mm to US Ring Size Conversion
| Diameter (mm) | US |
|---|---|
| 14.9 | 4 |
| 15.3 | 4.5 |
| 15.7 | 5 |
| 16.1 | 5.5 |
| 16.5 | 6 |
| 16.9 | 6.5 |
| 17.3 | 7 |
| 17.7 | 7.5 |
| 18.1 | 8 |
| 18.5 | 8.5 |
| 19 | 9 |
| 19.4 | 9.5 |
| 19.8 | 10 |
Sizing conventions vary by brand and by cut — this table shows the standard published conversion. Always check the garment's own size chart when one is available. See how these tables are compiled.
The most direct path to an accurate ring size
A millimetre measurement — either the ring's internal diameter or its internal circumference — is the most literal, unambiguous way to describe a ring size, since it's the actual physical measurement rather than a coded scale the way US, UK, or (to a lesser extent) EU sizing are. Converting mm to US sizing means translating a real number into the US arbitrary scale, the reverse of what most people do (measuring a US ring they already own and hoping the size transfers).
If you're converting a jeweler's stated internal diameter (rather than circumference) to a US size, remember diameter and circumference are two different figures for the same ring (circumference is roughly diameter multiplied by pi) — a common source of ordering mistakes is applying a circumference chart to a diameter figure or vice versa, so confirm which one you actually have before converting.
Who typically starts from a millimetre figure
This direction is most common for a first-time buyer with no existing US ring to reference — a gift-giver measuring a partner's sleeping ring, or someone using a printable paper-strip sizer at home for the first time — see this site's at-home ring measuring guide for the measuring method itself, including how temperature and time of day shift the true figure.
Why the US number you land on is a range, not a single point
Because the US scale steps in whole and half sizes (occasionally quarters at specialty jewelers), a millimetre figure that falls between two published US steps rounds to the nearest one — for a gift where you can't ask the recipient to try it on, rounding up slightly is generally the safer error than rounding down.
Why a printable at-home sizer is worth using alongside a tape measurement
Many jewelers offer a free printable ring sizer (a strip with graduated holes or a wrap-around gauge) that tends to be more accurate than a paper-strip-and-ruler estimate, since it’s calibrated directly to real ring sizes rather than requiring you to do the circumference-to-size arithmetic yourself.
Whichever method you use to reach a millimetre figure, measuring more than once and on a different day is worth doing before a significant purchase, given how much normal daily fluctuation affects the reading.
Frequently asked questions
What's the difference between a ring's diameter and its circumference in millimetres?
Diameter is the straight-line width across the inside of the ring; circumference is the distance around the inside edge, roughly diameter multiplied by pi — confirm which figure you have before converting, since applying the wrong chart is a common mistake.
I measured a partner's ring while they were asleep — how reliable is that figure?
Reasonably reliable if you measured the ring's actual inside diameter with a ruler rather than guessing, though a slightly worn or bent ring can measure a touch off — if the size matters a lot, rounding up half a size is the safer gift-buying error.
Why would I need my ring size in millimetres instead of just a US number?
Some jewelers, international retailers, or ring-sizing tools work exclusively in millimetres rather than the US system, so having an accurate millimetre figure avoids an extra layer of conversion uncertainty.
What if my millimetre figure falls exactly between two US sizes?
Round up rather than down for anything you cannot try on in person first — a US size that runs a touch loose is a far smaller problem than one that does not fit over the knuckle at all.