Ring Size Chart Converter
Convert between the ways ring sizes are written. Enter a circumference in centimeters (a 6 cm string measurement is 60 mm) or a US size number, and get millimeters around, inner diameter across, the US size, and the EU/ISO size — plus the full chart below.
Example: with Your measurement or size 6 · Convert from Circumference in cm → Circumference in mm: 60.0 mm around.
- US ring sizeUS 9 (exact 9.19)
- Inner diameter19.10 mm across
- EU / ISO sizeEU 60
Computed by the calculator below using its default values. Change any input to see your own numbers.
1 cm = 10 mm, and the EU/ISO ring size is the circumference in mm. US sizes step 0.8128 mm of diameter per size: diameter = 11.63 mm + 0.8128 × size.
One finger, four numbers
Every ring size system describes the same circle. EU/ISO sizes state the inner circumference in millimeters directly — a 6 cm measurement is EU 60, full stop. US sizes run a linear scale on the inner diameter instead: size 0 is 11.63 mm across and each full size adds 0.8128 mm (0.032 in). Circumference and diameter trade places through pi, so any one number pins down the other three.
That 6 cm string measurement therefore converts to 60 mm around, 19.10 mm across, and an exact US size of 9.19 — call it US 9. Going the other way, a US 7 is 17.32 mm across and 54.4 mm around, which is why charts list it next to EU 54.
How it’s calculated
From a cm measurement: circumference mm = cm × 10; diameter = circumference ÷ π; US size = (diameter − 11.63) ÷ 0.8128, rounded to the nearest half. From a US size: diameter = 11.63 + 0.8128 × size; circumference = π × diameter. EU/ISO size (ISO 8653) = circumference in mm, rounded to the nearest whole number.
String and paper-strip measurements stretch and tend to read about a half size large — confirm with a jeweler's sizer before resizing or buying an expensive ring.
US ring size chart
| US size | Inner diameter | Circumference | EU / ISO |
|---|---|---|---|
| 5 | 15.69 mm | 49.3 mm | 49 |
| 6 | 16.51 mm | 51.9 mm | 52 |
| 7 | 17.32 mm | 54.4 mm | 54 |
| 8 | 18.13 mm | 57.0 mm | 57 |
| 9 | 18.95 mm | 59.5 mm | 60 |
| 10 | 19.76 mm | 62.1 mm | 62 |
| 11 | 20.57 mm | 64.6 mm | 65 |
| 12 | 21.38 mm | 67.2 mm | 67 |
Diameter = 11.63 mm + 0.8128 mm × US size; circumference = π × diameter; EU/ISO size is the circumference in mm.
Common mistakes
- Reading 6 cm as 6 mm — a 6 cm circumference is 60 mm, about a US 9; nothing human is 6 mm around.
- Measuring only at the base of the finger: measure the knuckle too and size to whichever is larger, or the ring will not go on.
- Assuming the EU number is the diameter; EU/ISO sizes are the circumference in mm.
- Trusting a printed chart without checking its scale — print settings shrink paper sizers; verify a known distance with a ruler first.
Frequently asked questions
How do I convert cm to a ring size?
Multiply by 10 to get mm — that is already the EU/ISO size. For US, divide by π for the diameter and compute (diameter − 11.63) ÷ 0.8128. A 6 cm measurement is EU 60 and US 9.
What is 6 cm in mm for ring size?
60 mm of circumference, which is a 19.10 mm inner diameter — EU size 60, US size 9.
How big is a half size?
Each US half size is 0.4064 mm of inner diameter, about 1.3 mm of circumference. It sounds tiny but is clearly noticeable on the finger.
Which countries use which system?
The US and Canada use the numeric 3-13 scale; most of Europe uses the ISO circumference-in-mm size; the UK and Australia use letters. All of them map to the same mm measurements in the chart.