Ring size converter: US, UK, EU and millimetres

Convert a ring size

I know my size in

USUKEUmm

US size (numeric size)

US

6

UK

L

EU(circumference)

51.9

Diameter(inside)

16.50 mm

Full ring size chart

US sizes follow d = 11.63 mm + 0.8128 mm per size. EU (ISO) is the inner circumference. UK letters run from A at US ½, half a US size per letter.

Ring sizes in US, UK and EU with the matching inner diameter and circumference in millimetres

US	Diameter mm	UK	EU
3	14.07	F	44.2
3½	14.47	G	45.5
4	14.88	H	46.7
4½	15.29	I	48.0
5	15.69	J	49.3
5½	16.10	K	50.6
6	16.50	L	51.9
6½	16.91	M	53.1
7	17.32	N	54.4
7½	17.72	O	55.7
8	18.13	P	57.0
8½	18.54	Q	58.2
9	18.94	R	59.5
9½	19.35	S	60.8
10	19.76	T	62.1
10½	20.16	U	63.3
11	20.57	V	64.6
11½	20.98	W	65.9
12	21.38	X	67.2
12½	21.79	Y	68.4
13	22.19	Z	69.7
13½	22.60	Z	71.0

How the three systems relate

Each system describes the same thing, the inside of the band, in a different unit. US sizes are a linear scale on the inner diameter: 11.63 mm at size 0 and 0.8128 mm more for every size up. EU (ISO) sizes skip the scale entirely and give the inner circumference in millimetres, so an EU size is the diameter times π. UK sizes are letters, starting at A for about a US ½ and stepping roughly half a US size per letter.

That is why a US and an EU number never look alike, and why converting between them always goes through the diameter rather than from one label straight to the other.

Diameter or circumference

If you measure a ring you already own, measure across the inside, edge to edge, at the widest point. That is the diameter, and it maps straight onto US and millimetre sizes. If you wrap a strip of paper around a finger instead, the length of the strip is the circumference, which is the EU size directly.

When the number is not enough

A size is a starting point, not a fit. A wide band sits tighter than a narrow one at the same size, and fingers change through the day. Trying the ring on, even virtually, catches what a chart cannot.