# Ring Size Converter

URL: https://speedycalculator.com/calculators/everyday-life/ring-size/
Last updated: 2026-10-25

> The ring size converter turns a finger or ring measurement (inside diameter or circumference) into US & Canada, UK & Australia, French/ISO, German, Swiss and Japanese ring sizes.

## Inputs

- I measured
- Measurement (mm, in, cm)

## Results

- Inside diameter
- Inside circumference
- US & Canada size
- UK & Australia size
- ISO / France size
- Germany size (diameter mm)
- Switzerland size
- Japan size

## Formula

```
Diameter = circumference ÷ π
US size ≈ (diameter in mm − 11.63) ÷ 0.8128
UK letter: A = 37.8 mm circumference, each half size + 0.625 mm
ISO/French = circumference (mm); German = diameter (mm); Swiss = circumference − 40
```

## Explanation

Ring sizes are different in almost every country. The international standard (ISO 8653) simply uses the inside circumference in millimetres, which France also uses, while Germany uses the inside diameter. The US uses numbers in quarter steps and the UK uses letters with half sizes. Measure at the end of the day when fingers are largest, and remember wide bands fit tighter.

### How to use the Ring Size Converter

- **I measured**
- **Measurement**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Choose whether you measured the inside diameter of a ring or the circumference of your finger (or a strip of paper wrapped around it), and enter the measurement.

### Ring size chart

USDiameter (mm)Circumference (mm)UK414.946.8H515.749.3J616.551.9L717.354.4N½818.157.0P½918.959.5R½1019.862.1T½1120.664.6V½1221.467.2X½1322.269.7Z½

### Tips

- Measure over thin clothing with a soft tape held snug but not tight, and keep the tape level all the way round.
- Take each measurement twice; if you are between sizes, the larger size is usually the safer choice.
- Size charts vary by brand and country – check the maker's own chart when it is available.
- For sewing projects, pre-wash fabric before cutting and buy 10% extra for shrinkage, pattern matching and mistakes.

### Common mistakes to avoid

- Mixing inches and centimetres between the tape and the size chart.
- Forgetting seam and hem allowances when cutting fabric.
- Measuring an old, stretched garment instead of your body.

### Inputs explained

FieldWhat to enterI measuredChoose one: Inside diameter of a ring, Inside circumference / fingerMeasurementUnits: mm, in, cm

### What each result means

- **Inside circumference** = pi×[Inside diameter]
- **US & Canada size** = round(([Inside diameter]÷0.001 - 11.63)÷0.8128×4)÷4
- **UK & Australia size** = clamp(round(([Inside circumference]÷0.001 - 37.8)÷0.625), 0, 58)
- **ISO / France size** = round([Inside circumference]÷0.001)
- **Germany size (diameter mm)** = round([Inside diameter]÷0.001×2)÷2
- **Switzerland size** = round([Inside circumference]÷0.001 - 40)
- **Japan size** = max(round(([Inside diameter]÷0.001 - 13)÷0.334) + 1, 1)

## Worked example

Inputs — I measured: Inside circumference / finger; Measurement: 54.4 mm.

Results — Inside diameter = 17.32 mm; Inside circumference = 54.4 mm; **US & Canada size = 7**; UK & Australia size = N½; ISO / France size = 54; Germany size (diameter mm) = 17.5; Switzerland size = 14; Japan size = 14.

## FAQ

**How can I measure my ring size at home?**
Wrap a strip of paper around the finger, mark where it overlaps, measure the length in mm and enter it as circumference.

**What ring size is 54 mm?**
About US 7, UK N½–O, ISO 54, German 17.25.

**Should I size up for a wide band?**
Yes – bands wider than about 6 mm fit tighter, so go up a quarter to half size.

**What formula does the Ring Size Converter use?**
Diameter = circumference ÷ π.


## Sources

- [ISO 8653:2016 – Jewellery – Ring-sizes](https://www.iso.org/standard/66553.html)