# Ring Size Converter — EU, US, UK & JP Sizes > Convert ring sizes between EU/ISO, US, UK and Japanese systems instantly. Enter inner circumference, diameter, or a known size — get all four systems plus a to-scale circle diagram. Free. - **Category:** Conversions & Units - **Interactive calculator:** https://youcalc.com/en/conversions-units/ring-size-converter/ - **Price:** Free, no sign-up required ## Overview This converter translates a ring measurement into all four major sizing systems at once: EU/ISO 8653 (circumference in mm), US/Canada (numeric), UK/Australia (letters), and Japan (numeric). Enter an inner circumference, a diameter, or a size you already know from any system to get the equivalents instantly. ## How to read your result The result panel shows the same physical ring size expressed in each system. EU equals the inner circumference in millimetres exactly, so EU 57 means the inner circumference is 57 mm. US and JP sizes are numbers; the UK/Australia system uses letters with optional half-steps (for example N or N½). Ring sizing varies by country and brand and uses fine increments; treat conversions as approximate and confirm with a jeweler. ## Method All conversions are grounded in the physical inner circumference of the ring in millimetres, as defined by ISO 8653. EU size is the circumference directly. Diameter is derived as circumference divided by π. The US/Canada size uses a linear model anchored to published reference values (US 0 = 36.537 mm, each full size adds 2.5535 mm), solved for the nearest quarter step. The UK/Australia letter system uses a lookup table of circumference values derived from the Wikipedia ring-size comparison chart (A = 37.8 mm through Z = 69.5 mm, half sizes interpolated as midpoints). The Japanese system applies a linear offset: JP size = round(circumference − 39.84 mm), anchored so JP 1 = 40.84 mm and verified at JP 14 = 54 mm and JP 17 = 57 mm. ## Example - **Setup:** Enter an inner circumference of 57 mm (EU mode). - **Result:** EU 57, US 8, UK P, JP 17. The inner diameter is 18.1 mm. This is one of the most common ring sizes worldwide. ## Frequently asked questions ### What is EU ring size and why does it equal the circumference? EU ring size follows ISO 8653, which defines size as the inner circumference of the ring in millimetres. A ring marked EU 57 has an inner circumference of exactly 57 mm, so no formula is needed — the number is the measurement. ### How do I measure my ring size at home? Wrap a thin strip of paper snugly around the base of the finger you want to wear the ring on. Mark where the strip overlaps, then measure the length in millimetres — that is your inner circumference. Enter it here as EU or circumference to get all four systems. ### Why does my US size come out as a fraction like 6.75 or 8.25? US sizing uses quarter increments (for example 7, 7¼, 7½, 7¾) because finger circumferences do not fall on exact whole-number US sizes. The calculator rounds to the nearest quarter step, which is the precision jewelers actually stock. ### Are UK and Australian ring sizes the same? Yes. The UK and Australian letter system is identical — the same letter chart is used in both countries. New Zealand also follows this system. ### Why might a ring purchased abroad not fit even when the size matches? Ring width and shape affect perceived fit: a wide band feels tighter than a narrow one at the same size. Temperature, time of day, and dominant-hand swelling also change finger circumference slightly. Always try a ring on before purchasing or ask a jeweler to use a mandrel for a precise fitting. ## Related calculators - [Shoe Size Converter](https://youcalc.com/en/conversions-units/shoe-size-converter/) - [Unit Converter](https://youcalc.com/en/conversions-units/unit-converter/) ## Sources - https://www.iso.org/standard/70297.html - https://en.wikipedia.org/wiki/Ring_size --- Interactive version: https://youcalc.com/en/conversions-units/ring-size-converter/ · From YouCalc — https://youcalc.com