Ring Size Calculator
Measure a ring you own and convert its inside diameter or circumference in millimetres to an estimated US ring size using a documented linear approximation.
How to find your ring size
- Measure the inside of a well-fitting ring across the middle (diameter) or around the inside (circumference) in millimetres.
- Pick which measurement you have and enter it.
- Read the estimated US size, then confirm with a jeweller before buying.
Examples
From inside diameter
16.5 mm inside diameter
about US size 6 (51.84 mm circumference)
From circumference
54.5 mm inside circumference
about US size 7 (17.35 mm diameter)
Frequently asked questions
How accurate is this ring size estimate?
It uses a documented linear model where each US size adds about 0.8128 mm of inside diameter. Real sizing uses official mandrel tables, so treat the result as a close estimate.
Should I measure diameter or circumference?
Either works. Diameter is measured straight across the inside of a ring, circumference is measured around the inside. The tool converts between them using pi.
How do I measure a ring I already own?
Lay the ring flat and measure the inside edge to inside edge for diameter, or wrap a thin strip of paper around the inside and measure its length for circumference.
What inside diameter is a US size 7?
About 17.35 mm, with a circumference near 54.5 mm. Sizes step by roughly 0.8 mm of diameter, so a half size is about 0.4 mm.
Why does my finger size change during the day?
Fingers swell with heat, salt and activity, so measure at the end of the day when warm for a fit that will not feel tight later.
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