Ring sizes

21 sizes from US 3 to 13, with the UK letter, the ISO number, and the measurement each of them actually is.

The European size is the measurement

This is the fact that makes every chart unnecessary. An ISO or European ring size is the inside circumference in millimetres. A size 54 ring is 54 mm around the inside. So if you have a ring that fits:

measure the inside diameter in mm
× 3.1416
= the European size

The American scale is a straight line through the same quantity — circumference = 36.537 + 2.5535 × size — and the British scale advances one letter per half American size, with US 6 at L½. Between them those two rules reproduce every conversion table in print.

The full chart

US / Canada	UK / Australia	ISO / EU	Circumference	Diameter
3	F½	44	44.2 mm	14.07 mm
3½	G½	45	45.5 mm	14.47 mm
4	H½	47	46.8 mm	14.88 mm
4½	I½	48	48.0 mm	15.29 mm
5	J½	49	49.3 mm	15.69 mm
5½	K½	51	50.6 mm	16.10 mm
6	L½	52	51.9 mm	16.51 mm
6½	M½	53	53.1 mm	16.91 mm
7	N½	54	54.4 mm	17.32 mm
7½	O½	56	55.7 mm	17.73 mm
8	P½	57	57.0 mm	18.13 mm
8½	Q½	58	58.2 mm	18.54 mm
9	R½	60	59.5 mm	18.95 mm
9½	S½	61	60.8 mm	19.35 mm
10	T½	62	62.1 mm	19.76 mm
10½	U½	63	63.3 mm	20.16 mm
11	V½	65	64.6 mm	20.57 mm
11½	W½	66	65.9 mm	20.98 mm
12	X½	67	67.2 mm	21.38 mm
12½	Y½	68	68.5 mm	21.79 mm
13	Z½	70	69.7 mm	22.20 mm

Measuring, and why it goes wrong

Three things defeat a correctly taken measurement, and all of them are about the finger rather than the ring.

Time of day. Fingers swell. The difference between a cold morning and a warm evening is often more than a full size — larger than the half-size steps the whole scale is built on. Measure at the end of the day, at normal room temperature, and never straight after exercise or a hot bath.

The knuckle. A ring has to pass the knuckle and then sit on a narrower part of the finger. Where the knuckle is much larger, the ring that fits over it will spin once it is on, and the answer is a fitted bar inside the band rather than a smaller size.

Band width. A wide band touches more of the finger and reads as tighter. Anything over about 6 mm is usually ordered a quarter to a half size up. This is the most common reason a ring measured correctly still does not fit.

Sizing a ring you already own

Lay it on a ruler and measure the inside diameter at its widest — the number you want is across the hole, not across the metal. Multiply by π for the circumference, which is the European size, then read the American and British equivalents off the chart above. This is more reliable than printable paper sizers, which depend on the printer scaling the page exactly.

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