# What is my ring size?

Finds the US ring size and the ISO ring size from the inside diameter of a ring or the circumference of a finger, or the diameter and circumference of a US size.

- Page: https://www.acalculator.org/everyday-life/ring-size-calculator
- JSON spec: https://www.acalculator.org/everyday-life/ring-size-calculator.json
- Version: 92e58a474ae7

## Default answer

Example with the default inputs (I know the Ring diameter, Inside diameter 17.32 mm): An inside diameter of 17.32 mm is US ring size 7 (ISO 54.4).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| by | I know the | What you measured: the inside diameter of a ring that fits, the circumference of your finger, or a US size. |
| d | Inside diameter | The inside diameter of a ring that fits, edge to edge. |
| c | Finger circumference | The distance around the finger where the ring sits, measured with a strip of paper or string. |
| us | US size | A US or Canadian ring size, from 0 to 16 in steps such as 7, 7.25, 7.5. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| nearest | US ring size | The US and Canadian size to the nearest quarter size. |
| size | Exact US size | The US size before rounding: (inside diameter in mm − 11.63) ÷ 0.8128. |
| iso | ISO size | The ISO 8653 size: the inside circumference in millimetres. |
| diameterMm | Inside diameter (mm) | The inside diameter of the ring in millimetres. |
| diameterIn | Inside diameter (in) | The inside diameter in inches: millimetres ÷ 25.4. |

## Method

Inside diameter (mm) = 11.63 + 0.8128 × US size; US size = (diameter − 11.63) ÷ 0.8128; ISO size = inside circumference (mm) = π × diameter.

## Assumptions

- US and Canadian sizes follow the common scale: size 0 is 11.63 mm (0.458 in) inside, and each whole size adds 0.032 in (0.8128 mm) to the diameter.
- A finger circumference is taken as the ring’s inside circumference; the ISO size is that circumference in millimetres.
- Jewelers’ mandrels and ring makers differ slightly; try a ring on before you buy.

## Worked examples

1. by = diameter, d = 0.01732 gives nearest = 7, size = 7.000492, iso = 54.412385. Source: Ring size, Wikipedia: North American size to inside diameter d = 0.8128 × s + 11.63 mm (https://en.wikipedia.org/wiki/Ring_size, retrieved 2026-10-01); ISO 8653:2016 defines the size as the inside circumference in mm (https://www.iso.org/standard/66185.html).
2. by = us, us = 7 gives diameterMm = 17.3196, iso = 54.411128. Source: Ring size, Wikipedia (https://en.wikipedia.org/wiki/Ring_size): d = 0.8128 × 7 + 11.63 = 17.3196 mm.
3. by = circumference, c = 0.0544 gives nearest = 7, size = 6.995642. Source: Ring size, Wikipedia (https://en.wikipedia.org/wiki/Ring_size); ISO 8653:2016 (size = inside circumference in mm).

## FAQ

### How do I find my ring size at home?

Measure the inside diameter of a ring that fits the finger, edge to edge, in millimetres. Or wrap a thin strip of paper around the finger where the ring sits, mark where it overlaps, and measure that length: the circumference. Type either one.

### How do US ring sizes work?

Size 0 has an inside diameter of 11.63 mm (0.458 in), and each whole size adds 0.032 in (0.8128 mm). So size 7 is 11.63 + 7 × 0.8128 = 17.32 mm across, and half and quarter sizes sit in between.

### What is an ISO ring size?

ISO 8653 gives a ring size as the inside circumference in millimetres. A US size 7 ring (17.32 mm across) has an inside circumference of π × 17.32 = 54.4 mm, so it is ISO size 54.4, often marked 54.

### What ring size is a 54 mm circumference?

54 mm ÷ π = 17.19 mm across, which is US size (17.19 − 11.63) ÷ 0.8128 = 6.84, about 6 3/4.

### Should I round up or down between sizes?

The calculator shows the nearest quarter size. Fingers swell in heat and later in the day, and wide bands fit tighter, so many people pick the next quarter size up when in doubt.

### Are Canadian ring sizes the same as US sizes?

Yes. The US and Canada use the same number scale. The UK and Australia use letters, and much of Europe uses the ISO circumference in millimetres.

## Sources

- Ring size, Wikipedia: North American sizes, d = 0.8128 × size + 11.63 mm (whole sizes differ by 0.032 in of inside diameter), and ISO 8653:2016 defines a ring size as the inside circumference in millimetres, retrieved 2026-10-01. https://en.wikipedia.org/wiki/Ring_size
- ISO 8653:2016, Jewellery — Ring-sizes — Definition, measurement and designation (ring size = inside circumference in mm). https://www.iso.org/standard/66185.html