Ring Size Calculator
Enter a known ring size in any one system — US/Canada, UK/Ireland/Australia, France/Germany/Austria, Italy/Spain, Russia/CIS, or diameter and circumference in millimeters — and instantly see the equivalent size in every other system. Runs entirely in your browser.
Runs in your browser: every calculation uses plain arithmetic; nothing is uploaded to or stored by JewelModel.
One measurement, every sizing system
Ring sizing systems disagree on units, but they all describe the same physical thing: how big a circle fits your finger. This calculator uses that internal diameter as a shared reference point — enter a value in any one field, and it works out the diameter that value implies, then converts that diameter into every other listed system.
That makes it useful for the everyday cross-market questions a jeweler or shopper runs into: a customer quotes their ring size in the UK's letter scale but you need it in US numbers for a supplier's catalog, or a size measured in millimeters needs converting to what a customer in France, Russia, or the US would recognize as their own size.
Useful for
- Converting a customer's ring size from one country's sizing system to another before ordering or resizing
- Checking a finger measurement in millimeters against US, UK, European, and Russian size charts at once
- Quoting the same ring size correctly across marketplaces or suppliers that each use a different regional standard
How the calculation works
Internal diameter is the shared reference every system converts through.
- Circumference — which doubles as the France, Germany, and Austria size, all read directly with no offset — the Italy/Spain size, and the Russia/CIS size are exact, fixed conversions of the internal diameter in millimeters: circumference is diameter times pi, the Italy/Spain size is that same circumference minus 40, and the Russia/CIS size is the diameter itself, read to the nearest 0.5 mm.
- US and UK sizes are not evenly spaced in millimeters and don't come in arbitrary decimal steps in real jewelry stock, so both are matched to the nearest size on a reference chart rather than read off a smooth formula — US to the nearest listed number, UK to the nearest listed letter.
What this calculator does not do
The math is exact for a given diameter; getting that diameter right is the harder part.
- It converts a size you already know — it does not measure your finger. A ring sizer, a printable sizing chart, or a strip of paper wrapped snugly around the finger and measured against a ruler will get you the starting number.
- Finger size shifts slightly with temperature and time of day, so a measurement taken once may be half a size off from a proper in-store fitting.
- US and UK conversions are matched against a reference chart rather than a manufacturer-specific one — different jewelers' size charts can disagree by a fraction of a size.
How to use the calculator
One known size is all it needs; no account or upload required.
- Get a starting size
Use a ring sizer, a printable chart, or measure an existing ring's inner diameter with a ruler — or start from a size you already know in any system.
- Enter it in its matching field
Type that value into its field — US, UK, Italy/Spain, Russia/CIS, diameter, or circumference — and leave every other field empty.
- Read off every other system
All other fields fill in instantly, converted from the same underlying diameter.
Ring size calculator FAQ
Why do US, UK, French, Italian, and Russian ring sizes look so different for the same finger?
Each country standardized on a different way to describe the same physical circle: the US and Canada use an arbitrary numbered scale, the UK and Australia use letters, France (like Germany and Austria) quotes the internal circumference in millimeters directly, Italy and Spain quote that same circumference minus 40, and Russia/CIS quotes the internal diameter in millimeters directly. All of these describe the exact same ring.
How accurate is the US-to-UK (or UK-to-US) conversion?
It's matched to the nearest size on a standard reference chart, the same US/UK values already used elsewhere on this site — so the result is always a real, orderable size, never a decimal in between two sizes. Different manufacturers' charts can disagree by a fraction of a size, so treat the result as a close, reliable estimate rather than a certified measurement.
What if I only know my ring size in millimeters?
Enter it in the diameter or circumference field — whichever you measured — and every other system fills in from that same number, since diameter and circumference are exact, fixed conversions of each other.
Is my data sent to JewelModel?
No. Every calculation runs in this browser tab using plain arithmetic; nothing is uploaded or stored.
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