How this instrument works
Circumference is usually the easier of the two ring measurements to take at home, because it doesn't require the ring to sit flat under a caliper the way a diameter reading does. Wrap a strip of paper or a piece of string snugly around the inside of the ring, mark where the ends meet, then measure that length — that's the inside circumference, and it's exactly what this calculator expects, in centimeters.
From there the conversion is direct: multiply by 10 for millimeters, divide by π for the equivalent inside diameter, and run the millimeter circumference through a linear size formula — a step of 2.55 mm per whole US size — to get the approximate US/Canada ring size.
That 2.55 mm-per-size step is the same jewelers' trade convention behind this site's Ring Size Calculator page (which runs the US-size-to-mm conversion in the other direction), so results here land in close agreement with that page rather than being a separate, unrelated formula. Neither is governed by ISO or ANSI — US ring sizing is an industry convention refined over decades of shared manufacturing practice, not a written standard, which is worth knowing before treating any sizing chart as precise to the hundredth of a millimeter.
- Wrap a strip of paper or a piece of string snugly around the inside of the ring, mark the overlap, and measure that length in centimeters.
- Enter the measurement into Inside circumference (cm).
- Read Inside circumference (mm), a straight ×10 conversion.
- Read Inside diameter (mm), the circumference divided by π.
- Read the estimated US/Canada ring size, and round to the nearest quarter size your jeweler stocks.
Worked example — a 5.44 cm string measurement
Wrapping a strip of paper around the inside of a ring and marking the overlap gives a length of 5.44 cm. In millimeters that's 54.4 mm (5.44 × 10) — which happens to be a commonly published reference figure for a US size 7 inside circumference.
Running that through the size formula: (54.4 − 36.5) ÷ 2.55 = 7.02. That lands almost exactly on US size 7, confirming the string-and-ruler measurement against the published circumference table. Inside diameter, for reference, works out to 54.4 ÷ π = 17.32 mm.
Questions
Why measure circumference instead of diameter?
Circumference is easier to capture accurately with household tools — a strip of paper or string wraps snugly around a curved edge in a way a flat ruler can't measure a diameter through. It's also the same measurement a printable paper ring sizer or a string-method sizing guide uses, so if you've already done that at home, this circumference figure is the one you already have.
Does the string stretch or compress the measurement much?
A little, and it's the main source of error in this method — pull the string too tight and you'll undersize the ring; leave it loose and you'll oversize it. Wrap it snugly enough to sit against the inside edge without stretching, mark the overlap precisely, and measure the string flat against a ruler rather than trying to read a curved loop directly.
Is this US ring-size formula an official ISO or ANSI standard?
No — it's a jewelers' trade convention built from decades of converged industry practice, not a document from a formal standards body. The genuinely standardized system is ISO 8653, used widely across continental Europe, which defines ring size directly as inside circumference in millimeters rather than converting through a numbered US-style scale.
My result doesn't land exactly on a whole US size — what should I order?
Round to the nearest quarter size your jeweler stocks. A result like 7.02 is close enough to size 7 that ordinary measurement variance (from ruler reading, string tension, or even finger swelling that day) accounts for the gap — treat anything within about a tenth of a size as that whole or quarter size rather than chasing false precision.
I measured the ring's diameter instead of its circumference — what do I do?
Use this site's companion CM to MM Ring Size Converter, which takes an inside diameter measurement directly and derives circumference and US size from it, so you don't need to convert between the two by hand first.