Ring Size Calculator.

Carit's ring size calculator converts between US, UK, EU and Japan ring sizes, measures the finger with a device-calibrated on-screen sizer, and looks up any size from inner diameter or circumference in millimetres. This page explains exactly how the calculator works, what the industry-standard size chart looks like, and how to use it in your workshop.

How the Carit ring size calculator works

When you open the ring sizer in Carit, three tools live together in a single tabbed view. Understanding what each one does saves you from reaching for Google or a paper chart mid-job.

1. Size converter

Pick any of the four systems - US, UK, EU, Japan - and enter a known size. Every other system fills in instantly, alongside the inner diameter and circumference in millimetres. Useful when a client says "I'm a UK N" and you need to know the US and EU equivalents before you order grain or cut a shank.

2. True-to-life sizer circles

A set of circles drawn at real physical size on your phone screen, calibrated automatically to the device model you're using. Place an existing ring on top of the appropriate circle - the one that matches the inner diameter is your size. The circles are tested on every major iPhone model and Android device with known pixel densities, so what you see really is 17.3 mm across when it says US 7.

3. Finger bands

Digital finger bands that wrap around the finger on screen, sized in EU millimetres. Useful as a second reference when a client doesn't have an existing ring to compare against and you need a measurement on the spot.

How ring sizes actually work

The four major ring-size systems agree on one thing: the number is a proxy for the inner circumference of the band in millimetres. Everything else - the code, the scale, the fractional step - is convention. That's why a UK size R, a US size 8.75, a European 58.7 and a Japan 18 all fit the same finger. They're describing the same 58.7 mm of inner circumference.

The four systems

US sizes run from roughly 3 to 13 in quarter-size steps (3, 3¼, 3½, 3¾, 4…). Each quarter-size step equals about 0.2 mm of inner diameter. A US size 7 has an inner diameter of 17.3 mm, which is the most common women's size worldwide.

UK sizes use letters of the alphabet, from A (very small, ~12 mm diameter) through Z+ (very large, ~22 mm), in half-letter steps. UK half sizes are written with a fraction: L½, M½. The jump between each whole letter is about 0.4 mm of diameter. UK size O is the equivalent of US 7¼.

European (EU) sizes are the inner circumference in millimetres, rounded to the nearest whole. EU 54 fits where UK size N and US 6¾ fit. This makes the European system the clearest conceptually - it is the inner circumference - but it also has the loosest step, because jumping from 54 to 55 is a full millimetre of circumference where a US size moves in quarter-mm steps.

Japanese sizes are whole-number codes from roughly 1 to 30+, with each unit equal to about 1/3 mm of inner diameter. Japan size 14 is equivalent to US 7 and UK O.

UK to US ring size conversion

The fastest way bench jewellers learn the UK-to-US mapping: each whole UK letter is roughly half a US size. So UK L is US 5¾, UK M is US 6¼, UK N is US 6¾, UK O is US 7¼, and so on. But the mapping is not exact, which is why a chart matters.

The chart below is the industry-standard conversion table, rounded to the nearest quarter of a US size. It's anchored to inner-diameter and circumference in millimetres, so the conversions are consistent.

How to measure ring size at home

There are three methods that work without a professional ring sizer. In order of accuracy:

Method 1 - Measure a ring that already fits

Take a ring you already own that fits the correct finger. Measure the inside diameter with a ruler (or caliper if you have one), in millimetres. Match the number to the chart above. If it's between two sizes, go up - you can always size down later. A 17.3 mm inner diameter gives you a US 7 / UK O / EU 54.

Method 2 - Measure the finger with string or paper

Wrap a strip of non-stretch paper or a thin piece of string snugly around the base of the finger. Mark where the strip overlaps. Lay it flat against a ruler and measure the length in millimetres - that's your circumference. Match it to the EU column of the chart, or plug it into the calculator above.

Measure at the end of the day, when fingers tend to be slightly larger. Measure twice. Cold fingers read small. Warm fingers read accurate.

Method 3 - Print a ring sizer

Printable ring sizers exist, but they are only accurate if your printer is scaled at exactly 100%. A "fit to page" print will silently resize the chart and give you the wrong size. If you go this route, print a test page with a 100 mm reference bar and verify it with a ruler before trusting the size circles.

Common ring-size questions

Between sizes? Which way do you round?

Round up. A slightly loose ring can be sized down in about 15 minutes at the bench. A slightly tight ring requires cutting the shank, adding metal, and rebuilding - expensive and often the original ring's integrity is lost. This is doubly true for wide bands, eternity rings, and rings with stones set across the shoulders.

Wide bands run smaller - by how much?

Any band wider than about 4 mm should be ordered a quarter-size larger than your standard size. Wider bands (6 mm+) should be a half-size larger. This is because the skin and tissue of the finger can compress under a narrow band but can't around a wide one. For an 8 mm band, most setters recommend going a full size up.

Dominant hand vs non-dominant hand

Your dominant hand is typically a quarter to half size larger than your non-dominant hand. If you're sizing a wedding ring for the ring finger of the left hand (US/UK convention) and the measurement is borderline, and the customer is left-handed, nudge up.

Is ring size different for men vs women?

No. Size is size - anatomy varies across individuals, not systems. The perception that men's sizes are different is because men's ring finger measurements tend to be larger on average, not because men use a different scale.

Full ring size conversion chart

The table above is interactive and sortable. If you prefer a printable version, the values follow. All measurements derived from the ISO-adjacent industry-standard mm ring sizer charts used by Stuller, Rio Grande, and Cooksongold.

Ring size FAQ