This ring size calculator converts a finger measurement — inside diameter or wrapped circumference in millimetres — into US, UK and EU ring sizes.
| US size | – |
| UK size (approx.) | – |
| EU size (= circumference) | – |
| Diameter ↔ circumference | – |
Measure at day’s end (fingers swell slightly), at room temperature, over the knuckle — the ring must pass the knuckle and rest at the base. Wide bands (6 mm+) fit snugger: size up a quarter to half. Between sizes, jewelers round up.
Size Lives in Millimetres; Countries Just Rename It
The US formula size ≈ (diameter − 11.65) ÷ 0.8128 pins every standard chart point (16.5 mm → 6.0, 19.8 mm → 10.0). Fit conversions for feet are our Shoe Size Converter. Part of our Everyday Life Calculators collection.
Anchor Sizes (Verified Against Standard Charts)
- 16.5 mm diameter = US 6 (defaults) — circumference 51.8 mm, EU 52, UK L½; the most common women's size neighborhood (US 6–7) and men's (9–10) bracket most gifts
- Each full US size adds ≈ 0.81 mm of diameter and ≈ 2.55 mm of circumference — a single millimetre of measuring error is a full size of regret
- Half sizes shift diameter by ≈ 0.4 mm — well within daily finger swelling, which is why fit is a range wearing a point estimate's clothes
Measuring Discipline
- String method: wrap snug (not tight), mark, measure flat — then subtract nothing; charts already assume a working fit.
- Existing-ring method: measure the INSIDE diameter across the center, in mm, twice at different angles (rings go slightly oval).
- Cold fingers shrink a quarter-size or more; hot afternoons swell — measure the finger you'll have most of the year.
- Surprise proposals: borrow a ring she wears on the ring finger (or the middle — adjust down ~1 size), or trace its inner circle on paper for the diameter.
Related Tools
- Shoe Size Converter — US/UK/EU feet
- Wedding Budget Calculator — the rings in the budget
- Everyday Life Calculators — the full collection
Frequently Asked Questions
How do I find my ring size from diameter?
US size ≈ (diameter in mm − 11.65) ÷ 0.8128, rounded to the nearest quarter. 16.5 mm → 6.0; 19.8 mm → 10.0. This calculator also gives the UK letter and the EU size (which is simply the circumference in mm).
How do I measure my finger for a ring?
Wrap a strip of paper or string around the finger's base, over the knuckle, snug but not tight; mark the overlap and measure the length in mm — that's the circumference (EU size directly). Measure in the evening at room temperature, and repeat on two days if a purchase depends on it.
What is the average ring size?
Women's rings cluster around US 6–7 (16.5–17.3 mm); men's around US 9–10 (19.0–19.8 mm). Engagement averages skew to the middle of those bands — useful priors for a surprise, useless as substitutes for one measurement.
Should I size up or down between sizes?
Up, almost always: a slightly loose ring can be sized down or wear a guard; a tight one is a circulation complaint. Wide bands (6 mm and broader) effectively fit a quarter-to-half size smaller — size those up accordingly.
Do UK and EU sizes convert exactly?
EU size = circumference in mm (exact by definition). UK letter sizes step in half-size increments that track US closely (US 6 ≈ UK L½, US 10 ≈ UK T½) — the calculator's table covers the standard range; vintage UK charts occasionally sit a half-step off, so jewelers confirm with a mandrel.
Is this ring size calculator free?
Yes — free, instant, no sign-up.