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Ring Size Converter

Converters · Added

Every ring sizing system is a different label for one measurement: the distance around the inside of the band. Each has a published definition, so this computes rather than looking up — and it shows the circumference behind whatever size you entered.

Numeric, in quarter and half sizes

A wide band fits tighter at the same nominal size, so it needs a larger one.

How to use the ring size converter

  1. 1Pick the system you know the size in, or measure the inside of a ring that fits.
  2. 2Enter the size — UK sizes are letters, so N or N½.
  3. 3Read the equivalent in every other scale, and the circumference underneath them all.
  4. 4Add the band width if the ring is wide; a wide band needs a larger size.

Examples

A US size

Input
US 7
Result
54.4 mm circumference · UK N½ · EU 54 · JP 14

From a measurement

Input
17.3 mm inner diameter
Result
54.3 mm circumference · US 7 · UK N½ · EU 54

About the ring size converter

One measurement, four labels

Every scale here encodes the inner circumference, and the differences are only in where each starts counting and how big a step it takes. The US scale begins at a circumference of 1.4325 inches and adds 0.1015 inch per size. The UK scale steps half a US size per letter, with half sizes between those. Japan uses its own numeric scale keyed to diameter. Europe skips the encoding and states the millimetres.

Because they are all functions of the same quantity, converting between any two is exact — which is not the same as saying a size that fits in one country will fit in another, since that depends on the ring rather than the number.

Measuring an existing ring, and its trap

Measuring the inside of a ring that already fits is the most reliable method available at home: measure the inside diameter across the widest point, or wrap a strip of paper inside the band and measure its length. Both are more repeatable than measuring a finger.

The trap is that this measures the ring, not the finger. If the ring being measured is a loose one worn on a different finger, or was itself bought slightly wrong, the error carries straight through. Where the ring matters — an engagement ring, a wedding band — a jeweller's sizing set is worth the trip, because it tests the fit rather than inferring it.

Frequently asked questions

Why is the EU size just the circumference?
Because that is what ISO 8653 defines it as. A size 54 ring is 54 mm around the inside, which makes the European scale the only one that states the measurement rather than encoding it. The US and UK scales count fixed steps up from an origin, so converting either one requires going through a length.
Where does the UK letter scale come from?
It runs at exactly two letter positions to one US size, which is the relationship every published conversion chart uses: US 5 is J½, US 6 is L½, US 7 is N½, US 8 is P½. Because the US scale has an exact definition in inches, the whole letter scale can be derived from it rather than fitted separately — and deriving it reproduces the chart values exactly instead of drifting by half a size across the range.
Does a wide band need a different size?
Usually yes, and the reason is that a wider band grips more of the finger and so fits tighter at the same nominal size. The trade rule of thumb is a quarter size up for a band around 6 mm and a half size up above 8 mm. Below about 4 mm no allowance is made.
When should I measure?
At the end of a normal day, at a normal temperature. Fingers are smallest in the morning and in the cold and can vary by a full size between a winter morning and a summer evening. Measure the knuckle as well as the base of the finger: the ring has to pass the knuckle and then sit, so the larger measurement is the one that governs — while still being small enough that the ring does not turn.

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