Free MM to Ring Size Converter

Measurement Type

Value

Circumference = Diameter x π

1 cm = 10 mm · 1 in = 25.4 mm
Ring sizes are approximate and may vary by manufacturer.

Enter a ring measurement above to see your ring size

Converting a finger measurement from millimeters to the right ring size doesn't have to be complicated. Whether you've taken the circumference with a piece of string or noted the inner diameter of an existing ring, this ring size converter provides a fast and accurate way to match millimeters with international ring sizing standards. The tool accepts both circumference and diameter inputs, making it suitable for any measurement method you choose.

How to Obtain Your Finger Measurement

Measuring the Circumference

Take a non‑elastic string or a thin strip of paper and wrap it around the base of the finger you intend to size. Mark the point where the end meets the string, then lay the string flat against a ruler and record the length in millimeters. Enter this value into the Inside circumference field of the calculator; the tool immediately displays the corresponding US & Canada size, as well as equivalent sizes for UK & Australia, France, Germany, Switzerland, and Japan.

Measuring the Diameter of an Existing Ring

If you already have a ring that fits well, measure its inner diameter by placing it on a ruler, aligning the inside edge with the zero mark, and reading the diameter at the opposite inner edge. Select the closest value in the Inside diameter dropdown of the converter to find the ring size.

Reading the Ring Size Conversion Table

The comprehensive table below lists the internal diameter and circumference in millimeters alongside their equivalents in the most common ring sizing systems. After you have your measurement, locate the nearest circumference (or diameter) row and read across to find the size for your country.

Internal Diameter (mm)	Internal Circumference (mm)	US & Canada	UK & Australia	France	Germany	Japan	Switzerland
11.53	36.22	—	—	0	—	—	—
11.95	37.54	—	A	½	—	1	—
12.37	38.86	—	B	1	—	—	—
12.78	40.15	—	C	1½	—	—	—
13.21	41.50	—	D	2	41½	13½	2
13.61	42.76	—	E	2½	42¾	13¾	3
14.05	44.14	—	F	3	44	14	4
14.36	45.11	—	G	3⅜	45¼	—	5
14.45	45.40	—	G¼	3½	—	14½	—
14.56	45.74	—	G½	3⅝	—	6	—
14.65	46.02	—	H	3¾	46½	—	6½
14.86	46.68	—	H½	4	—	15	7
15.04	47.25	—	I	4¼	47¾	—	7¾
15.27	47.97	—	I½	4½	—	15¼	8
15.40	48.38	—	J	4⅝	49	15½	—
15.70	49.32	—	J½	5	—	15¾	9
15.80	49.64	—	K	5⅛	50	—	10
16.00	50.27	—	K½	5⅜	—	10	—
16.10	50.58	—	L	5½	51¾	16	—
16.51	51.87	—	M	6	52¾	16½	12
16.92	53.16	—	N	6½	54	17	13
17.35	54.51	—	O	7	55¼	17¼	14
17.75	55.76	—	P	7½	56½	17¾	15
18.19	57.15	—	Q	8	57¾	18	16
18.53	58.21	—	Q¾	8½	—	18½	17
18.61	58.47	—	R	8⅝	59	—	19
18.89	59.34	—	R¾	9	—	19	18
19.10	60.00	—	S	9⅛	60¼	—	20¼
19.41	60.98	—	S¾	9½	—	19½	19
19.51	61.29	—	T	9⅝	61½	—	21½
19.84	62.33	—	T½	10	—	20	—
20.02	62.89	—	U	10¼	62¾	—	21
20.20	63.46	—	U½	10½	—	20¼	22
20.32	63.84	—	V	10⅝	63	—	23¾
20.68	64.97	—	V½	11	—	20¾	23
20.76	65.22	—	W	11⅛	65	—	25
21.08	66.22	—	W¾	11½	—	21	24
21.18	66.54	—	—	11⅝	66¼	—	26¼
21.49	67.51	—	Y	12	67½	21¼	25
21.89	68.77	—	Z	12½	68¾	21¾	26
22.33	70.15	—	Z+1	13	—	22	27

How to interpret the table: Suppose you wrapped a strip of paper around your finger and the length came to 54.0 mm. The closest higher circumference in the table is 54.51 mm (row with Diameter 17.35 mm), which points to US size 7, UK size O, French size 55¼, German size 17¼, Japanese size 14, and Swiss size 15¼.

Why You Should Always Choose the Next Larger Size

Finger knuckles are not perfectly round; a ring must pass over the knuckle to sit comfortably on the finger base. Therefore, when your measurement falls between two entries on the chart, always select the next larger circumference (and corresponding size) rather than the nearest smaller one. This ensures the ring slides over the knuckle without being too loose on the finger.

The Relation Between Diameter and Circumference

If you only have the diameter of a ring (or your finger) but the chart lists circumferences, you can easily convert between them using the formula:

C = \pi \times d

Where $C$ is the circumference and $d$ is the diameter. For example, a diameter of 16.00 mm gives a circumference of approximately $50.27\ \text{mm}$ ( $\pi \times 16.00$ ), which corresponds to US size 5⅜.

Putting It All Together

Whether you measure the circumference with a piece of string or the diameter of an existing ring, this ring measurement converter bridges your millimeter reading to the correct size in US, UK, French, German, Japanese, Swiss, and other systems. The conversion table above and the built‑in calculator work together to deliver a precise ring size without guesswork.

FAQ

1. How can I convert a finger measurement in millimeters to a ring size step by step?

First, wrap a non‑stretch string or paper strip around your finger and mark the overlap. Measure the length between the mark and the end in millimeters (this is the circumference). Enter that value into the ring size converter’s Inside circumference field, or find the nearest circumference in the conversion table. Because knuckles are not perfectly round, always pick the next larger size if your measurement falls between two chart entries.

2. What US ring size corresponds to a diameter of 16 mm?

A diameter of 16.00 mm corresponds to US & Canada size 5⅜ (five and three‑eighths), UK size K½, French size —, German size —, Japanese size 10, and Swiss size —. (See the table row where Diameter = 16.00 mm and Circumference = 50.27 mm.)

3. Why should I round up to the next ring size when my measurement is between two values?

Finger knuckles are wider than the base of the finger. If you select the closest smaller size, the ring may get stuck at the knuckle. Choosing the next larger circumference ensures the ring passes the knuckle easily while still fitting comfortably on the base.

4. Can I use the converter if I only know the diameter of an existing ring?

Yes. The tool accepts inside diameter as input. Simply select the closest diameter value in the Inside diameter dropdown, and it will display corresponding ring sizes across multiple international systems. You can also use the conversion table directly by matching your diameter measurement to the nearest entry.

5. How do the French and German ring sizing systems differ from US sizes?

French ring size is derived from the inner circumference in millimeters (size = circumference – 40). German sizes use a different numbering system based on diameter or circumference. The conversion table shows equivalent values so you can easily cross‑reference between US, UK, French, German, Japanese, and Swiss standards without manual math.

How to Use

Measure your finger by wrapping a string or strip of paper around it and mark where it overlaps.
Enter the length in millimeters as the circumference, or measure the diameter of an existing ring.
View your ring size across US, UK, EU, and other international standards instantly.