Ring size chart: inner diameter and five scales

What the size actually names

One circle, and nothing else. The European size is the length of that circle: 53 mm around. The Russian size is the diameter of the same circle: a size 17 ring is 17 mm across inside. Between the two sits exactly π, so no conversion table is needed — it would be multiplication, printed.

The US, UK and Japanese scales work differently: they are step counters, each from its own zero. The US scale starts at a diameter of 11.63 mm and steps by 0.032 inch. The UK scale runs along the same physical line at half that step and prints a letter, so one US size covers two letters. The Japanese scale starts at 13 mm and steps by a third of a millimeter.

Ring size chart

How to measure your finger

The most reliable way is to measure a ring already worn on that finger: lay a ruler across the inside edge and read the diameter. That reading is the inner diameter — a Russian size outright, and the row you want in the chart.

With no ring to hand, wrap the finger in string or a strip of paper — snug, but not pulled tight — mark where the ends meet and measure to the mark with a ruler. That gives you the circumference, the second column of the chart. Measure in the evening and when you are warm: a finger swells over the day and shrinks in the cold, and morning and evening readings can differ by half a size. And measure the finger you mean, on the hand you mean — fingers on your dominant hand usually run slightly larger.

What no chart knows

Whether the ring will clear your knuckle. A finger is not a cylinder: the widest circle the ring has to squeeze past is not the circle it settles on afterward. If the knuckle is wider than the base, you size to the knuckle, and the ring sits looser than the chart says. One diameter cannot tell you that — and the chart above, like every other chart, cannot answer it either: it takes a single number in.

The ring scale asks twice. Hold the base of your finger to the screen, then the knuckle — the same gesture, just higher up — and you get the size for the larger of the two circles, plus how loosely the ring will sit at the base, in millimeters and in steps of whichever scale you picked. If the gap is more than a size and a half, the app says so outright: what you need here is not a bigger size but a fitting inside the band — beads or a spring that hold the ring at the base without stopping it from clearing the joint.

And the width of the band itself. At the same size, a wide ring sits tighter than a narrow one because it touches more of the finger; jewelers usually add a quarter to half a size once the band is wider than about six millimeters. That is a property of the ring, not of the finger, which is why the measuring screen puts it on a slider: the answer is recalculated for the ring you have in mind, not for an average one.

The second measurement takes as long as the first, and one tap skips it. But it is the one that answers the single question behind a correctly sized ring that will not go on — and no size chart, and no set of sizing rings, asks that question at all.

Where each column’s number comes from

Two of the five scales are definitions, and the only way to get them wrong is rounding. The other three are straight lines fitted to the anchor points every chart prints: US 6 = 16.51 mm = ISO 52, US 7 = N½ in UK letters, Japanese 13 = 17.0 mm. Hitting those points is the check: a line fitted wrong would drift away from them at the ends of the range.

If there is no ring to hand

Then you measure the finger, and there is a separate screen for it: lay the finger across the field and bring two lines in to its edges. The answer there is approximate, for the reason given above — the screen knows nothing about the knuckle. From the same place the result goes out as a link, for when the size is not for you but for the person you are buying the ring for.