Ring Size Converter

How to use Ring Size Converter

1. Select a size you know in any scale.
2. Read the equivalents and the millimetre measurements.
3. Check against a ring that already fits by measuring its inside diameter.
4. EU/ISO size is the inside circumference in millimetres — the most directly useful number.
5. Measure in the evening at room temperature; fingers vary by half a size through the day.
6. Wide bands usually need half a size up; comfort-fit bands often half a size down.

Size is inside circumference, and the EU scale says so directly

A ring’s size is defined by the inside circumference of the band. The EU/ISO scale simply states that circumference in millimetres, which is why it is the only one that is self-explanatory: EU 54 is a 54 mm inside circumference. US sizes are a numeric scale with half sizes, UK sizes use letters with halves. Diameter and circumference relate by π, so if you have measured one you have the other.

Measuring accurately is harder than it sounds

The reliable method is to measure an existing ring that fits: lay it on a ruler and read the inside diameter across the widest point. A paper strip around the finger works but consistently reads large, because paper does not have to pass over a knuckle. If the knuckle is much wider than the base of the finger — which is common — the ring must fit over the knuckle and will then be loose at the base, which is a fit problem no measurement solves.

Fingers change size, by more than a size

Warmth makes fingers swell and cold shrinks them, so the same finger can vary by half a size or more across a day, and more across seasons. Salt, alcohol, exercise, pregnancy and time of day all move it. Measure in the evening at normal room temperature, which is the middle of the range. Do not size a ring after a run or on a cold morning, which are the two extremes.

Band width changes the size you need

A wide band sits against more of the finger and feels tighter than a narrow one of the same measurement. The usual allowance is half a size up for bands over about 6 mm. Comfort-fit bands, which are domed on the inside, run the other way and often need half a size down. This is why the same nominal size from two jewellers can fit differently, and why resizing is common even when the measurement was right.