full ring size chart (US 4–13)
- Resizing limits: how far can a ring be changed?
- Keep exploring
- Ring size FAQs
Ring Size Calculator
Find your ring size in seconds. Measure around your finger, or measure a ring that already fits, and convert it to US, UK, EU, French and Japanese sizes.
1. What are you measuring?
Wrap a strip of paper around the base of your finger, mark where it meets, then measure that length flat.
Millimetres give the most accurate ring size — even half a millimetre can shift a half size.
How to Measure Your Ring Size Accurately
The string or paper method (circumference)
Cut a thin strip of paper about 10 cm long, or use a piece of non-stretch string. Wrap it snugly around the base of the finger the ring is for — snug, not tight; it should feel like a well-fitting ring, and it must be able to slide over your knuckle. Mark where the end meets, lay it flat, and measure the length in millimetres. That length is your inner circumference, the most direct route to your size.
The existing-ring method (diameter)
If a ring already fits that exact finger comfortably, measure straight across the inside of the band with a ruler or calipers, from inner edge to inner edge. Do not measure the outside, and do not include the thickness of the metal. Enter that diameter and the calculator works out the circumference for you. For reference, a 17.3 mm inner diameter works out to about a 54.4 mm circumference — a US 7.
Timing matters more than people think
Fingers are measurably smaller in the morning and in cold weather, and larger at the end of the day, in warm rooms, and after exercise or salty food. Measure in the evening, with warm hands, and repeat the measurement two or three times across a day if the ring is expensive or cannot be resized. If your results straddle two sizes, the larger one is usually the safer choice.
Common Ring Sizing Mistakes
Ignoring the knuckle. Many fingers taper: the base is slimmer than the knuckle the ring has to pass over. If your knuckle is noticeably larger, measure at the knuckle and accept a slightly looser fit at the base — sizing beads or a spring insert can steady it once it is on.
Forgetting the wide-band rule. Wide bands (about 6 mm and broader) sit on more skin and feel tighter than a slim band of the same size. For a wide band, go up a quarter to a half size from your calculator result.
Measuring the wrong finger or hand. Your dominant hand is usually slightly larger, and every finger is its own size. A size taken from your right ring finger will not necessarily fit your left.
Using a stretchy measure. Fabric tape measures and string that stretches quietly add millimetres — and roughly 1.3 mm of circumference is a whole half size.
US, UK, EU, French and Japanese Sizes Explained
The US system uses numbers from about 3 to 13, in half-size steps; each half size adds roughly 1.3 mm of circumference. The UK system uses letters — H for a US 4, stepping through the alphabet to Z for a US 13. The EU (ISO) system is the most literal: the size is simply the inner circumference in millimetres, rounded, so a 54 mm finger is a size 54. The French system uses the same circumference number. The Japanese system runs on its own scale; as a rule of thumb, the Japanese size is approximately the circumference in millimetres minus 40.4, so a 54 mm finger is about a Japanese 14.
Full Ring Size Chart (US 4–13)
| US size | Circumference (mm) | Diameter (mm) | UK | EU / FR | JP |
|---|---|---|---|---|---|
| 4 | 46.8 | 14.9 | H | 47 | 6 |
| 4.5 | 48.0 | 15.3 | I | 48 | 8 |
| 5 | 49.3 | 15.7 | J | 49 | 9 |
| 5.5 | 50.6 | 16.1 | K | 51 | 10 |
| 6 | 51.9 | 16.5 | L | 52 | 12 |
| 6.5 | 54.0 | 17.2 | M | 54 | 14 |
| 7 | 54.4 | 17.3 | N | 54 | 14 |
| 7.5 | 55.7 | 17.7 | O | 56 | 15 |
| 8 | 57.0 | 18.1 | P | 57 | 17 |
| 8.5 | 58.3 | 18.5 | Q | 58 | 18 |
| 9 | 59.5 | 19.0 | R | 60 | 19 |
| 9.5 | 60.8 | 19.4 | S | 61 | 20 |
| 10 | 62.1 | 19.8 | T | 62 | 22 |
| 10.5 | 63.4 | 20.2 | U | 63 | 23 |
| 11 | 64.6 | 20.6 | V | 65 | 24 |
| 11.5 | 65.9 | 21.0 | W | 66 | 26 |
| 12 | 67.2 | 21.4 | X | 67 | 27 |
| 12.5 | 68.5 | 21.8 | Y | 28 | |
| 13 | 69.7 | 22.2 | Z | 70 | 29 |
Chart values are the standard published midpoints for each size; the calculator matches your measurement to the nearest size band, which is why a 54.0 mm circumference returns US 6.5 while a 54.4 mm circumference returns US 7.
Resizing Limits: How Far Can a Ring Be Changed?
Most simple metal bands can be resized about one to two full sizes up or down. Beyond that, the shape distorts, settings loosen, and patterns or pavé stones around the band make resizing risky or impossible. Eternity bands, titanium and tungsten rings generally cannot be resized at all. If you are between sizes and the ring has stones all the way around, order the larger size and use sizing assistants; if it is a plain band, ordering closer to size and resizing once is usually fine. Always confirm the retailer’s exchange policy before a surprise purchase.
Quick tip for surprise proposals: borrow a ring the person already wears on the correct finger, trace its inner circle on paper, and measure that diameter. It is far more accurate than guessing — and discreet.
Keep Exploring
Find all of our converters and calculators on the tools hub. If you are converting clothing sizes too, try the dress size converter and the body shape calculator. Planning the proposal look? These engagement nail ideas are made for ring photos, and our wedding hairstyles guide completes the picture.
Ring Size FAQs
What is the average women’s ring size?
The most commonly purchased women’s ring size is US 6 to 7, with US 7 often cited as the single most common size. Averages vary by country and age, so treat them as a last-resort guess only — a measurement always beats an average.
How do I find someone’s ring size in secret?
Borrow a ring they wear on the same finger and measure its inner diameter, or press it into a bar of soap or soft clay and measure the imprint. Asking a close friend or family member who may already know is the other classic route.
Is it better to size up or down if I am between sizes?
Size up. Rings can be made to fit more snugly with inexpensive sizing aids, while a ring that is too small may not pass the knuckle at all. This is doubly true for wide bands.
Do ring sizes change with age or weight?
They can. Knuckles often enlarge with age and arthritis, and fingers respond to weight changes, pregnancy and temperature. Re-measure before any significant purchase rather than trusting a size from years ago.
What if my size is below US 4 or above US 13?
Many jewellers make sizes outside the standard chart on request, and the calculator’s EU size (your circumference in millimetres) is the universal number to give them, since it needs no conversion.
