Find your ring size

A ring size can be read two ways. The circumference is the length of the thread that goes around the finger: between 52 and 70 millimetres on an adult. The inner diameter is the width of the ring seen face on: between 16 and 22 millimetres.

Both numbers are correct, but they do not stand in for each other. A circumference is roughly three times the diameter. Someone who measures 52 millimetres around and looks for that number in a list of diameters finds nothing, or worse, finds the highest value and orders three sizes too big.

The simple rule: a string gives you a circumference, a ruler laid on an existing ring gives you a diameter. Work out which one you have first, and only then read the chart.

Cut a piece of string, thread, or a strip of paper about a centimetre wide. Wrap it around the base of the finger without pulling: it should hold without marking the skin, and still turn.

Mark where the strip overlaps, take it off, lay it flat and measure from the end to the mark. That gives you a circumference, in millimetres.

One detail decides the outcome: the ring has to clear the knuckle. If your knuckle is wider than the base of the finger, measure the knuckle too and keep the larger of the two. A ring that will not pass the knuckle cannot be worn, however perfect it is at the base.

This is the most reliable, and the most discreet. Take a ring that sits comfortably on the same finger, lay it on a ruler, and measure the inner diameter, from one inner edge to the other, through the centre.

Read that number in the diameter column of the chart below. Half a millimetre changes the size: measure twice, in daylight.

It swells with heat, exercise, salt and altitude. It shrinks in the cold and first thing in the morning. The gap can reach a full size between a finger measured on an empty stomach in January and the same finger on a summer evening after dinner.

Measure at the end of the day, at normal temperature, neither after exercise nor with cold hands. If two attempts give two different results, keep the larger one.

For a gift the question turns delicate, since asking gives the surprise away. Three approaches work.

Quietly borrowing a ring they already wear, long enough to measure it with a ruler, remains the safest. Failing that, a ring can be set on a sheet of paper and its inner circle traced in pencil: the diameter can then be measured at leisure.

With no ring available at all, comparison is what is left. A finger close to the size of your own gives a starting point, and you adjust one size up or down by eye. It is approximate, and that is exactly what the next piece of advice is for.

That advice holds for any ring, but it holds doubly here. A smart ring reads the heartbeat through the skin: too tight, it compresses, and the reading degrades before the moment even starts. Comfortable, it reads better.

The rings are yours once delivered, and they are not returned. A slightly loose ring can be worn; a ring that pinches cannot.

A ring that does not work, on the other hand, is replaced. That is a matter of defect, not of size.