What unit should I use for the ring size calculator?

Millimetres (mm) are the most precise and widely used unit for ring sizing. If you have measured in inches or centimetres, simply select that unit from the dropdown and the calculator will convert automatically.

Is the ring size calculator accurate?

Yes. The calculator uses the standard international ring sizing tables and matches your input to the nearest recognised ring size. Because ring sizes are quantised (not continuous), the calculator returns the closest standard size to your measurement.

Can I calculate my ring size from an existing ring?

Yes. Measure the internal diameter of the ring in millimetres and enter it into the diameter field. The calculator will return all equivalent international sizes.

Ring Size Calculator — Convert Diameter & Circumference to US, UK, EU, JP

Q: How do I use the ring size calculator?

Enter either your ring diameter or inner circumference into the calculator. Select the unit (mm, cm, or inches) and the calculator will instantly display your ring size in US/CA, UK, EU/DK, and Japanese systems.

Enter Your Measurement

Inner Diameter

— or —

Inner Circumference

Enter a diameter or circumference above to see your ring sizes.

How the Calculator Works

Our ring size calculator takes your measurement and finds the closest match in our standard sizing database, which covers US sizes 3 through 13 — the full range used by most jewellers worldwide.

Using diameter

The inner diameter is the straight-line measurement across the inside of a ring. It is the most common measurement jewellers use. To measure it yourself, place an existing ring flat and measure straight across the opening with a ruler.

Using circumference

The inner circumference is the distance around the inside of the ring. This is what you get when you wrap a strip of paper around your finger and measure the length. The EU ring size system is based directly on this number in millimetres.

Which should I use? Both give the same result. Use whichever measurement you have. If you are measuring a finger directly, circumference is easier. If you are measuring an existing ring, diameter is usually simpler.

Quick Reference: Common Sizes

US / CA	UK	EU / DK	JP	Diam (mm)	Circ (mm)
5	J½	49	7	15.7	49.3
5.5	K½	51	8	16.1	50.6
6	L½	52	10	16.5	51.9
6.5	M½	53	11	16.9	53.1
7	N½	54	12	17.3	54.4
7.5	O	56	13	17.7	55.7
8	P	57	15	18.2	57.2
9	R	60	17	19.0	59.7
10	T	62	19	19.8	62.2

View the full ring size chart →

Frequently Asked Questions

How do I use the ring size calculator?

Type your diameter or circumference measurement into the appropriate field. Select the unit (mm, cm, or inches) from the dropdown. The calculator will instantly display your ring size in US/CA, UK, EU/DK, and Japanese sizing systems, along with the exact diameter and circumference for that size.

What unit should I use?

Millimetres (mm) offer the greatest precision for ring sizing. If your ruler or tape shows centimetres or you prefer inches, simply choose the matching unit in the dropdown and the calculation will automatically convert for you.

Is the calculator accurate?

The calculator matches your input to the nearest standard ring size using official international sizing tables. Because ring sizes are not a continuous scale, it always returns the closest recognised size. For the most accurate fit, always verify with your jeweller before purchasing.

Can I measure a ring I already own?

Yes. Place the ring on a flat surface and measure the inner diameter straight across the hole in millimetres. Enter that value into the diameter field above. You can also try our online ring sizer to visually match a circle on screen to your existing ring.