How to use the converter
- Type a size in any box: US / Canada, UK / AU, EU, Diameter (mm) or Circumference (mm). The other four update and the nearest chart row is highlighted.
- To size from a measurement, use Circumference for a string or paper strip around your finger, or Diameter for a ring that already fits.
- Read the sentence under the boxes. For an in-between measurement it also shows the exact US size.
How the sizes are calculated
The US and Canada scale is a straight line: inside diameter in mm = 0.8128 x size + 11.63, and circumference = 3.1416 x diameter. EU sizes follow ISO 8653, which defines the ring size as the inner circumference in millimeters, so the EU number is the circumference rounded to a whole millimeter. UK and Australian sizes are letters; each letter adds about 1.25 mm of circumference, and a half letter is allowed. The tool uses the common retailer chart (US 7 is UK N½) from F to Z½, interpolating for quarter sizes.
Worked example: US 7
The tool opens on US 7. Diameter = 0.8128 x 7 + 11.63 = 17.32 mm, shown as 17.3 mm. Circumference = 3.1416 x 17.32 = 54.4 mm, so the EU size is 54 and the UK letter is N½. The sentence under the boxes reads:
US 7 = UK N½ = EU 54. Inner diameter 17.3 mm, inner circumference 54.4 mm.
Measuring tips
Fingers swell when warm and in the evening and shrink when cold, so measure on two or three different days and average. If your knuckle is wider than the base of the finger, pick a size between the two readings or ask for sizing beads. Wide bands feel tighter than thin ones; many jewelers suggest going up a quarter size above about 6 mm and a half size above about 8 mm. Measure the finger you will wear the ring on, since the dominant hand is often slightly larger.
Limits and edge cases
- Range. The chart covers US 3 to 13: 14.1 to 22.2 mm diameter, 44.2 to 69.7 mm circumference, EU 44 to 70 and UK F to Z½. Anything outside gets a message.
- EU rounding. EU sizes are whole millimeters, so converting EU to US can be off by up to a quarter size.
- Other systems. Japan, India and China use their own scales, which this tool does not include.
- Ordering. If a seller publishes its own chart, order from that one.
Buying a gift for someone with other measurements? The shoe size converter and hat size converter work the same way, and the on-screen ruler can help with a quick millimeter reading.
Frequently asked questions
How do I convert a US ring size to UK?
Type the US size in the US / Canada box and read the UK / AU box. US 6 is UK L½, US 7 is UK N½ and US 8 is UK P½. Jewelers' charts can differ by about half a size, so check the seller's own chart before ordering.
What is the most common ring size?
Jewelers most often cite US 6 to 7 for women (about 16.5 to 17.3 mm inside diameter) and US 9 to 10 for men (about 19.0 to 19.8 mm). There is no official statistic, and finger size varies with build and hand.
How do I find my ring size from a ring I already own?
Measure the inside diameter of a ring that fits the right finger, across the widest point, in millimeters. Type it in the Diameter (mm) box. A 17.3 mm ring is US 7.
Can a ring be resized after purchase?
Many plain bands can be sized up or down by a size or two. Eternity bands, tension settings and rings with intricate side patterns often cannot. A jeweler will quote a fee that depends on the metal, the design and whether the ring goes up or down.
Sources: Wikipedia, Ring size (formulas and ISO 8653) and Queensmith ring size guide (UK letters).