Ring size chart

US 7 is 17.32 mm of inner diameter and 54.42 mm around. That is UK N½, EU/ISO 54, Japan 14 and India 14. The diameter is the figure to give a jeweller, because every one of those national sizes is derived from it.

No, and they are not even the same kind of scale. US sizes are numbers on a scale with no governing standard; UK sizes are letters defined by BS EN 28653, one letter per 1.25 mm of inner circumference. US 6 is UK L½, and US 7 is UK N½. Because the two scales step by different amounts, half of all US sizes land between two UK letters.

UK N½. Some charts round that to N instead. Our value is computed from the standard: 54.42 mm of circumference, minus the 37.5 mm the scale starts at, divided by 1.25 mm per letter.

17.32 mm across the inside. Around the inside it is 54.42 mm — the number a paper strip measures, and the one EU/ISO sizes are literally equal to.

Nothing wearable. 10 mm of inner diameter is well below US 0, which starts at 11.63 mm, so no adult ring scale defines it. If you measured 10 mm you almost certainly measured across the wrong axis, or measured a child's ring. If 10 mm was a circumference rather than a diameter, that is smaller still — about 3.2 mm across.

Measure the inner diameter of a ring that fits: lay it on the calibrated circle in our sizer and match the circle to the inside edge, or measure it with a ruler across the widest point of the hole. If you are measuring a finger rather than a ring, you get circumference instead — divide by π (3.1416) for diameter, or let the tool switch for you.

Japanese size 14. Japan uses JIS S 4700, which runs from 1 to 35 and steps by a third of a millimetre of diameter — so Japanese sizes are roughly three times as fine-grained as US sizes, and land closer to your true measurement.

Indian size 14. State plainly, though: India has no ring size standard. The figure comes from the scale four large Indian jewellers agree on — inner circumference in millimetres minus 40 — which is the same arithmetic France, Italy, Spain and Brazil use. Check against your jeweller's own chart before buying.

By a number that tracks inner circumference: Indian size = circumference in millimetres − 40. US 6 is 51.87 mm around, so Indian 12. There is no Indian standards body defining it, which is why Indian charts disagree with each other more than any other country's.

Multiply by 25.4 to get millimetres, then read the size off that. 0.68 in of diameter is 17.32 mm, which is US 7. One warning: a quarter US size is about 0.008 in, so inches written to two decimal places cannot express one — measure in millimetres if you have the choice.