Units

Ring size in inches

Inches are the unit most US rulers and tapes show first — and the worst unit for ring sizes, because the fractions people actually use are far too coarse. Convert to millimetres first, and keep the decimals.

The conversion — and why fractions fail

Multiply by 25.4, keep three decimals

1 inch = 25.4 mm exactly. A diameter of 0.682 inches is 17.32 mm (US 7); a circumference of 2.142 inches is 54.4 mm (also US 7). Three decimal places in inches is about 0.025 mm of precision — more than enough. Two decimals (0.68 in) is still fine: 17.27 mm, within a tenth of a millimetre of the true row.

Never round to the nearest eighth

An eighth of an inch is 3.18 mm. A half-size step is only 0.41 mm of diameter and 1.3 mm of circumference, so rounding to ⅛ in can move the answer by two full sizes. Even a sixteenth (1.59 mm) is more than a half-size of diameter. If your ruler only shows eighths, it is not a ring-measuring ruler — use the millimetre side, or the printable guide.

The same diameter-vs-circumference test, in inches: a number under 1 inch is a diameter; a number between 1.7 and 2.8 inches is a circumference.

The full table, in inches

Ring sizes with inner diameter and circumference in inches
US / Canada	Inner diameter (in)	Inner circumference (in)	EU / ISO
3	0.554	1.740	44
3.5	0.570	1.791	46
4	0.586	1.839	47
4.5	0.602	1.890	48
5	0.618	1.941	49
5.5	0.634	1.992	51
6	0.650	2.043	52
6.5	0.666	2.094	53
7	0.682	2.142	54
7.5	0.698	2.193	56
8	0.714	2.244	57
8.5	0.730	2.291	58
9	0.746	2.343	60
9.5	0.762	2.394	61
10	0.778	2.445	62
10.5	0.794	2.496	63
11	0.810	2.543	65
11.5	0.826	2.594	66
12	0.842	2.646	67
12.5	0.858	2.697	69
13	0.874	2.744	70
13.5	0.890	2.795	71

Worked examples

Ruler reads 0.682 in across a ring. That is 17.32 mm of diameter: US 7, EU 54.
Tape reads 2.142 in around a finger. That is 54.4 mm of circumference: US 7, EU 54 — the same size, reached from the other number.
A tape reads “2⅛ in” around a finger. 2.125 × 25.4 = 53.98 mm: between US 6 (51.9 mm) and US 7 (54.4 mm), closer to US 7, and flagged between sizes — which is honest, because a fraction reading cannot resolve a half-size on its own.

Convert an inch measurement

Your measurement

Decimal numbers are welcome. Use a dot for decimals, for example 16.5.

What you measured

Circumference

Unit

Changing the unit reinterprets the number you typed in the new unit — it does not convert it. Re-enter the number if you measured in a different unit.

Note: Tension: snug, not tight — the strip should slide over the knuckle with slight resistance, and should not dent the finger. Knuckle: if your knuckle is wider than the finger base, size for the knuckle, or the ring will not go on. Measure in the evening, with warm hands, and repeat twice, taking the largest reading.

Try an example

Ring band width, mm (optional — for width guidance only)

Highlight my country's size

Enter a measurement to see your size.

Formulas: circumference = π × diameter; diameter = circumference ÷ π. Matching uses your full, unrounded value; displayed diameter is rounded to 2 decimal places and circumference to 1 decimal place.

For a diameter reading in inches, use the diameter calculator instead.

Related guides

Ring size in millimetres
The same table in mm, the unit the systems are built from.
Ring size in centimetres
When your reading is in cm, and why rounding matters there too.
Measure ring size with a tape measure
How to take the inch reading in the first place.
Ring circumference calculator
Convert any circumference to diameter and sizes.