Ring Size Converter
Convert ring sizes across US, UK, EU/ISO, Japan, plus inner circumference and diameter in millimetres. Anchored to circumference — the measurement that doesn't change between countries.
Convert any ring size
How to measure & use it
Measure an existing ring
Take a ring that fits the right finger and measure its inner diameter across the middle in mm. Enter it under “Diameter”.
Or wrap a strip
Wrap a thin strip of paper around the finger, mark where it overlaps, and measure the length in mm. That's your circumference — or print our ring sizer.
Read every system
The converter shows US, UK, EU, JP, circumference and diameter together. EU equals the circumference, so it's the safest cross-check.
Between sizes? Size up
If you land between two, choose the larger — a slightly loose ring can be resized down more easily than a tight one can be stretched.
Reusable finger-gauge set to measure your true size at home before ordering.
View on Amazon →Measure an existing ring's inner diameter in mm to the tenth for an exact match.
View on Amazon →Invisible silicone inserts that take up slack when a ring sits a touch loose.
View on Amazon →How these conversions are calculated
Ring sizes are anchored to inner circumference in millimetres. The EU/ISO size equals that circumference (ISO 8653); US sizes follow the standard inner-diameter formula (0.032in per size); UK letters and Japanese numbers follow cross-checked international charts. UK half-letter conventions vary slightly between jewellers, so treat UK letters as the closest equivalent and confirm with a physical sizer.
Sources: ISO 8653:2016 — EU ring size = inner circumference (mm); US/Canada standard — inner diameter, 0.032in per size from 0.458in base; Cross-checked international ring charts (ringchart.co.uk, antoanetta.com, miniwebtool, 2026).
Frequently asked questions
How do I measure my ring size at home?
Measure the inner diameter of a ring that already fits across the middle in millimetres, or wrap a strip of paper around your finger and measure the length (that's the circumference). You can also print our ring sizer.
What is the EU/ISO ring size?
The EU/ISO ring size is simply the inner circumference in millimetres — an EU 54 measures 54 mm around the inside. That makes it the most reliable cross-reference between countries.
What is a US 7 in UK ring size?
A US 7 is approximately a UK N½, with an inner circumference around 54 mm. UK half-letter conventions vary slightly between jewellers, so confirm with a physical sizer.
Should I size up or down if I'm between ring sizes?
Size up. A slightly loose ring can be resized down or fitted with an insert, whereas a tight ring is harder and riskier to stretch. Also measure at the end of the day when fingers are largest.