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Ring Size Calculator
Use this free ring size calculator to find your ring size from a finger measurement. Enter your finger circumference in millimeters, or the diameter in millimeters, and it shows your US, UK, EU, and JP sizes at once, rounded to the nearest half size — plus a printable size chart below so you can check any size at a glance.
Ring size chart
| US size | UK size | EU size | JP size | Circumference (mm) | Diameter (mm) |
|---|---|---|---|---|---|
| 3 | F½ | 44 | 4 | 44.2 | 14.1 |
| 3.5 | G½ | 45 | 5 | 45.5 | 14.5 |
| 4 | H½ | 47 | 7 | 46.8 | 14.9 |
| 4.5 | I½ | 48 | 8 | 48.0 | 15.3 |
| 5 | J½ | 49 | 9 | 49.3 | 15.7 |
| 5.5 | K | 51 | 11 | 50.6 | 16.1 |
| 6 | L | 52 | 12 | 51.9 | 16.5 |
| 6.5 | M | 53 | 13 | 53.1 | 16.9 |
| 7 | N | 54 | 14 | 54.4 | 17.3 |
| 7.5 | O | 56 | 15 | 55.7 | 17.7 |
| 8 | P | 57 | 16 | 57.0 | 18.1 |
| 8.5 | Q | 58 | 18 | 58.3 | 18.6 |
| 9 | R | 59 | 19 | 59.5 | 18.9 |
| 9.5 | S | 61 | 20 | 60.8 | 19.4 |
| 10 | T | 62 | 21 | 62.1 | 19.8 |
| 10.5 | U | 63 | 22 | 63.4 | 20.2 |
| 11 | V | 65 | 23 | 64.6 | 20.6 |
| 11.5 | W | 66 | 25 | 65.9 | 21.0 |
| 12 | X | 67 | 26 | 67.2 | 21.4 |
| 12.5 | Y | 68 | 27 | 68.5 | 21.8 |
| 13 | Z | 70 | 28 | 69.7 | 22.2 |
| 13.5 | Z+1 | 71 | 30 | 71.0 | 22.6 |
Sizes vary slightly between makers, so treat the chart as a guide and confirm a valuable ring with a jeweler's sizer. EU sizes are close to the circumference in millimeters, rounded to a whole number.
How it works
Ring sizes are just different ways of naming the same finger. The US system numbers sizes from about 3 to 13.5 in half steps, the UK system uses letters, the EU system uses numbers close to the circumference in millimeters, and the JP system uses its own numbered scale. Underneath them all is one physical measurement: the circumference of your finger in millimeters, or equivalently the diameter, since circumference equals π times diameter.
The calculator takes your measurement, converts a diameter into a circumference when needed, and finds the nearest entry on the standard chart. A circumference of 49.3 mm is US 5, 51.9 mm is US 6, 54.4 mm is US 7, 57.0 mm is US 8, 59.5 mm is US 9, 62.1 mm is US 10, 64.6 mm is US 11, 67.2 mm is US 12, and 69.7 mm is US 13. Between those anchor points, half sizes fill the gaps: 50.6 mm is US 5.5, 53.1 mm is US 6.5, and so on. If your measurement falls exactly halfway between two half sizes, the calculator picks the nearer one and tells you how far off the chart value you are.
Measuring well matters more than the arithmetic. Fingers change size during the day: they are usually smallest in the morning and largest in the evening, and cold weather shrinks them while heat swells them. Measure in the evening at a comfortable room temperature, wrap the paper or string snugly — not tight enough to dent the skin — and measure twice. If the two readings disagree by more than a millimeter, measure a third time.
The knuckle is the other thing to think about. A ring has to pass over the knuckle, which is often wider than the base of the finger where the ring sits. If your knuckle is much wider, choose the size that slides over the knuckle even if it feels a touch loose at the base. Wide bands — roughly 6 mm or wider — also fit tighter than narrow bands of the same size, so many people go up half a size for a wide band. A jeweler can confirm the fit in seconds, and for an engagement ring or any expensive piece that check is worth doing.
Worked example
Someone measures a finger circumference of 54.4 mm. What is the ring size?
Step 1: Compare with the chart — 54.4 mm is listed exactly as US 7.
Step 2: Read the same row — UK N, EU 54, JP 14.
Step 3: Find the diameter — 54.4 ÷ 3.1416 = 17.3 mm.
Step 4: Enter 54.4 in the calculator with "Circumference" selected, and it shows US 7 · UK N · EU 54 · JP 14, an exact match.
A second check: a diameter of 18.1 mm gives a circumference of 18.1 × 3.1416 = 56.9 mm, which rounds to the nearest chart entry, 57.0 mm — US 8, UK P, EU 57, JP 16.
FAQ
How do I measure my ring size at home?
Wrap a strip of paper around the base of your finger, mark where it overlaps, and measure the length in millimeters — that is your circumference. Measure in the evening, when fingers are slightly larger, and repeat twice to be sure.
What ring size is 54.4 mm circumference?
A circumference of 54.4 mm is US size 7, UK size O, EU size 54, and JP size 14. Enter 54.4 in the calculator and it shows all four sizes at once.
Should I size up or down if I am between sizes?
Size up if your knuckle is much wider than the base of your finger, if you live in a hot climate, or if the ring is a wide band, because wide bands fit tighter. Otherwise, the nearest half size is usually right.
Is ring size the same as finger diameter?
They describe the same finger in different ways: circumference is the distance around the finger, and diameter is the distance straight across. Divide circumference by 3.1416 to get diameter, and this calculator accepts either measurement.