Ring size chart: US sizes in mm (inner diameter and circumference)

Short answer: a US size 7 ring has an inner diameter of about 17.32 mm and an inner circumference of about 54.4 mm. Each whole US size adds 0.8128 mm (0.032 in) of inner diameter.^{[1 · Oct 29, 2026]}

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US ring size chart

Diameter and circumference below are our own arithmetic from the US/Canada formula. The last column shows the rounded diameter printed on GIA's international chart so you can see how closely they match.^{[2 · Oct 29, 2026]}

US size	Inner diameter	Inner diameter (in)	Inner circumference	GIA chart diameter
3	14.07 mm	0.554 in	44.2 mm	14.1 mm
3.5	14.47 mm	0.570 in	45.5 mm	14.5 mm
4	14.88 mm	0.586 in	46.8 mm	14.9 mm
4.5	15.29 mm	0.602 in	48.0 mm	15.3 mm
5	15.69 mm	0.618 in	49.3 mm	15.7 mm
5.5	16.10 mm	0.634 in	50.6 mm	16.1 mm
6	16.51 mm	0.650 in	51.9 mm	16.5 mm
6.5	16.91 mm	0.666 in	53.1 mm	16.9 mm
7	17.32 mm	0.682 in	54.4 mm	17.3 mm
7.5	17.73 mm	0.698 in	55.7 mm	17.7 mm
8	18.13 mm	0.714 in	57.0 mm	18.1 mm
8.5	18.54 mm	0.730 in	58.2 mm	18.5 mm
9	18.95 mm	0.746 in	59.5 mm	19.0 mm
9.5	19.35 mm	0.762 in	60.8 mm	19.4 mm
10	19.76 mm	0.778 in	62.1 mm	19.8 mm
10.5	20.16 mm	0.794 in	63.3 mm	20.2 mm
11	20.57 mm	0.810 in	64.6 mm	20.6 mm
11.5	20.98 mm	0.826 in	65.9 mm	21.0 mm
12	21.38 mm	0.842 in	67.2 mm	21.4 mm
12.5	21.79 mm	0.858 in	68.5 mm	21.8 mm
13	22.20 mm	0.874 in	69.7 mm	22.2 mm

The math (our own arithmetic): inner diameter d = 11.63 mm + 0.8128 mm × US size (the US/Canada scale described on Wikipedia).^{[1 · Oct 29, 2026]} Circumference C = π × d. Example for size 7: d = 11.63 + 0.8128 × 7 = 17.32 mm; C = 3.1416 × 17.32 = 54.4 mm; in inches, 17.32 ÷ 25.4 = 0.682 in. Size 9 by formula is 18.95 mm; GIA's chart prints 19.0 mm, a difference of 0.05 mm from rounding.

Using the chart

Measure an existing ring that fits the same finger across the inside, edge to edge, in millimetres, then find the nearest diameter above.
If you fall between two sizes, both GIA and Jewelers Mutual suggest choosing the larger one.^{[2 3 · Oct 29, 2026]}
Wider bands fit more snugly; Brilliant Earth suggests at least a half size up for bands over 4 mm.^{[4 · Oct 29, 2026]}

Average ring size for men and women

Unverified. We couldn't confirm a reliable source for this yet.

Sources: 1. Wikipedia, Ring size. 2. GIA 4Cs, How to determine ring size. 3. Jewelers Mutual, How to measure ring size. 4. Brilliant Earth, How to measure ring size. Checked Oct 29, 2026.