Ring size 10
A US 10 ring has an inner diameter of 19.76 mm and an inner circumference of 62.1 mm. Below it is the same circle as every other scale prints it, and the sizes either side of it.
Ring size 10 in every scale
- RU
- 20
- ISO
- 62
- US
- 10
- UK
- T½
- JP
- 21
The scales do not share a step, so some land on a slightly different circle than this page's 19.76 mm: RU 20 → 20, JP 21 → 19.67. Where two sizes are that close, size up for a wide band.
What 10 measures
Size 10 is a circle 19.76 mm across — 62.1 mm around, or 0.78 inches across. A thread wrapped around the finger measures the circumference; a ring you already own is measured across the inside.
What no diameter can tell you is whether the ring will pass the knuckle. A finger is not a cylinder, and the widest circle a ring has to cross is not the circle it will sit on. If your knuckle is much wider than the base of the finger, buy for the knuckle and have the ring sized down.
One step away
9½, 10½ — the sizes either side of 10 on the same scale.
Where these numbers come from
Every ring scale is a straight line through the inner diameter, so nothing here is looked up in a table of equivalents — each label is computed from its own scale's definition. How the five scales are anchored, one by one, is on the ring size page.
Sources
Two of the five scales are definitions rather than conventions: ISO 8653 defines the size as the inner circumference in millimetres, and the Russian size is the inner diameter in millimetres. The other three are straight lines pinned to published anchor points.
- iso.org ISO 8653:2016 "Jewellery — Ring-sizes — Definition, measurement and designation": defines ring size as the inner circumference in millimetres
- en.wikipedia.org a summary of the scales: the US step of 0.032 in, anchor size 6 = 0.650 in, the British letter as half a US step, the Japanese scale from 13 mm in ⅓ mm steps
- gia.edu GIA correspondence table: US 6 = 16.51 mm = ISO 52, US 7 = 17.32 mm, US 8 = 18.14 mm
Checked: no date on record