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 actually use: US/Canada numbers, UK/Australia letters, EU numbers, plus inner circumference and inner diameter in millimetres. Type into any field and every other system updates instantly, with a from/to result line, a live “closest standard size” readout and a full reference chart. It is ideal before ordering a ring online, buying a surprise gift, or translating a size quoted by an overseas jeweller.

How it works

The converter is anchored on inner circumference in millimetres, following the ISO 8653 standard in which the EU size simply equals the circumference. It holds a lookup table mapping each US size to its circumference, with the matching UK/Australia letter, derived from the common adult jeweller charts. From the circumference it computes the inner diameter with the relation diameter = circumference ÷ π.

Circumference and diameter are treated as continuous values, so they cross-fill each other exactly to one or two decimal places. US, UK and EU are discrete standard sizes, so when you type a measurement the tool snaps those labels to the nearest row and tells you which standard size you are closest to. Whichever field you edit — a US number, a UK letter, an EU number, a circumference, or a diameter — the rest of the grid and the reference chart update together, and the matching row is highlighted.

Measuring guide

You do not need a special tool to find your size. There are two reliable home methods:

Measure a ring that already fits. Lay it flat and measure the inside diameter in millimetres across its widest point, then type that into the diameter field.
Measure your finger. Wrap a thin strip of paper or string snugly around the base of the finger, mark where it overlaps, lay it flat and measure the length in millimetres — that is the circumference.

For the best fit, measure at the end of the day when fingers are warm and at their largest, take two or three readings, and size up slightly for wide bands (about 6 mm or wider).

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	L½	~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.