GuidesRing size

How to measure your ring size, and how far off each method can be

From one ring size to the next there are 0.81 mm of diameter, barely more than the thickness of a credit card, which is 0.76 mm. With margins that small, a thick string or a badly printed sheet can move you a whole size without you noticing, so instead of the usual tips, this guide puts a number on how far off each method can be, and shows how to avoid it.

Updated September 24, 2026

The short answer. The most reliable way at home is to measure, with calipers, the inside of a ring that already fits that same finger well, and to convert the measurement with the calculator on this page. If you have neither calipers nor a reference ring, leave the size on «To be confirmed» in the configurator and we confirm it with you before casting.

What a ring size is

The US scale, also used in Canada, Mexico and Colombia, adds 0.8128 mm to the inside diameter of the ring with each whole size, and size 0 measures 11.63 mm. In Europe, the ISO 8653 standard defines the size as the inside circumference in millimeters, so a US 7 is a European 54. The chart comes from those two rules; jewelers' charts can differ by a tenth of a millimeter depending on how they round.

US size	Inside diameter	Inside circumference	European size
5	15.7 mm	49.3 mm	49
5 1/2	16.1 mm	50.6 mm	51
6	16.5 mm	51.9 mm	52
6 1/2	16.9 mm	53.1 mm	53
7	17.3 mm	54.4 mm	54
7 1/2	17.7 mm	55.7 mm	56
8	18.1 mm	57.0 mm	57
8 1/2	18.5 mm	58.2 mm	58
9	18.9 mm	59.5 mm	60
9 1/2	19.4 mm	60.8 mm	61
10	19.8 mm	62.1 mm	62
10 1/2	20.2 mm	63.3 mm	63
11	20.6 mm	64.6 mm	65
11 1/2	21.0 mm	65.9 mm	66
12	21.4 mm	67.2 mm	67
12 1/2	21.8 mm	68.5 mm	68
13	22.2 mm	69.7 mm	70

How far off each way of measuring can be

Because sizes move in fractions of a millimeter, any measuring error gets multiplied: one millimeter of diameter is 1.2 sizes, and one millimeter of circumference is 0.4 of a size. With that, you can work out how far off each method can be.

Method	Where the error comes from	Error in sizes
A ring that already fits, measured with a ruler	Where the error comes from: You can read half a millimeter, up or down	Error in sizes: 0.6 of a size, either way
The same ring, measured with calipers	Where the error comes from: Five hundredths of a millimeter	Error in sizes: Less than 0.1
A 1 mm string around the finger	Where the error comes from: Its thickness adds 3.1 mm to the circumference	Error in sizes: 1.2 sizes too big
A paper strip around the finger	Where the error comes from: Its thickness adds 0.3 mm	Error in sizes: 0.1 too big
String or strip pulled 2 mm too tight	Where the error comes from: The skin gives and the circumference drops	Error in sizes: 0.8 too small
An A4 sizer printed on Letter paper with «fit to page»	Where the error comes from: Everything comes out at 94%	Error in sizes: 1.3 too small at size 7

A string fools you with its own thickness: wrapped around the finger, the length you mark runs along the middle of the string and not along the skin, so you end up measuring the circumference of a bigger circle. With a 1 mm cord that is already more than a whole size, while with a paper strip the difference barely shows. The opposite error is pulling tight, because the skin gives, and 2 mm less of circumference is enough to drop almost a whole size.

Printable sizers have their own trap across the Americas, where the everyday sheet is Letter and not the European A4: if the file comes in A4 and «fit to page» is on when you print, everything comes out at 94% or less, and the size 7 circle ends up the size of a 5 3/4. That is why those sizers carry a control line; measure it with a real ruler before using the sheet and, if it does not measure what it says, print again at actual size, 100%.

Your finger does not measure the same all day

In the cold, the blood vessels in your fingers close and the finger slims down; in the heat, or after a long walk, it swells. Half a size is only 1.3 mm of circumference, so it does not take much: measure first thing on a cold morning, then travel somewhere hot, and the ring will feel tight. What jewelers do is measure in mid afternoon, with hands at a normal temperature, and repeat the measurement on another day before deciding.

The knuckle and the band width

The ring has to pass over the knuckle, and on many hands the knuckle is thicker than the base of the finger. If that is your case, measure both and choose a size in between, closer to the knuckle, so the ring slides on without forcing and does not fall off.
A wide band covers more of the finger's length and feels tighter than a thin one of the same size. The Hinge Ring is 5.5 mm wide, so if you measure with a thin ring, 2 or 3 mm wide, and you land right between two sizes, choose the larger one.
Measure the same finger of the same hand you will wear it on: the fingers of one hand and the other do not always match.

Why the Hinge Ring cannot be resized afterwards

A plain ring is resized by cutting it and removing or adding metal, or by stretching it on a mandrel. The Hinge Ring is eight links joined by hinges, each with its own bezel-set cubic zirconia, all the way around: there is no plain stretch to cut, and stretching it would deform the links and loosen the stones, which is exactly what happens with eternity rings. That is why the size is confirmed before casting, not after.

Convert your measurement to a size

Type the inside diameter of a ring that already fits you, or the circumference of your finger if you measured it with a paper strip.

Calculator

Inside diameter, in mm

Type the measurement in millimeters.

Would you rather not measure? In the Hinge Ring configurator, leave the size on «To be confirmed»: before casting, we tell you how to measure it with a ring that already fits, or we take it with you in Bogotá.

Sources