Ring Size in mm

A ring size in mm is its inner diameter (across the inside of the band) or its inner circumference (around it); the two are linked by π. US 7 is 17.32 mm across and 54.4 mm around. Type a size on any scale for its millimetres, or read the tables below.

In millimetres

17.32 mm

inner diameter of US 7; circumference 54.4 mm

US 7 is an inner circumference of 54.4 mm and an inner diameter of 17.32 mm. Nearest sizes: US 7, UK N½, EU 54.5, Japan 14; the brackets show how far each is from exact.

US / Canada 7 (exact)
UK / Australia N½ (exact)
EU / ISO 54.5 (exact)
Japan 14 (exact)
Inner diameter 17.32 mm (0.682 in)
Inner circumference 54.4 mm (2.14 in)

US	Diameter mm	Circumference mm	UK	EU / ISO	Japan
3	14.07	44.2	F½	44	4
3½	14.47	45.5	G½	45.5	5
4	14.88	46.8	H½	47	7
4½	15.29	48	I½	48	8
5	15.69	49.3	J½	49.5	9
5½	16.1	50.6	K½	50.5	10
6	16.51	51.9	L½	52	12
6½	16.91	53.1	M½	53	13
7 ◀	17.32	54.4	N½	54.5	14
7½	17.73	55.7	O½	55.5	15
8	18.13	57	P½	57	16
8½	18.54	58.2	Q½	58	18
9	18.95	59.5	R½	59.5	19
9½	19.35	60.8	S½	61	20
10	19.76	62.1	T½	62	21
10½	20.16	63.3	U½	63.5	22
11	20.57	64.6	V½	64.5	24
11½	20.98	65.9	W½	66	25
12	21.38	67.2	X½	67	26
12½	21.79	68.5	Z	68.5	27
13	22.2	69.7	Z+1	69.5	29
13½	22.6	71	Z+2	71	30
14	23.01	72.3	Z+3	72.5	31
14½	23.42	73.6	Z+4	73.5	32
15	23.82	74.8	Z+5	75	33

How are the millimetres worked out?

US: diameter = 11.63 + 0.8128 × size. UK: circumference = 37.5 + 1.25 × letter (A = 0). EU / ISO: the size is the circumference. Japan: diameter = 13 + (size − 1) ÷ 3. Always: circumference = π × diameter.

Worked example. US 7: 11.63 + 0.8128 × 7 = 17.32 mm; × π = 54.4 mm. UK N (letter 13): 37.5 + 1.25 × 13 = 53.75 mm around, ÷ π = 17.11 mm across. Japan 14: 13 + 13 ÷ 3 = 17.33 mm across.

US sizes in mm

US 3 to 15, inner diameter and circumference
US	Diameter mm	Diameter in	Around mm
3	14.07	0.554	44.2
3½	14.47	0.57	45.5
4	14.88	0.586	46.8
4½	15.29	0.602	48
5	15.69	0.618	49.3
5½	16.1	0.634	50.6
6	16.51	0.65	51.9
6½	16.91	0.666	53.1
7	17.32	0.682	54.4
7½	17.73	0.698	55.7
8	18.13	0.714	57
8½	18.54	0.73	58.2
9	18.95	0.746	59.5
9½	19.35	0.762	60.8
10	19.76	0.778	62.1
10½	20.16	0.794	63.3
11	20.57	0.81	64.6
11½	20.98	0.826	65.9
12	21.38	0.842	67.2
12½	21.79	0.858	68.5
13	22.2	0.874	69.7
13½	22.6	0.89	71
14	23.01	0.906	72.3
14½	23.42	0.922	73.6
15	23.82	0.938	74.8

UK letters in mm

UK A to Z, whole letters (half sizes add 0.625 mm)
UK	Around mm	Diameter mm
A	37.5	11.94
B	38.75	12.33
C	40	12.73
D	41.25	13.13
E	42.5	13.53
F	43.75	13.93
G	45	14.32
H	46.25	14.72
I	47.5	15.12
J	48.75	15.52
K	50	15.92
L	51.25	16.31
M	52.5	16.71
N	53.75	17.11
O	55	17.51
P	56.25	17.9
Q	57.5	18.3
R	58.75	18.7
S	60	19.1
T	61.25	19.5
U	62.5	19.89
V	63.75	20.29
W	65	20.69
X	66.25	21.09
Y	67.5	21.49
Z	68.75	21.88

Japanese sizes in mm

Japan 1 to 30
Japan	Diameter mm	Around mm
1	13	40.8
2	13.33	41.9
3	13.67	42.9
4	14	44
5	14.33	45
6	14.67	46.1
7	15	47.1
8	15.33	48.2
9	15.67	49.2
10	16	50.3
11	16.33	51.3
12	16.67	52.4
13	17	53.4
14	17.33	54.5
15	17.67	55.5
16	18	56.5
17	18.33	57.6
18	18.67	58.6
19	19	59.7
20	19.33	60.7
21	19.67	61.8
22	20	62.8
23	20.33	63.9
24	20.67	64.9
25	21	66
26	21.33	67
27	21.67	68.1
28	22	69.1
29	22.33	70.2
30	22.67	71.2

Questions about sizes in mm

What is a size 7 ring in mm?

US 7 is 17.32 mm inner diameter and 54.4 mm inner circumference. UK N, the nearest letter, is 53.75 mm around.

Is a ring measured by diameter or circumference?

Both are used. US and Japanese scales are defined by inner diameter; UK and EU/ISO scales by inner circumference. The two are linked by π: circumference = 3.1416 × diameter, so 17 mm across is 53.4 mm around.

How do I measure a ring in mm?

Lay the ring on a ruler and read the widest distance between the inside edges of the band, or use callipers. Read to a quarter of a millimetre if you can: one US quarter size is 0.2 mm of diameter.

What does a jeweller mean by “size 54”?

An EU/ISO size: the ring is 54 mm around inside, 17.19 mm across, close to US 7. Sizes quoted as two-digit numbers in the 40s to 70s are almost always circumferences.

Why does the UK table step by 1.25 mm?

Because the British standard of 1987 fixed each letter at 1.25 mm of circumference, with size C at 40 mm. Before that the letters were spaced by diameter in sixty-fourths of an inch, which is why old and new UK charts differ slightly.