Ring Size Converter — US, UK, EU, JP and millimetres
Convert ring sizes between US numbers, UK letters, EU/ISO, Japanese sizes and the inside diameter or circumference in millimetres.
All systems describe the same inside measurement: EU/ISO 8653 is the inside circumference in mm, US counts from the inside diameter, UK runs A–Z in ~1.25 mm steps and JP numbers from about 40 mm. Measure in the late afternoon at room temperature, and go up a quarter to a half size for bands wider than 6 mm.
About the Ring Size Converter
Every ring sizing system is describing the same thing — the inside of the band — but each one reports it differently. The EU or ISO 8653 size is the most literal: it is simply the inside circumference in millimetres, so a size 52 ring measures 52 mm around the inside. The US system counts in quarter and half numbers derived from the inside diameter, the UK system runs through the alphabet from A upwards in steps of about 1.25 mm of circumference, and the Japanese system numbers sizes from roughly 40 mm of circumference, so a JP 13 is about 53 mm.
Because the underlying measurement is the same, any size can be converted to any other as long as you go through the millimetres. That is what this tool does: whatever you enter is turned into an inside circumference first, then read back out in every system. It also prints the inside diameter, which is the number you need if you are measuring a ring you already own rather than your finger.
Two things move a finger between sizes. Temperature and time of day: fingers are measurably smaller in the cold and first thing in the morning, and largest in the late afternoon and in heat, so measure at the end of a normal day. And band width: a wide band sits on more of your finger and feels tighter, so for anything over about 6 mm wide it is usual to go up a quarter to a half size. Everything here is calculated in your browser and nothing is sent to a server.
How to use it
- Choose what you already know — a US size, a UK letter, a Japanese number, or a measurement in millimetres.
- To measure a ring you already own, lay it flat and measure straight across the inside from edge to edge: that is the inside diameter in mm.
- To measure your finger, wrap a strip of paper or a piece of string around the base of the finger, mark where it overlaps and measure the length: that is the inside circumference in mm.
- Read your size in every system from the result panel, or find your row in the full chart below.
- Measure late in the day at normal room temperature, and add a quarter to a half size for bands wider than about 6 mm.
Frequently asked questions
What is a US ring size 7 in UK sizing?
A US 7 is a UK N½, an ISO/EU 54 and a Japanese 14. The inside diameter is about 17.3 mm and the inside circumference about 54.4 mm.
How do I measure my ring size at home?
Two ways. If you own a ring that fits the right finger, measure straight across the inside from edge to edge with a ruler — that is the inside diameter in millimetres. If you do not, wrap a thin strip of paper or a piece of string snugly around the base of the finger, mark where it overlaps, lay it flat and measure the length — that is the inside circumference. Enter either number above.
What is the EU or ISO ring size?
ISO 8653, used across most of Europe, defines the size as the inside circumference in millimetres. A size 52 ring has a 52 mm inside circumference. It is the least ambiguous system, which is why it is worth converting into when you buy from an unfamiliar country.
What time of day should I measure?
Late afternoon or early evening, at normal room temperature. Fingers swell over the course of the day and in the heat, and shrink in the cold, with a spread of up to half a size between a cold morning and a warm evening. Measuring at your largest means the ring will never be too tight.
Does the width of the band change the size I need?
Yes. A wide band covers more of the finger and feels tighter than a narrow one of the same nominal size. For bands wider than roughly 6 mm, go up a quarter to a half size. Comfort-fit bands, which are domed on the inside, run slightly loose and may need the opposite adjustment.
How do I convert millimetres to a ring size?
If you have the inside circumference, that number is already the EU/ISO size. To get the diameter, divide the circumference by 3.1416. Enter either figure above and the converter will print the US, UK and Japanese equivalents.
Can a ring be resized if I get it wrong?
Most plain metal bands can be resized a size or two by a jeweller. Rings with stones set all the way round, and rings in tungsten, titanium or ceramic, usually cannot — with those it is worth being measured professionally before ordering.