mm to Ring Size Converter

How to Measure Your Ring Size at Home

Finding your accurate ring size is essential when purchasing rings online or as a surprise gift. While professional jewelers have specialized tools, you can measure your ring size at home using simple materials you likely already have.

Different Measurement Methods

Using an Existing Ring

If you have a ring that fits the intended finger well, you can measure its inside diameter directly:

1. Place the ring on a ruler
2. Measure from one inside edge to the other inside edge
3. This gives you the inside diameter in millimeters
4. Enter this value in our calculator with "Inside Diameter" selected

Using String or Paper

The string method measures circumference, which is the distance around your finger:

• Cut a thin strip of paper or use non-stretchy string
• Wrap it around your finger at the base
• Mark where it overlaps
• Measure the marked length - this is your circumference

International Ring Sizing Systems

US and Canada

The US/Canadian system uses numerical sizes, typically ranging from 3 to 13 for women and 6 to 16 for men. Half and quarter sizes provide a finer fit. Size 6 corresponds to an inside diameter of approximately 16.5 mm.

UK and Australia

The UK and Australian system uses letters from A to Z, where A is the smallest size. Half sizes are indicated with "1/2" after the letter (e.g., K 1/2). This system is also used in Ireland, New Zealand, and South Africa.

European

European sizes are based on the inner circumference of the ring in millimeters. A size 50 ring has an inner circumference of approximately 50 mm. This makes conversion straightforward once you know your circumference.

Japan

Japanese ring sizes use a numerical system starting from 1, which corresponds to a 13 mm diameter. Each increase of 1 in Japanese size equals an increase of 1/3 mm in diameter.

The Conversion Formula

Converting between diameter and circumference uses the mathematical constant pi (π ≈ 3.14159):

Tips for Accurate Measurements

• Measure at the right time: Fingers swell during hot weather and shrink in cold. Measure at room temperature, preferably in the evening.
• Consider the knuckle: Your ring must pass over the knuckle, so measure at the widest point.
• Account for ring width: Wide bands fit tighter than thin bands. Go up half a size for bands wider than 6mm.
• Measure multiple times: Take 3-4 measurements and use the average for best accuracy.
• Check dominant hand: Fingers on your dominant hand may be slightly larger.
• Don't measure after exercise: Physical activity can temporarily change finger size.

Frequently Asked Questions

What ring size measures 16 mm in diameter?

A 16 mm inside diameter corresponds to approximately US size 5 3/8, UK size K 1/2, and European size 50. This is a common size for women's rings.

How do I convert circumference to ring size?

First, divide your circumference by π (3.14159) to get the diameter. Then use a ring size chart or our calculator to find your size. For example, a 50 mm circumference equals approximately 15.9 mm diameter, which is about US size 5 1/4.

What is the average ring size for women?

The average women's ring size in the US is between 5 and 7, with size 6 being most common. This corresponds to an inside diameter of approximately 16.5 mm.

What is the average ring size for men?

The average men's ring size in the US is between 9 and 11, with size 10 being most common. This corresponds to an inside diameter of approximately 19.8 mm.

Should I size up or down if between sizes?

If you're between sizes, it's generally better to size up. A slightly loose ring is more comfortable than a tight one, and rings can always be sized down by a jeweler more easily than sized up.

Do finger sizes change?

Yes, finger sizes can change due to temperature, time of day, weight changes, pregnancy, arthritis, and age. It's a good idea to remeasure periodically, especially before purchasing expensive rings.