MM to Ring Size

To convert mm to ring size, measure the inner diameter or inner circumference of the ring in millimetres and match it to the chart. A 16.51 mm diameter or 51.9 mm circumference is US size 6. The formula is US size = (diameter in mm – 11.63) / 0.8128.

MM Ring Size Chart (US 3 to 13)

US size	UK size	EU size	Inner diameter (mm)	Inner circumference (mm)
3	F	44	14.07	44.2
3.5	G	45	14.47	45.5
4	H	47	14.88	46.7
4.5	I	48	15.29	48.0
5	J½	49	15.69	49.3
5.5	K½	51	16.10	50.6
6	L½	52	16.51	51.9
6.5	M½	53	16.91	53.1
7	N½	54	17.32	54.4
7.5	O½	56	17.73	55.7
8	P½	57	18.13	57.0
8.5	Q½	58	18.54	58.2
9	R½	60	18.95	59.5
9.5	S½	61	19.35	60.8
10	T½	62	19.76	62.1
10.5	U½	63	20.16	63.3
11	V½	65	20.57	64.6
11.5	W½	66	20.98	65.9
12	X½	67	21.38	67.2
12.5	Z	68	21.79	68.5
13	Z½	70	22.20	69.7

EU size equals circumference in mm, rounded, following ISO 8653:2016 as summarised on Wikipedia. See the ring size conversion chart for other conversions.

Diameter or Circumference: Which MM Number Do You Have?

Diameter is the width across the inside of the ring. Circumference is the distance around it. Circumference = 3.1416 x diameter. If your measurement is between 14 and 22 mm it is almost certainly a diameter; between 44 and 70 mm it is a circumference.

Range Lookup: Which Size Covers Your Measurement

Each size covers a small range around its exact value. Find your measurement in the range to get the closest size.

US size	Diameter range (mm)	Circumference range (mm)
3	13.87 to 14.27	43.6 to 44.8
3.5	14.27 to 14.67	44.8 to 46.1
4	14.68 to 15.08	46.1 to 47.4
4.5	15.09 to 15.49	47.4 to 48.7
5	15.49 to 15.89	48.7 to 49.9
5.5	15.90 to 16.30	49.9 to 51.2
6	16.31 to 16.71	51.2 to 52.5
6.5	16.71 to 17.11	52.5 to 53.8
7	17.12 to 17.52	53.8 to 55.1
7.5	17.53 to 17.93	55.1 to 56.3
8	17.93 to 18.33	56.3 to 57.6
8.5	18.34 to 18.74	57.6 to 58.9
9	18.75 to 19.15	58.9 to 60.2
9.5	19.15 to 19.55	60.2 to 61.4
10	19.56 to 19.96	61.4 to 62.7
10.5	19.96 to 20.36	62.7 to 64.0
11	20.37 to 20.77	64.0 to 65.3
11.5	20.78 to 21.18	65.3 to 66.5
12	21.18 to 21.58	66.5 to 67.8
12.5	21.59 to 21.99	67.8 to 69.1
13	22.00 to 22.40	69.1 to 70.4

How to Measure Inner Diameter in MM

1. Lay a ring that fits you flat on a ruler in millimetres.
2. Line the zero mark up with the inside left edge.
3. Read the mark at the inside right edge.
4. Find the number in the chart.Measure the inside, because the outer edge adds the band thickness. For measuring your finger, see how to measure your ring size or print the printable ring size chart. Related charts: US ring size chart, UK ring size chart, men’s ring size chart and women’s ring size chart.

Frequently Asked Questions

What is size 10 ring in mm?

US size 10 has an inner diameter of 19.76 mm and an inner circumference of 62.1 mm. UK size is T½ and EU size is 62.

What is size 7 ring in mm?

US size 7 is 17.32 mm across and 54.4 mm around. UK is N½ and EU is 54.

What size is a 16.5 mm ring?

16.5 mm is the inner diameter, which is US size 6 (16.51 mm).

Is ring size diameter or circumference?

Both describe the same size. US sizes are usually given by diameter and EU sizes by circumference in mm. Multiply diameter by 3.1416 for circumference.

How many mm is one ring size?

One full US size is 0.81 mm of diameter and 2.55 mm of circumference. A half size is 0.41 mm and 1.28 mm.

Where can I convert cm or inches instead?

Use the ring size chart in cm or the inches to ring size chart.