Ring size to finger width

US ring sizes are a simple scale: each whole size adds 0.8128 mm to the ring's inside diameter. That makes the conversion exact. Going from there to the width of a finger — what a diamond sits against — is where an approximation comes in.

A 1 ct round on fingers as wide as sizes 4, 6, 8 and 10 imply, to scale.

The chart

US size	Inside ⌀ (mm)	Circumference (mm)	Slender finger	Average finger	Broad finger
3	14.07	44.20	14.77	15.05	15.33
3.5	14.47	45.47	15.20	15.49	15.78
4	14.88	46.75	15.63	15.92	16.22
4.5	15.29	48.03	16.05	16.36	16.66
5	15.69	49.30	16.48	16.79	17.11
5.5	16.10	50.58	16.91	17.23	17.55
6	16.51	51.86	17.33	17.66	17.99
6.5	16.91	53.13	17.76	18.10	18.44
7	17.32	54.41	18.19	18.53	18.88
7.5	17.73	55.69	18.61	18.97	19.32
8	18.13	56.96	19.04	19.40	19.76
8.5	18.54	58.24	19.47	19.84	20.21
9	18.95	59.52	19.89	20.27	20.65
9.5	19.35	60.79	20.32	20.71	21.09
10	19.76	62.07	20.75	21.14	21.54
10.5	20.16	63.35	21.17	21.58	21.98
11	20.57	64.63	21.60	22.01	22.42
11.5	20.98	65.90	22.03	22.45	22.87
12	21.38	67.18	22.45	22.88	23.31
12.5	21.79	68.46	22.88	23.32	23.75
13	22.20	69.73	23.31	23.75	24.19

Why a finger is wider than the ring

A ring's inside circumference matches the finger's. But a finger's cross-section is an oval, not a circle — so its width is more than the diameter of a circle with the same circumference. Solving Ramanujan's approximation for an ellipse's perimeter gives roughly:

width : depth of 1.10 → factor 1.047
width : depth of 1.15 → factor 1.068
width : depth of 1.20 → factor 1.089

Real fingers fall in about that range, which is where these come from:

slender — ×1.05
average — ×1.07
broad — ×1.09

This factor is the largest single source of error in estimating size from a photograph, and it errs in a known direction: understating it makes every stone render larger than it really is. The tool sits mid-range on purpose, not at the flattering end.

To skip the assumption entirely, put a bank card beside your hand in the photo — they're a fixed 85.60 mm wide worldwide, which makes one an accurate ruler.

Measure your finger from a photo

Tell the tool your ring size, or put a bank card in the picture, and every diamond is drawn at its real size on your hand.

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