Ring Size Calculator

US ring sizes are a linear scale defined against the inside circumference: US size = (circumference in mm − 36.5) ÷ 2.55. Each full size is 2.55 mm of circumference (about 0.81 mm of diameter); half and quarter sizes subdivide from there. The European “size” is simply the circumference in millimetres, which is why a US 7 is an EU 54. UK and Australian sizes run alphabetically — roughly one letter per half US size — and Japanese sizes are a numbered scale offset from the US one.

The two measurement methods feed the same formula. The string method measures circumference directly: wrap a strip of paper around the base of the finger, mark the overlap, measure the length in millimetres. The existing-ring method measures the inside diameter of a ring that already fits — multiply by π (3.14159) to get circumference. Diameter is easier to measure accurately with a ruler, but you must measure the inside edge, not the outside of the band.

Cut a strip of paper about 10 mm wide and 100 mm long — paper, not string: thread and yarn stretch several millimetres under tension and will read small. Wrap the strip around the base of the finger, where the ring will sit, with the end overlapping. Pull it snug — touching the skin all the way round, but not digging in — and mark where the end meets the rest of the strip with a fine pen. Lay the strip flat and measure from the end to the mark in millimetres. That length is the finger’s circumference; enter it in the calculator above, or read it straight off the conversion chart below (54.4 mm, for instance, is US 7).

Three details decide whether the number is trustworthy. First, the strip must pass the knuckle test: slide the looped paper over the knuckle before marking, because the ring has to make that journey every day. Second, measure three times and take the middle value — wrapping tension varies more than most people’s fingers do. Third, write the number down immediately; a half-millimetre of misreading is a fifth of a size. A cut cable tie works even better than paper: it is stiff, does not stretch, and holds a circular shape while you mark it.

If a ring already fits the finger in question, measure its inside diameter: lay it on a ruler and read straight across the centre from inside edge to inside edge, to the nearest half millimetre. Multiply by π for circumference, or just switch the calculator to the existing-ring method and it does that step. A 17.3 mm inside diameter, for example, is 54.4 mm around — US 7. This is also the reliable way to size someone else’s finger secretly: borrow a ring they wear on the correct finger and measure the ring, not the person.

No ruler precise enough? Use the on-screen sizer below: calibrate your display against a bank card, then lay the ring over the true-size circles until one matches the inside edge exactly. It is accurate to about a quarter size once calibrated — good enough to confirm a measurement, though the printable strip further down beats it for anything expensive.

Fingers are not a fixed size — they change by up to half a US size across a single day. They are smallest first thing on a cold morning and largest in the evening, in heat, after exercise and after salty food. Measure at the end of the day, at room temperature, with warm but not hot hands; a ring sized on a cold morning will feel tight every summer afternoon. If your knuckle is noticeably wider than the base of the finger, measure both and size to the knuckle — the ring must pass it going on, and a slightly loose fit at the base beats a ring that will not go on at all. Some jewellers fit such fingers a quarter size down from the knuckle measurement so the ring does not spin.

Band width changes the fit too: a wide band (6 mm and up) contacts more skin and grips harder, so jewellers routinely recommend going a quarter to half size up for wide bands and comfort-fit profiles. Measure the finger on the hand that will wear the ring — dominant-hand fingers typically run a quarter to half size larger. And treat any single measurement as a draft: three readings on two different days that agree is a size; one reading is a guess.

A ring that is half a size out is not a small problem. Too loose and the ring spins, wears unevenly, and eventually comes off in cold water — fingers shrink noticeably when chilled, which is how so many rings are lost swimming. Too tight and it compresses the finger, leaves a groove, and in hot weather can need cutting off. The sweet spot: the ring slides on easily, then resists slightly coming back over the knuckle.

Resizing is not always an undo button. Plain gold and silver bands resize readily (typically one to two sizes, for a modest bench fee), but eternity bands, tungsten, titanium and tension-set rings generally cannot be resized at all — the stone setting or the metal’s hardness leaves no room to move. Engraving can be damaged by stretching, and plated rings may need replating after the seam is reworked. For anything engraved, plated, tension-set or full-eternity, the measurement is the only chance to get it right — which is worth the three readings and a printed sizer.

Home methods — paper, the on-screen tool, a printed strip — agree with a jeweller’s mandrel within about a quarter size when done carefully, which is enough for most fashion rings and online orders. Go to a counter instead when the stakes rise: an engagement ring, a wedding band worn for decades, anything over a few hundred dollars, or any of the non-resizable styles above. A jeweller sizes with a set of metal sizing rings you actually wear for a minute, in the correct band width — wide sizing rings for wide bands — which catches fit issues a paper strip cannot.

Sizing at the counter is usually free and takes five minutes; you do not need to be buying from that shop, though it is polite to say so. Ask for the size in the scale the seller uses (US numbers in North America, letters in the UK and Australia, millimetre circumference in most of Europe) and note the answer in all four systems from the chart below. If a jeweller’s size disagrees with your home measurement by more than half a size, trust the jeweller — then check whether you measured a different finger, a different hand, or a colder day.

The printable sizer below is a cut-out strip with the US scale printed true to size — the same wrap-and-read design jewellers post out, free. Print it at 100% scale (“actual size”, never “fit to page”), check the 100 mm verification line against a ruler, cut the strip and its slit, then wrap it around the finger numbers-out and pull the pointed end through. The slit edge lands on your size; long numbered ticks are full sizes, short ticks are half sizes. It prints on its own page, sized correctly for both Letter and A4.

A printed strip is the most accurate home method — stiff paper, no stretch, and the scale cannot drift once the 100 mm line checks out. Measure in the evening, do the knuckle test, and if you land between two ticks take the larger for a wide band and the smaller for a thin one.

A paper strip around the finger measures 54.4 mm. The formula: (54.4 − 36.5) ÷ 2.55 = 7.02 — a US size 7. That is 54 mm around in European sizing, UK N½, and Japanese 14. If the same finger measured with a ring: 54.4 ÷ π = 17.3 mm inside diameter, so a ring measuring 17.3 mm across the inside is the same size 7.

Between sizes? Fingers swell through the day and in heat, so measure in the evening and at room temperature. A wide band (6 mm+) fits tighter than a thin one — jewellers usually recommend going up a quarter to half size for bands over 6 mm. And if the knuckle is noticeably wider than the base, size to the knuckle: the ring has to get over it every day.

The full table the calculator matches against, US 3–13 in half sizes. Circumference and diameter follow the ISO formula; UK and JP equivalents are the standard published conversions.