Free ring size converter. Convert ring sizes between US, UK, EU, and millimeter (diameter/circumference) systems instantly. Includes a full ring size chart for buying engagement rings and jewelry online. Accurate, instant, no sign-up.
US • UK • EU • diameter & circumference (mm)
Buying a ring as a surprise or from an overseas jeweler? Convert sizes confidently to get the perfect fit.
International jewelers list sizes in their local system. Convert US to UK letters or EU numbers before ordering.
Use diameter (mm) of a well-fitting ring as the most accurate basis, then convert to any sizing system.
Ring sizing differs significantly by country, which makes converting essential when shopping for engagement rings or jewelry online. The US and Canada use a numeric scale (typically 3 to 13.5, including half sizes), where each full size adds about 0.4 mm to the inside diameter. The UK, Ireland, and Australia use an alphabetic scale (A to Z+), so a US 7 is roughly a UK N½. The EU/ISO system is based directly on the inside circumference in millimeters — a US 7 has a circumference of about 54.4 mm, so its EU size is 54.
The most reliable basis for any conversion is the physical measurement: inside diameter or circumference in millimeters. A US size 7 corresponds to a 17.3 mm inside diameter (54.4 mm circumference). To measure at home, either measure the inside diameter of a ring that already fits well, or wrap a thin strip of paper around the base of your finger and measure its length (the circumference). Fingers swell in heat and shrink in cold, so measure at the end of the day at normal temperature, and size up slightly for wide bands, which fit more snugly.
US 5 = 15.7 mm. US 6 = 16.5 mm. US 7 = 17.3 mm. US 8 = 18.1 mm. US 9 = 19.0 mm. US 10 = 19.8 mm. Each size ≈ +0.4 mm.
US 5 = UK J½. US 6 = UK L½. US 7 = UK N½. US 8 = UK P½. US 9 = UK R½. US 10 = UK T½.
Measure inside diameter of a fitting ring in mm, or wrap paper around your finger for circumference. Convert from the mm value for best accuracy.
Measure at day's end (fingers swell). Avoid when cold. Size up for wide bands (4mm+). Knuckle larger than base? Size between the two.
Ring sizing systems all describe the same physical quantity — the inside circumference of the band — but express it differently. US sizes run numerically with half sizes, incrementing by about 0.8 mm of circumference each half size. UK and Australian sizing uses letters. European systems often quote the circumference in millimetres directly, which is the most transparent approach. Fingers change size measurably through the day and with temperature, swelling in heat and after salt, so measuring once is unreliable and evening measurements are the most representative.
Circumference = π × inside diameterUS size ≈ (circumference in mm − 36.5) ÷ 2.55Each full US size ≈ 2.55 mm of circumference (0.81 mm of diameter)EU size = circumference in mmwhere:
Assumptions: Fingers can vary by a full size between morning and evening and between summer and winter. Measure at the end of the day at normal room temperature, and take the reading three times.
Convert a measured diameter into circumference and then into sizes.
Result17.35 mm diameter = US 7 = EU 54.5 = UK N½
The band-width adjustment is the detail most often missed and the most common reason a correctly-sized ring feels tight. A wide band contacts more of the finger and cannot pass the knuckle as easily, which is why wedding bands are often ordered a quarter size larger than an engagement ring for the same finger.