Convert instantly between US, UK, Japan and international sizing — or measure from scratch with what's already in your drawer.
Sizing methodology reviewed for accuracy against industry references
Calibrated to the ISO 8653:2016 international ring-sizing standard.
Pick the system and size printed inside a ring you already own, and we'll convert it across every other system.
Best measured at the end of the day, when fingers are at their largest — cold hands measure slightly small.
Lay a ring that already fits flat on a ruler and measure straight across the inside edge — that's the internal diameter.
Wide bands (6mm+) tend to fit tighter — many people size up half a size for comfort.
Pick a size above to see it converted here.
Finger size shifts a little with temperature and time of day — check anything that applied.
Select any conditions above for a quick calibration tip based on when you measured.
Nothing saved yet — convert a size above and tap "Save this size" to keep it here for next time.
Wider bands and comfort-fit inner edges fit differently than a thin, flat band of the same labeled size.
A 2mm standard band at your usual size fits true to size — no adjustment needed.
A rough, private estimate for planning a surprise proposal — answer what you can, skip what you can't.
What's their height / body frame?
How do their hands compare to average?
Shoe size (optional — just a loose reference)
Shoe size alone doesn't predict ring size reliably — it's just a light supporting clue, not a factor we calculate with.
Estimated range
This is a starting point for a surprise purchase, not a guarantee — many jewelers offer free resizing within the first 30–60 days for exactly this reason.
Preview the calibration line below, then print at 100% scale — no "fit to page" — for an accurate strip.
Full charts, printable tools, and step-by-step guides for every region.
Full US sizing table with fit notes.
Letter-based sizing, explained.
Numeric sizing used across Japan.
Every major system, side by side.
A calibrated strip you print at home.
Every method, with common mistakes.
Size a ring you already own.
Swelling, half sizes, gifting, and more.
Sizing tips, buying guides, and answers to the questions we hear most.
Prefer to watch instead of read? Here's a quick video walkthrough.
Every size we support, cross-referenced across every major sizing system.
| US / Canada | UK / AU / NZ | Japan / China / Asia | EU / ISO (circumference mm) | Inside diameter (mm) | Inside circumference (mm) |
|---|
No sizes match that search.
Most of the world doesn't share one ring-sizing standard. The US and Canada use a numeric scale, the UK and Ireland use letters, Japan uses its own numeric scale based on internal circumference, and much of continental Europe measures diameter in millimeters directly. All of them describe the same thing — the internal circumference of the ring — just on different scales.
That's why a size that fits perfectly at home can come out oddly specific abroad, like a US 7 landing between a UK N and O. This converter cross-references all of those scales against the same physical measurement, so the sizes it gives you should match, regardless of which system a jeweler local to you uses.
Finger size shifts slightly with temperature, time of day, and salt intake, so treat any home measurement as a strong estimate. For an engagement ring or a size you can't easily change, confirming with a local jeweler's ring gauge is worth the extra step.
Every size on this site is derived from a single physical measurement: the ring's inside circumference. Circumference and diameter are related by the standard formula C = π × d, so once we know a ring's inside diameter in millimeters, its circumference — and therefore its size on any national scale — follows directly from that constant.
Our conversion table is aligned with the ISO 8653:2016 international ring-sizing standard, which defines ring size as a function of inside circumference in millimeters and provides the reference points that most national systems (including the UK's Sizing Standard 148:2016 and Japan's JIS-based scale) are calibrated against.
Home measurement — whether with a string, paper strip, or an existing ring — typically carries a margin of error of around ±0.1mm to ±0.3mm in measured diameter, which can shift the resulting size by a small fraction of a step. That's well within normal day-to-day finger-size fluctuation, but for a purchase that's expensive or difficult to resize, we recommend confirming the final measurement with a jeweler's calibrated metal ring gauge, which holds a tighter tolerance than any string or paper method.
The questions we hear most from people sizing a ring for the first time.
Size up rather than down. A ring that's a touch loose can be resized smaller fairly easily and cheaply; a ring that's too tight often can't be resized larger at all, especially with eternity bands or channel-set stones.
No — each system uses its own scale, so the numbers or letters don't match up directly. A US 7 lines up with a UK size N and a Japan size 14, because all three describe the same physical internal circumference, just labeled differently. That's exactly what the converter above handles for you.
Yes. Fingers swell slightly in heat and shrink in the cold, and most people measure a touch larger in the evening than first thing in the morning. For the most reliable result, measure at room temperature, later in the day, and avoid measuring right after exercise or a salty meal.
It's a strong estimate, usually within half a size, which is enough for most everyday rings. For something you can't easily resize — an engagement ring or a wide eternity band — confirm with a jeweler's metal ring gauge before ordering.
Measure both the knuckle and the base of the finger, then size for something in between, leaning slightly toward the knuckle measurement so the ring can still pass over it. A jeweler can also suggest a comfort-fit inner edge, which makes it easier to get over a larger knuckle without sizing up as much.
Most rings in precious metals like gold or platinum can be resized up or down by about two sizes. Rings with eternity settings, inlays, or certain alternative metals like tungsten or titanium often can't be resized at all, so it's worth checking with the jeweler before you buy.
Free ring size conversion and measurement tools for US, UK, Japan, and international sizing systems.
© Rings Sizer. Sizes are estimates — confirm in person for a permanent purchase.