What this calculator does
A ring size converter turns one sizing system into another, because the US/Canada numeric scale, the UK/Australian/Irish letter scale, and the EU numeric scale each measure the same physical ring differently. This calculator works from the standard published jewellery-industry sizing charts, in both directions: give it a ring size and it returns the inside circumference and diameter, or give it a circumference or diameter and it returns the matching size on all three scales.
The underlying measurement is always the inside circumference of the ring, the distance around the inside edge. Ring size charts convert that single measurement into whichever scale a retailer happens to use, which is why the same ring can be labelled "size N" in the UK, "size 7" in the US, and "size 54" in the EU.
The formula
Circumference and diameter relate by circumference = π × diameter. The US/Canada size is read from the standard chart relationship between size and diameter, which increases in close to a straight line: roughly 0.813 mm of diameter per half US size. The UK/Australian/Irish letter is read from the same standard chart by nearest match, since that scale runs in fixed letter steps rather than a smooth number line. The EU size is the inside circumference in millimetres, rounded to the nearest whole number, which is how EU (and French) ring sizing is defined.
| Term | Meaning |
|---|---|
| US/Canada size | The numeric scale most familiar from US and Canadian retailers, running from about 3 to 13 for adult rings in half-size steps. |
| UK/Australian/Irish size | The alphabetic scale (A through Z and beyond) shared by the UK, Ireland, Australia, New Zealand and South Africa. |
| EU size | The European scale, defined as the inside circumference in millimetres, rounded to the nearest whole number. |
| Inside circumference | The distance around the inside edge of the ring, the measurement every other sizing scale is derived from. |
The inputs explained
| Field | What to enter |
|---|---|
| Convert | Choose whether you are starting from a known ring size, or from a measurement of circumference or diameter. |
| US/Canada ring size | The US/Canada ring size to convert from, used when "Ring size to measurement" is selected. |
| Measurement type | Whether the measurement you have is the inside circumference or the inside diameter of the ring. |
| Measurement value | The measurement value itself, used when "Measurement to ring size" is selected. |
| Unit | The unit the measurement was taken in. |
When to use it
Buying a ring online in a different country’s sizing system
A ring listed in EU or UK sizing needs converting to the US/Canada scale, or vice versa, before ordering, since the numbers on each scale are not interchangeable.
Sizing a ring from a piece of string or paper strip
Wrapping a strip of paper or string around a finger and measuring its length gives the inside circumference directly, which converts straight into a ring size on any of the three scales.
Checking a ring that already fits
Measuring the inside diameter of an existing ring that fits well, with a ruler laid across the inside edge, gives a reliable size without needing to measure a finger at all.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What size does each US/Canada ring size correspond to?
The matching circumference, diameter, UK/Australian letter and EU size for a run of common adult US/Canada ring sizes.
| US/Canada size | Inside circumference | Inside diameter | UK/Australian/Irish size | EU size |
|---|---|---|---|---|
| 5 | 49.3 mm | 15.70 mm | J | 49 (approx) |
| 6 | 51.9 mm | 16.51 mm | L | 52 (approx) |
| 7 | 54.4 mm | 17.32 mm | N | 54 (approx) |
| 8 | 57.0 mm | 18.13 mm | P | 57 (approx) |
| 9 | 59.5 mm | 18.95 mm | R | 60 (approx) |
| 10 | 62.1 mm | 19.76 mm | T | 62 (approx) |
| 11 | 64.6 mm | 20.57 mm | V | 65 (approx) |
| 12 | 67.2 mm | 21.39 mm | X | 67 (approx) |
What ring size does a 7 cm or 8 cm circumference measurement give?
A range of circumference measurements around 7 to 8 cm, the kind of figure a wrapped paper strip or string measurement commonly gives, converted to each sizing scale.
| Circumference | US/Canada ring size | UK/Australian/Irish size | EU size |
|---|---|---|---|
| 5 cm | 5.3 (nearest half size 5.5) | K | 50 (approx) |
| 6 cm | 9.2 (nearest half size 9) | R | 60 (approx) |
| 6.5 cm | 11.1 (nearest half size 11) | V | 65 (approx) |
| 7 cm | 13.1 (nearest half size 13) | Z | 70 (approx) |
| 7.5 cm | 15.1 (nearest half size 15) | Z | 75 (approx) |
| 8 cm | 17.0 (nearest half size 17) | Z | 80 (approx) |
Questions
What is 8cm in ring size?
Treated as an 8 cm (80 mm) inside circumference, that converts to roughly a US/Canada size 17, well beyond the standard adult chart, which tops out around US 13 (about 7 cm). An 8 cm reading usually means the measurement was actually a diameter, or was taken in a unit other than centimetres, so it is worth re-measuring before ordering.
What is ring size 7cm?
A 7 cm (70 mm) inside circumference converts to approximately a US/Canada size 13, or a UK/Australian size Z, right at the top edge of the standard adult sizing chart.
How accurate is a paper-strip or string measurement?
It is a reasonable estimate but not as precise as a set of ring sizers or an in-store measurement, since the strip can be pulled too tight or left slightly loose. Measuring an existing ring that already fits well, using its inside diameter, is usually more reliable.
Why do UK and Australian ring sizes use letters instead of numbers?
It is simply a different historical convention, shared by the UK, Ireland, Australia, New Zealand and South Africa, that grades ring sizes in fixed letter steps rather than a numeric scale. Both systems measure the same underlying circumference; only the labelling differs.
For other everyday unit conversions, see the paper weight calculator. To work out a date rather than a measurement, the date calculator covers everyday date arithmetic.