Ring Size Converter maps a size in any national system onto all the others. Every scale measures the same thing — the inside of the band — but dresses it up differently: the EU and ISO size is literally the inside circumference in millimetres, the US scale is an abstract number in half steps, the UK uses letters, and Japan runs its own integer scale. Because the systems do not line up exactly, conversion here is a lookup against the standard chart rather than a formula, with the nearest stocked size reported when a measurement falls between two. Enter a measured diameter or circumference and the same chart resolves it, with the sizes either side shown for context.
Features
Converts between US, UK, EU/ISO and Japanese sizing at once
Accepts a measured inside diameter or circumference in millimetres
Reports the nearest stocked size and how far off the measurement was
Shows the size above and below for a between-sizes measurement
Understands UK half sizes written as N½, N 1/2 or N0.5
Full conversion chart with the matched row highlighted
Diameter also given in inches
Everything looked up on your device
How to use the Ring Size Converter
Pick the system your size is in, or choose diameter or circumference to enter a measurement
Type the size or the millimetre measurement
Read the equivalent in every other system
Check the neighbouring sizes if your measurement fell between two
Example
Input
US size 7
Output
UK N½ · EU 54 · Japan 14 · 17.3 mm diameter · 54.4 mm around
The EU size is simply the inside circumference rounded to a whole millimetre.
Common errors & troubleshooting
A UK letter is not recognised. — UK sizes run from F to Z. Half sizes need the marker — N½, N 1/2 or N0.5 all work, but a bare N is a different size from N½.
The measured circumference lands between two sizes. — That is normal, and the tool reports how far off you are. Jewellers generally size up rather than down, especially for a wide band.
A paper strip measurement gives an unexpectedly large size. — Pull the strip snug rather than tight, and measure the base of the finger rather than over the knuckle unless the knuckle is much wider — in which case size for the knuckle so the ring goes on.
The same finger measures differently at different times. — Fingers swell in heat, after exercise and late in the day, and shrink when cold. Measure warm and at rest, and take the reading more than once.
Frequently asked questions
How do I measure my ring size at home?
Wrap a thin strip of paper around the base of the finger, mark where it overlaps, and measure the length in millimetres. That length is the inside circumference — enter it directly and the matching size comes back.
What is a US size 7 in UK ring sizes?
N½. The two scales do not line up neatly, which is why a chart is needed rather than an offset — a US 8 is P½ and a US 9 is R½.
Is the EU ring size the same as the circumference in millimetres?
Yes. The EU and ISO 8653 size is the inside circumference in millimetres, rounded to a whole number, so a 54 mm circumference is a size 54. It is the most directly physical of the scales.
Should I size up for a wide band?
Usually. A wide band sits on more of the finger and feels tighter than a narrow one of the same nominal size, so half a size up is a common recommendation for bands over about 6 mm.
Is the size I enter recorded anywhere?
No. The conversion is a table lookup running in your browser, and nothing you type is transmitted or saved.
Related tools
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