Quick Ring Size
Size chart

mm to ring size

Enter a millimetre measurement and read the ring size in US, UK, EU, and Japan. Works from either the inner diameter or the circumference.

US / Canada
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UK / AU
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EU / ISO
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Japan
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Diameter
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Circumf.
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Diameter or circumference: get this right first

To convert millimetres to a ring size, the one thing you must get right is which millimetre measurement you have. Inner diameter is the straight-line distance across the inside of a ring; circumference is the distance all the way around the inside. They're related by pi (circumference = diameter × 3.14), but they're very different numbers: a 17.3 mm diameter and a 54.4 mm circumference are the same ring (a US 7), while a “17 mm” read off the wrong scale is a full-blown error. Enter your number into the sizer on the homepage and tell it which one you measured, or read across the correct column below.

Measuring an existing ring? Use inner diameter. Lay the ring on a millimetre ruler or, better, span the inside with calipers, and read edge-to-edge through the centre. Quick anchors: 16.5 mm = US 6, 17.3 mm = US 7, 18.1 mm = US 8, 19.0 mm = US 9, 19.8 mm = US 10. Each full US size is about 0.81 mm of diameter, so half a size is only about 0.4 mm, so precision matters.

Measuring a finger? Use circumference. Wrap paper or string around the base, mark the overlap, and measure the length. Anchors: 51.8 mm = US 6, 54.4 mm = US 7, 56.9 mm = US 8, 59.5 mm = US 9, 62.0 mm = US 10. Each full US size is about 2.55 mm of circumference. To work in diameter instead, divide by 3.14.

Caliper tips for the most accurate reading

  • Use digital calipers set to millimetres and measure the inner diameter with the internal jaws or the jaw tips.
  • Measure across the true centre of the ring, not a chord. Rock the calipers slightly and take the largest stable reading.
  • Take three measurements and average them; a single reading can be off by enough to shift a half size.
  • Reading a finger with calipers isn't reliable, because soft tissue compresses. Wrap for circumference instead.

Millimetre lookup table

The nearest standard size for a range of inner measurements. For a value between two rows, round up.

Diameter (mm) Circumf. (mm) US / CA UK / AU EU / ISO Japan
14.1 44.2 3 F 44 4
14.5 45.4 3.5 G 45 5
14.9 46.7 4 H 47 7
15.3 48.0 4.5 I 48 8
15.7 49.3 5 J½ 49 9
16.1 50.5 5.5 K½ 51 10
16.5 51.8 6 L½ 52 12
16.9 53.1 6.5 M½ 53 13
17.3 54.4 7 N½ 54 14
17.7 55.6 7.5 O½ 56 15
18.1 56.9 8 P½ 57 16
18.5 58.2 8.5 Q¼ 58 18
18.9 59.5 9 R½ 59 19
19.4 60.7 9.5 S½ 61 20
19.8 62.0 10 T½ 62 21
20.2 63.3 10.5 U½ 63 22
20.6 64.6 11 V½ 65 24
21.0 65.8 11.5 W½ 66 25
21.4 67.1 12 X½ 67 26
21.8 68.4 12.5 Y½ 68 27
22.2 69.7 13 Z½ 70 29
22.6 70.9 13.5 Z1½ 71 30

Once you have a size, see how it converts on the ring size chart and international conversion pages, or measure directly with the calibrated sizer.

Common questions

How do I convert mm to a ring size?

First decide which millimeter measurement you have. If you measured across the inside of a ring, that's the inner diameter; a 17.3 mm diameter is a US 7. If you measured around a finger or the inside of a ring, that's the circumference; a 54.4 mm circumference is also a US 7. Match your number to the right column on a chart, or enter it into a converter that asks which one you measured.

Should I measure inner diameter or circumference?

Measure whichever fits your situation. An existing ring is easiest to measure by inner diameter: lay it on a ruler and read straight across the inside. A finger is easiest to measure by circumference: wrap paper or string around it. Be sure you tell the converter which one you measured, because a 17 mm diameter and a 17 mm circumference are wildly different sizes.

What ring size is 18 mm?

It depends on whether 18 mm is diameter or circumference. An 18 mm inner diameter is about a US 7.75 to 8. An 18 mm circumference isn't a wearable ring size at all. It's far too small, roughly a 5.7 mm diameter. This is the most common mm-to-size mistake, so always confirm which measurement you took.

How do I use calipers to measure ring size?

Digital calipers are the most precise home tool. To measure an existing ring, use the inside (internal) jaws or the tips to span the inner edges and read the inner diameter to 0.1 mm. Measure across the true center, take three readings, and average them. Then convert the diameter to a size.

How do I convert circumference to diameter?

Divide the circumference by pi (about 3.14). A 54.4 mm circumference divided by 3.14 is about 17.3 mm of diameter, a US 7. To go the other way, multiply the diameter by 3.14. Many home methods measure circumference (wrapping a finger), so this conversion comes up often.