About

Ordering a ring without accurate sizing risks a costly return or, worse, a lost piece during wear. This converter maps a measured finger circumference C (in cm) to standardized ring sizes across seven international scales: US/Canada, UK/Australia, EU, Japan, Swiss, India, and China. The underlying relationship is geometric: C = π × d, where d is the inner diameter. Between tabulated sizes the tool applies linear interpolation, reporting the nearest half-size rather than rounding blindly. Measurements should be taken at the widest knuckle point, at room temperature, because fingers swell by up to 0.5mm in heat.

Pro tip: measure three times at different times of day and average the results. Note that wide-band rings (above 6mm width) typically require a half-size increase. This tool assumes a standard comfort-fit band.

Formulas

The core geometric relationship between finger circumference and diameter:

C = π × d

Where C = inner circumference of the ring in mm, d = inner diameter in mm, and π ≈ 3.14159.

When the input circumference falls between two tabulated sizes, linear interpolation determines the nearest match:

size = s₁ + C − C₁C₂ − C₁ × (s₂ − s₁)

Where s₁ and s₂ are the bounding size values, and C₁ and C₂ are their corresponding circumferences. The result is then snapped to the nearest 0.5 increment for US sizing.

Reference Data

Circumference (mm)	Diameter (mm)	US / CA	UK / AU	EU	Japan	Swiss	India	China
44.2	14.07	3	F	44	4	4	3	4
45.5	14.48	3.5	G½	46	5	5.5	4	5
46.8	14.88	4	H½	47	7	7	5	7
48.0	15.29	4.5	I½	48	8	8	6	8
49.3	15.70	5	J½	49	9	9.5	7	9
50.6	16.10	5.5	K½	51	10	11	8	10
51.9	16.51	6	L½	52	12	12	9	12
53.1	16.92	6.5	M½	53	13	13.5	10	13
54.4	17.32	7	N½	54	14	14.5	11	14
55.7	17.73	7.5	O½	56	15	16	12	15
57.0	18.14	8	P½	57	16	17	13	16
58.3	18.54	8.5	Q½	58	17	18.5	14	17
59.5	18.95	9	R½	60	18	19.5	15	18
60.8	19.35	9.5	S½	61	19	21	16	19
62.1	19.76	10	T½	62	20	22	17	20
63.4	20.17	10.5	U½	63	22	23.5	18	21
64.6	20.57	11	V½	65	23	25	19	22
65.9	20.98	11.5	W½	66	24	26	20	23
67.2	21.39	12	Y	67	25	27.5	21	24
68.5	21.79	12.5	Z	69	26	29	22	25
69.7	22.20	13	Z+1	70	27	30	23	26

Frequently Asked Questions

Fingers expand in warm conditions and contract in cold. A measurement taken in a heated room can differ by up to 0.5 mm in circumference from one taken outdoors in winter. Always measure at a neutral room temperature (approximately 20 °C) for the most accurate result. If your hands feel cold or swollen, wait until they normalize.

Yes. Bands wider than 6 mm create more friction against the skin surface, making the ring feel tighter. Jewelers recommend going up by half a US size (roughly 0.6 mm in circumference) for bands between 6-8 mm, and a full size up for bands exceeding 8 mm.

The US system uses a numeric scale with 0.5 increments based on a linear diameter progression starting at size 0 = 11.63 mm. The UK system uses alphabetic letters (A - Z and beyond) mapped to slightly different circumference breakpoints defined by the British Standard BS 6820. These two standards were developed independently, so identical circumference values map to different labels.

The EU (European) ring size is simply the inner circumference of the ring in millimeters, rounded to the nearest whole number. A ring with a 52.3 mm circumference is sold as EU size 52. This makes it the most straightforward system but lacks the half-size granularity of the US scale.

This tool supports both. When you enter a diameter, it is multiplied by π (≈ 3.14159) to produce the circumference, which is then matched against the size chart. The conversion is exact assuming a perfectly circular cross-section. Real rings have slight manufacturing tolerances of ±0.2 mm.

A wrapped string method introduces ±1 mm of error due to overlap, stretch, and marker imprecision. For critical purchases (engagement rings, expensive bands), use a jeweler's ring mandrel or a set of sizing rings. This tool assumes you provide an accurate circumference. A 1 mm error can shift the result by half a US size.