Ring Size in mm – Diameter and Circumference for Every Size

Ring size in mm is the inside diameter of the ring, or its inside circumference. US size 6 is 16.5 mm across (51.8 mm around), size 7 is 17.3 mm (54.3 mm) and size 8 is 18.1 mm (56.9 mm). Each whole US size adds about 0.8 mm of diameter and 2.5 mm of circumference. Measure your ring in mm on screen with the online ring sizer, or use the charts below.

Ring size chart in mm

Diameter or circumference?

Jewellers use both, so it helps to know which one you have:

• Inner diameter is the distance straight across the inside of the ring. You get it by measuring a ring you own with a ruler or caliper. US sizes are defined by diameter, for example US 7 = 17.3 mm.
• Inner circumference is the distance around the inside, which is what you measure when you wrap a thread or paper strip around your finger. European (ISO) sizes are simply the circumference rounded to the nearest mm: EU 54 is 54 mm around.

Convert one to the other with π: circumference = diameter × 3.1416, and diameter = circumference ÷ 3.1416. A 17.3 mm ring is 17.3 × 3.1416 = 54.3 mm around.

I measured my ring in mm – what size is it?

Find your measurement in the first column. Values are rounded to the nearest half millimetre; if yours falls between two rows, use the nearest one or check the full chart above.

Popular sizes in mm

• Size 5 ring in mm: 15.7 mm diameter, 49.3 mm circumference.
• Size 5.5 in mm: 16.1 mm, 50.6 mm.
• Size 6 ring in mm: 16.5 mm, 51.8 mm.
• Size 6.5 in mm: 16.9 mm, 53.1 mm.
• Size 7 ring in mm: 17.3 mm, 54.3 mm.
• Size 7.5 in mm: 17.7 mm, 55.6 mm.
• Size 8 ring in mm: 18.1 mm, 56.9 mm.
• Size 9 ring in mm: 19.0 mm, 59.7 mm.
• Size 10 ring in mm: 19.8 mm, 62.2 mm.

How to measure a ring in mm

With a ruler

1. Choose a ring that fits the finger you are sizing.
2. Lay it on a ruler with mm marks and measure the inside edge straight across the centre.
3. Read to the nearest half millimetre. A ruler is accurate to about one half size, so if the result is between two sizes, measure again or use the on-screen sizer.

With a vernier or digital caliper

Use the inside jaws of the caliper to measure the inner diameter at two points at right angles and average them. This gives 0.1 mm accuracy, which is enough to tell quarter sizes apart.

On your phone screen

The online ring sizer draws a true-size circle once you calibrate it with a bank card. Place the ring on the screen, match the circle to the inside edge, and read the diameter and circumference in mm along with US, UK, EU and Indian sizes.

How to measure your finger in mm

1. Cut a thin strip of paper about 1 cm wide and 10 cm long, or use non-stretch thread.
2. Wrap it around the base of the finger, snug but not tight, and mark where it overlaps.
3. Measure from the end to the mark in mm. That is your finger circumference.
4. Find the circumference in the chart. If the knuckle is larger than the base of the finger, measure both and choose a size in between, so the ring passes the knuckle but does not spin.

String and paper stretch and slip, so this method can be off by half a size. Measure two or three times, at the end of the day, when fingers are at their largest.

Why each size is 0.8 mm

The US system increases the inner circumference by about 2.55 mm (0.1 inch) for every whole size, which is 0.81 mm of diameter. Half sizes are therefore 0.4 mm apart and quarter sizes 0.2 mm. UK letters step in half-size increments of about 1.25 mm circumference. That is why a measurement error of just half a millimetre can move you up or down a half size, and why measuring the inside of a ring that already fits gives a better result than measuring your finger.

Band width changes the fit

Two rings with the same inner diameter can feel different. Wide bands (6 mm or more) cover more of the finger and feel tighter, so many jewellers recommend going up ¼ to ½ size. Comfort-fit bands, which are rounded on the inside, feel slightly looser than flat bands of the same size. Thin stacking rings under 2 mm can be bought at your exact size or even ¼ size down.

Ring sizes in mm for men and women

Women’s ring sizes most often fall between 15.7 mm and 17.7 mm (US 5–7.5), with US 6–7 (16.5–17.3 mm) the most common. Men’s sizes usually fall between 18.5 mm and 20.6 mm (US 8.5–11), with US 9–10 (19.0–19.8 mm) the most common. Children’s rings start around 13–14 mm.

Need other systems? See the full ring size chart, the ring size converter or the ring size chart for Pakistan. Have a measurement? Look it up by diameter or circumference.

Frequently asked questions

What is ring size in mm?

Ring size in mm is either the inner diameter of the ring (US 7 = 17.3 mm) or its inner circumference (US 7 = 54.3 mm). European sizes use the circumference.

How many mm is each ring size?

Each whole US size adds about 0.8 mm of inner diameter and 2.5 mm of inner circumference. Half sizes are about 0.4 mm apart.

What ring size is 18 mm?

An 18 mm inner diameter is very close to US 8 (18.1 mm), UK P½ and EU 57.

What ring size is 20 mm?

A 20 mm inner diameter is between US 10 (19.8 mm) and 10.5 (20.2 mm); most people choose US 10.5, UK U½.

How do I measure my ring size in mm at home?

Measure the inside diameter of a ring that fits with a ruler, or wrap a paper strip around your finger and measure its length for the circumference. For better accuracy, match a ring on screen with our ring sizer.