The conversion is simple arithmetic. What decides whether the answer is right is the measurement you give it — and a few things about the ring itself.
Enter a circumference or an inside diameter in millimetres.
Your size
This page explains how the conversion works and how far to trust it. For a complete overview, see the ring size calculator itself.
A ring size calculator does one job: it converts a millimetre measurement into the size labels used in different countries. The arithmetic is trivial. What determines whether the answer is right is the measurement you feed it, and whether the ring you are buying behaves the way the chart assumes.
This page covers how the conversion works, how much confidence the result deserves, and what to do when the number is not clean. If you simply want the answer, the calculator on the homepage gives it in one screen.
Two steps, both simple.
There is no model, no estimation and nothing proprietary. Any calculator doing something more elaborate than this is adding assumptions rather than accuracy.
A US half size is about 1.25 mm of circumference, or 0.4 mm of diameter. Those are small distances, and that is the single most useful thing to understand about ring sizing.
| US / Canada | Inside circumference | Inside diameter | UK / AU / IE | EU / ISO |
|---|---|---|---|---|
| 6 | 51.9 mm | 16.5 mm | L½ | 52 |
| 6.5 | 53.1 mm | 16.9 mm | M½ | 53 |
| 7 | 54.4 mm | 17.3 mm | N½ | 54 |
| 7.5 | 55.7 mm | 17.7 mm | O½ | 56 |
| 8 | 57.0 mm | 18.1 mm | P½ | 57 |
It means a measuring error you would not notice — a strip pulled slightly tight, a ruler read a millimetre out, a finger measured before breakfast — is enough to move the result by a half size or a full one. It also means the difference between two adjacent sizes is real and worth getting right, not a rounding detail.
Not every retailer stocks them, but many jewellers will make one on request. European sizing offers finer steps still: because the ISO size is the circumference in millimetres, EU half sizes are half a millimetre apart rather than 1.25 mm. How EU sizing works
Your measurement is within about a quarter of a half size of a standard size. That is as clean a result as this method produces. If the measurement itself was taken carefully, you can order from it.
Your circumference sits meaningfully between two listed sizes. This is common and not a fault in the measurement. Sizing up is usually right, because a loose ring can be worn now and adjusted later while a ring that will not clear your knuckle cannot be worn at all — but there are genuine exceptions. Working through the decision
You entered both a circumference and a diameter, and they do not describe the same ring. Since circumference is always diameter multiplied by pi, one of the two was measured wrong. The tool follows whichever field you edited last, but the honest answer is to measure again. Common measuring errors
Your reading falls outside US 3 to 13. Usually this means a circumference was entered in the diameter field, or a measurement was taken in centimetres. Genuine sizes outside this range do exist, but check the input first.
The obvious alternative to measuring is picking a common size and hoping. Here is what the extra five minutes buys you.
What it does not buy you is certainty. The section below is the honest other half of that list.
The conversion is reliable. The things it has no visibility of are what cause bad fits:
These are not competing options so much as three points on a scale of reliability, and it is worth knowing where each one sits.
Because everything depends on the input, it is worth spending the extra few minutes on it.
Whichever you use, take three readings in the late afternoon at normal room temperature. Readings within about half a millimetre of each other mean the measurement is stable enough to order from. All four methods compared
Ring sizing figures get copied between websites without anyone verifying them, so an error introduced once spreads quietly. Rather than ask you to take these numbers on faith, here is what stands behind them — each of which you can check.
Ring sizing is not governed by a single enforced standard. Published charts differ between manufacturers by up to roughly a quarter of a size, and a home measurement adds error of its own. Treat every figure here as a close estimate. For a ring that cannot easily be resized, have the finger measured on a metal ring sizer by a jeweller before you order.
The conversion itself is exact — it is arithmetic against a published table. The accuracy of the answer is entirely inherited from your measurement, and a home measurement can easily be half a size out. For an engagement ring or anything hard to resize, confirm with a jeweller.
About 1.25 mm of inside circumference, or roughly 0.4 mm of inside diameter. A full US size is about 2.5 mm of circumference. Those distances are small enough that ordinary measuring errors can move you a whole size.
You entered both a circumference and a diameter that do not describe the same ring. Circumference is always diameter multiplied by pi, so if the two do not reconcile, one was measured incorrectly. The result uses whichever field you edited last, but measuring again is the better answer.
For an everyday ring, yes. For an engagement ring, a wedding band, or anything in tungsten, ceramic or a full eternity setting, no — a jeweller with metal sizing rings lets you feel the actual fit and test knuckle clearance, which no calculator can do. Sizing is normally free and takes a few minutes.
Whichever you measured. Circumference is the distance around your finger, from a paper strip. Inside diameter is the distance across the inside of a ring you own. If you enter both, the calculator uses whichever field you edited last and flags any disagreement between them.
The tool itself, with the full conversion chart underneath it.
Get a measurement firstFour methods, ranked by how much error each introduces.
When the result is between sizesHow to decide which way to round, and when to size down.
The full size chartEvery size from US 3 to 13 in all systems, printable.