How do I measure ring size in mm?

Measure the inner diameter of a ring that fits across its widest point in millimetres, or wrap a strip of paper around your finger and measure the circumference. Enter either value to convert.

What is a US size 7 ring in UK?

A US size 7 is about a UK N½, an EU 54–55, an inner circumference of roughly 54.4 mm and a diameter near 17.3 mm.

Is anything uploaded?

No. The converter works entirely in your browser with no network requests.

What does the EU ring size mean?

In the ISO 8653 system used across Europe, the EU ring size is simply the inner circumference of the ring in millimetres. So an EU 54 ring has a 54 mm inside circumference.

How is diameter related to circumference?

Circumference = π × diameter, so diameter = circumference ÷ π. A 54.4 mm circumference gives a diameter of 54.4 ÷ 3.14159 ≈ 17.3 mm. The converter computes this for you.

When is the best time to measure my finger?

Measure at the end of the day at normal body temperature, when fingers are at their largest. Cold fingers shrink, which can lead to a ring that is too tight later.

Should I size up for wide bands?

Yes. Wide bands feel tighter, so jewellers often suggest going up a quarter to half a size for bands wider than about 6 mm.

Ring Size Converter — Gera Tools

Match ring sizes across regions and the physical measurements jewellers use: US/Canada, UK/Australia letters, EU, plus inner circumference and inner diameter in millimetres. It is ideal before ordering a ring online or buying a gift.

How it works

The converter is anchored on inner circumference in millimetres, following the ISO 8653 standard where the EU size equals the circumference. It holds a lookup table mapping each US size to its circumference, with the matching UK/Australia letter. From the circumference it derives the inner diameter using diameter = circumference ÷ π. Whichever system you enter — US, EU, circumference, or diameter — it finds the matching row and shows all the equivalents. For the most accurate result, measure the inside diameter of a ring you already own, or wrap a strip of paper around your finger and measure its length (the circumference).

Example

A US size 7:

Inner circumference = 54.4 mm (so EU ≈ 54)
Inner diameter = 54.4 ÷ π ≈ 17.3 mm
UK/Australia letter = N½

US	UK	Circumference	Diameter
6	M	~51.9 mm	~16.5 mm
7	N½	54.4 mm	~17.3 mm
8	P½	57.0 mm	~18.1 mm
9	R½	~59.5 mm	~18.9 mm

All lookups happen locally in your browser — nothing is uploaded.