How to use the Ring Size
- Wrap a strip of paper around the finger and mark where it overlaps.
- Measure the length in millimetres — this is the circumference.
- Enter it to get US, UK, EU and Japanese sizes.
- Measure at the end of the day when fingers are at their largest.
How the calculation works
US sizes start at size 3 for a 44.2 mm circumference and increase by about 0.8 mm per half size. UK sizes use letters from A upward on a similar scale, EU sizes are the circumference in millimetres minus 40, and Japanese sizes run on their own numeric scale. Inner diameter is circumference ÷ π, which is how printable ring sizers work when measured against an existing band.
Fingers change size by up to a full size across a day and a season: heat, salt, exercise and pregnancy all swell them, and cold shrinks them. Measure in the evening at normal temperature, and measure three times. Band width matters too — a wide band fits more tightly than a narrow one at the same nominal size, so add a quarter to a half size for anything over 6 mm wide. The knuckle must also clear, so the ring should pass it with mild resistance.
Diameter = circumference / π ; EU size = circumference mm − 40 ; US ≈ (circumference − 36.5) / 2.55 + 3Source: ISO 8653 ring sizing standard based on inner circumference in millimetres.
Worked example
A finger measuring 54.5 mm in circumference, for a 7 mm wide band.
- Inner diameter = 54.5 / π = 17.35 mm.
- EU size = 54.5 − 40 = 54.5, so EU 54–55.
- US size ≈ 7.
- Add half a size for the wide band: US 7.5.
About a US 7, going to 7.5 for the 7 mm band width.
Frequently asked questions
When should I measure?+
In the evening at normal room temperature, when fingers are at their largest.
Does band width change the size?+
Yes. Bands over about 6 mm fit tighter, so go up a quarter to half size.
Can rings be resized?+
Plain gold and silver bands usually can. Full eternity rings and tension settings often cannot.
How do I measure secretly for a gift?+
Borrow a ring she already wears on that finger and measure its inner diameter.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.