Units

Ring size in centimetres

Centimetres are the unit most flexible tapes show first — and the unit most likely to be rounded badly. A centimetre is 10 millimetres, so a reading rounded to the nearest centimetre can be off by a whole ring size or more.

The conversion — and the rounding trap

Multiply by 10, keep one decimal

1 cm = 10 mm exactly. A diameter of 1.73 cm is 17.3 mm; a circumference of 5.44 cm is 54.4 mm. Keep at least one decimal place in centimetres — that is one millimetre of precision, which is the minimum a size decision needs.

Never round to a whole centimetre

A half-size step is about 0.13 cm of circumference. Rounding 5.44 cm to 5 cm throws away 4.4 mm — more than three half-sizes. If your tape only shows whole centimetres, read the millimetre marks between them, or use the millimetre side.

The same diameter-vs-circumference test as in millimetres applies, divided by 10: a number under 2.5 cm is a diameter; a number between 4.0 and 7.5 cm is a circumference.

The full table, in centimetres

Ring sizes with inner diameter and circumference in centimetres
US / Canada	Inner diameter (cm)	Inner circumference (cm)	EU / ISO
3	1.41	4.42	44
3.5	1.45	4.55	46
4	1.49	4.67	47
4.5	1.53	4.80	48
5	1.57	4.93	49
5.5	1.61	5.06	51
6	1.65	5.19	52
6.5	1.69	5.32	53
7	1.73	5.44	54
7.5	1.77	5.57	56
8	1.81	5.70	57
8.5	1.85	5.82	58
9	1.90	5.95	60
9.5	1.94	6.08	61
10	1.98	6.21	62
10.5	2.02	6.34	63
11	2.06	6.46	65
11.5	2.10	6.59	66
12	2.14	6.72	67
12.5	2.18	6.85	69
13	2.22	6.97	70
13.5	2.26	7.10	71

Worked examples

Tape reads 5.44 cm around a finger. That is 54.4 mm of circumference: US 7, EU 54.
Ruler reads 1.65 cm across a ring. That is 16.5 mm of diameter: US 6, EU 52.
A tape shows “5 cm” and nothing finer. Do not convert 5 cm — re-measure with the millimetre marks. 5.0 cm of circumference would be US 5.5, but the true reading could be anywhere from 4.5 to 5.5 cm, which spans US 3 to US 8.

Convert a centimetre 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 centimetres, use the diameter calculator instead — the same number typed into the wrong tool gives a nonsense size, because a diameter and a circumference in the same unit are different quantities.

Related guides

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