Ring Size Calculator
Find US, UK, EU, and JP ring sizes from inner diameter or circumference in mm or inches.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this ring size calculator
- Pick what you measured: inner diameter or inner circumference, in mm or inches.
- Enter the measurement. Wrap a thin strip of paper around the base of the finger and measure its length for circumference, or measure a ring you already own across the inside for diameter.
- Press Find size to see US, UK, EU, and Japanese sizes plus the matching diameter and circumference.
- Expand "Show full size chart" to browse the entire conversion table.
About this ring size calculator
Ring sizes are derived from one physical measurement: the inner circumference (or its equivalent inner diameter) of the band. The ring size calculator uses the Jewelers of America standard chart (which aligns with ISO 8653) to map inner diameter or circumference in millimetres to the four common regional scales: US (numeric, half-step), UK (letter), EU (numeric), and Japan (numeric). Each whole US size up represents roughly 0.4 mm of extra diameter, so the chart is finely-grained — a 0.5 mm difference can move you a full size.
Worked example: you wrap a paper strip around the base of your finger and measure the strip as 54.4 mm. That is the inner circumference. The chart's closest row is 54.4 mm = 17.3 mm inner diameter = US 7 / UK N½ / EU 54 / JP 14. If instead you measure an existing ring across the inside as 18.2 mm, the row above (US 8 / UK P½ / EU 57 / JP 16) is your size. Fingers swell with heat and at night, so measure at room temperature. For a snug-but-removable fit, go up a half size if the band is thick (≥ 4 mm) — wider bands feel tighter.
FAQ
- Which measurement is more reliable, diameter or circumference?
- Circumference, if you measure with a soft tape or paper strip on the actual finger. Diameter from an existing ring is good too — but the inside of a worn ring can deform, so measure at the thinnest cross-section.
- Do I need to account for the season?
- Yes — fingers can swell up to half a size between cold winter mornings and hot summer afternoons. Measure twice, at different times of day, and pick the larger value.
- Why does the UK chart use letters?
- British (and Irish, Australian, South African) ring sizes use single letters A–Z plus half-sizes (e.g. M½). The order parallels the US numeric scale: A is smallest, Z+ is largest.
- Are EU and Japanese sizes the same?
- No. The EU size is the inner circumference in millimetres (approximately). Japan uses its own numeric scale that ascends roughly twice as fast as the US scale.
- How accurate is the conversion?
- To the nearest half-step. The tool snaps your input to the closest row of the standard chart and reports the diameter / circumference of that row so you can double-check.
- Is the ring size calculator free?
- Yes — free, no signup, runs entirely in your browser.