Read your size across every system at once
Ring sizes are not standardised worldwide. The US and Canada number them, the UK and Australia use letters, the EU uses the inside circumference in millimetres, and Japan has its own numbers. The one measurement they all agree on is the band's inside diameter, so start there.
| US / Canada | UK / Aus | EU | Japan | Diameter | Circumference |
|---|---|---|---|---|---|
| 3 | F | 47 | 5 | 14.8 mm | 46.6 mm |
| 4 | H | 49 | 7 | 15.6 mm | 49.1 mm |
| 5 | J | 52 | 9 | 16.5 mm | 51.7 mm |
| 6 | L | 54 | 11 | 17.3 mm | 54.2 mm |
| 7 | N | 57 | 13 | 18.1 mm | 56.8 mm |
| 8 | P | 59 | 15 | 18.9 mm | 59.3 mm |
| 9 | R | 62 | 17 | 19.7 mm | 61.9 mm |
| 10 | T | 64 | 19 | 20.5 mm | 64.5 mm |
| 11 | V | 67 | 21 | 21.3 mm | 67.0 mm |
| 12 | X | 70 | 23 | 22.1 mm | 69.6 mm |
| 13 | Z | 72 | 25 | 23.0 mm | 72.1 mm |
Half sizes fall halfway between the rows. UK letters and Japan numbers vary by a step or two between jewellers; the diameter in millimetres is the value to trust.
The two conversions the chart is built on
Each US size adds 0.8128 mm to the inside diameter. The EU size needs no formula at all: it is the inside circumference rounded to the nearest millimetre, so a 54 mm band is simply EU 54.
Before you settle on a size
- Fingers change through the day. They swell in heat and by evening, shrinking again in the cold and first thing in the morning — a swing of up to half a size. Measure a few times and take the average.
- Wide bands sit tighter. A thick or wide ring feels smaller than a thin one at the same size, so size up by a quarter to half for chunky bands.
- Measure the ring, not the finger. A finger is thicker than the ring that slides over the knuckle, so a string around the finger reads too large.
Common questions
What ring size is an 18 mm inside diameter?
About a US 7. An 18 mm inside diameter is a 56.5 mm inside circumference, which is EU size 56, UK N and Japan 13.
What is the difference between diameter and circumference?
Diameter is the straight measurement across the inside of the band; circumference is the distance all the way around, and equals diameter times pi (about 3.14159). US, UK and Japan sizes track the diameter, while the EU size is simply the inside circumference in millimetres.
How do I measure my ring size at home?
Measure the inside diameter of a ring that already fits with a ruler, in millimetres, then read across the chart. Measuring your finger with string is less reliable because a finger is thicker than the ring and string stretches.
