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Find your ring size

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Ring size converter

Look up the published crosswalk between UK, US, ISO, Asian and Swiss sizes. If you are starting from a ring rather than a size, the size finder measures its inside diameter first.

The sourced international conversion chart

GIA's published rows run from US 3 to 13.5 and include UK half sizes. ISO size is the nearest whole-millimetre inner circumference. A blank cell means the source gives no equivalent for that row. Each whole UK size links to its own page.

UKUSISO / EUAsiaSwissCircumferenceDiameter
F 3 44 4 4 44.3 mm 14.1 mm
F½ 3.25 45 5 5.25 44.9 mm 14.3 mm
G 3.5 46 45.6 mm 14.5 mm
G½ 3.75 46 6 6.5 46.2 mm 14.7 mm
H 4 47 7 46.8 mm 14.9 mm
H½ 4.25 47 7.75 47.4 mm 15.1 mm
I 4.5 48 8 48.1 mm 15.3 mm
I½ 4.75 49 9 48.7 mm 15.5 mm
J 5 49 9 49.3 mm 15.7 mm
J½ 5.25 50 10 50.0 mm 15.9 mm
K 5.5 51 10 50.6 mm 16.1 mm
K½ 5.75 51 11.75 51.2 mm 16.3 mm
L 6 52 11 12.75 51.8 mm 16.5 mm
L½ 6.25 53 12 52.5 mm 16.7 mm
M 6.5 53 13 14 53.1 mm 16.9 mm
M½ 6.75 54 53.7 mm 17.1 mm
N 7 54 14 15.25 54.3 mm 17.3 mm
N½ 7.25 55 55.0 mm 17.5 mm
O 7.5 56 15 16.5 55.6 mm 17.7 mm
O½ 7.75 56 56.2 mm 17.9 mm
P 8 57 16 17.75 56.9 mm 18.1 mm
P½ 8.25 58 57.5 mm 18.3 mm
Q 8.5 58 17 58.1 mm 18.5 mm
Q½ 8.75 59 19 59.1 mm 18.8 mm
R 9 60 18 59.7 mm 19.0 mm
R½ 9.25 60 20.25 60.3 mm 19.2 mm
S 9.5 61 19 60.9 mm 19.4 mm
S½ 9.75 62 21.5 61.6 mm 19.6 mm
T 10 62 20 62.2 mm 19.8 mm
T½ 10.25 63 21 62.8 mm 20.0 mm
U 10.5 64 22 22.75 63.5 mm 20.2 mm
U½ 10.75 64 64.1 mm 20.4 mm
V 11 65 23 64.7 mm 20.6 mm
V½ 11.25 65 25 65.3 mm 20.8 mm
W 11.5 66 24 66.0 mm 21.0 mm
W½ 11.75 67 66.6 mm 21.2 mm
X 12 67 25 27.5 67.2 mm 21.4 mm
X½ 12.25 68 67.9 mm 21.6 mm
Y 12.5 69 26 28.75 68.5 mm 21.8 mm
Z 12.75 69 69.1 mm 22.0 mm
Z½ 13 70 27 69.7 mm 22.2 mm
13.25 70 70.4 mm 22.4 mm
Z1 13.5 71 71.0 mm 22.6 mm

Conversion questions

What is a UK size N in US sizes?
A UK N is a US 7. The UK uses letters on the Wheatsheaf scale and the US uses numbers, and they do not line up neatly: roughly two UK letters to one US size, which is why converting by eye goes wrong.
How do European and ISO ring sizes work?
ISO 8653 defines ring size as the inner circumference in millimetres. A 54 designation therefore means 54mm around the inside. This converter rounds the measured GIA chart row to the nearest whole-millimetre ISO size.
Are international ring-size conversions exact?
No. Regional scales are separate measuring systems, so a conversion chart is a nearest crosswalk rather than a mathematical identity. Keep the circumference and diameter with the converted label, and use a jeweller's matching ring gauges for a final order.
Why are some Swiss and Asian conversions blank?
Because the published GIA chart does not assign a regional label to every US quarter size or UK half size. A blank is more honest than inventing a number between two listed sizes.
Can I convert a ring size from a diameter measurement?
Yes, provided the ring is round. Multiply the inside diameter in millimetres by pi to get the circumference, then find the nearest row. A 17.3mm diameter is 54.3mm around, which the GIA chart maps to UK N and US 7.