Ring Size Calculator
Inner diameter of a ring from the finger circumference, in millimetres, centimetres and inches.
Results
What this tool does
The ISO 8653 size is simply the inside circumference in millimetres, which is what most of Europe uses directly and what jewellers measure with a mandrel. The US, UK and Japanese scales are arbitrary letter or number sequences with no formula behind them, so converting to those needs a published chart rather than arithmetic.
Formula
diameter = circumference ÷ π
Variables
| Symbol | Meaning | Unit |
|---|---|---|
cf | Finger circumference | mm |
DI | Diameter | mm |
IS | Ring size (ISO 8653) | — |
DC | Diameter | cm |
IN | Diameter | in |
CW | Finger circumference | mm |
Worked example
- Finger circumference54.5 mm
- Diameter17.348 mm
- Ring size (ISO 8653)54.5
- Diameter1.7348 cm
- Diameter0.6830 in
- Finger circumference56.0 mm
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
How do I measure my finger properly?
Wrap a strip of paper snugly round the base of the finger, mark where it overlaps and measure the length in millimetres — that is the circumference. Measure at the end of the day when fingers are largest, and in normal temperature: cold fingers can read a whole size small. If a knuckle is wider than the base, use the fifth result, which adds a little so the ring passes over it.