Your U.S. ring size, in two minutes

American sizes are a scale: each size stands for one inner diameter. Size 6 is an inner diameter of 16.5 mm, size 13 is 22.2 mm — 0.65 to 0.87 of an inch, end to end across the opening. Our rings cover that whole range; outside it, write to us before ordering.

A tape wrapped around the finger gives you something else entirely: the circumference, roughly three times the diameter. Size 6 measures about 2.04 inches around, size 13 about 2.74. Both numbers are right, and they measure two different things: mixing them up shows only once the box arrives.

On a ring you already own, the diameter is read on the INSIDE: inner edge to inner edge, straight through the middle. Measuring the outside adds a millimetre or two, which is one to three sizes.

So: a strip of paper gives a circumference, a ruler laid across an existing ring gives a diameter. Establish whether you hold a circumference or a diameter before you look at any chart.

Cut a strip of paper about a third of an inch wide, or use a length of thread. Wrap the strip around the base of the finger without pulling — snug enough to stay, loose enough to turn.

Mark the overlap, then lay the strip flat against a ruler. Most American rulers show sixteenths of an inch on one edge and millimetres on the other; read the millimetre edge if you have it, since the chart is built on millimetres and no rounding is needed.

The knuckle decides the rest. If your knuckle is wider than the base of the finger, measure the knuckle as well and keep the larger of the two readings: the ring has to pass the knuckle to be worn.

Measuring a ring that already fits is the most accurate route, and the quietest. Take a ring worn comfortably on the same finger, set the ring on a ruler, and read the inner diameter across the opening.

Find that figure in the diameter column below and take the size beside that figure. Half a millimetre — two hundredths of an inch — moves you a full size, so measure twice, in daylight.

Heat, salt, exercise and altitude make a finger swell; cold and early mornings make a finger shrink. Between a January morning and a July evening the same finger can move a full size. If you live somewhere with real summers, measure in the season you will wear the ring.

Measure late in the day, at room temperature, away from a workout and with warm hands. Two attempts, two answers: keep the larger one.

For a gift, asking outright gives the surprise away. Three routes work.

Borrowing a ring they already wear, long enough to lay the ring on a ruler, stays the safest. Failing that, press the ring onto a sheet of paper and trace the inside circle — the line can be measured later, at your own pace.

With no ring at hand, compare. A finger close in size to one of your own gives a starting point, and you adjust a size up or down from there. This estimate is approximate, and a size slightly large still gets worn.

Once delivered, the rings are final and yours to keep.

A ring with a defect is replaced; the fit is yours to choose, once, when you order.