Ring Size Converter

Why no two charts agree

Look up a ring size conversion and then look up another, and the two will usually differ. That is not carelessness. The scales are structurally incapable of lining up.

A US size step is 2.55 mm of inner circumference. A British letter step is exactly half that, 1.28 mm. Which means the two scales are offset, and every whole US size lands on a UK half-letter:

And it works the other way too: every whole UK letter lands on a US quarter. UK L is US 5.75. UK N is US 6.75. UK P is US 7.75.

So a chart that prints “UK L = US 6” has rounded up by a quarter size, and one that prints “UK L = US 5.5” has rounded down by a quarter. Both are defensible, neither is exact, and the two charts disagree by half a size while describing the same finger.

Which is the practical reason to work in millimetres wherever you can. The circumference is a measurement; the letters and numbers are translations of it, and something is lost in every translation.

Measure the circumference, not the size

The international standard, ISO 8653, defines a ring size as the inner circumference in millimetres. That is what the European scale is — a size 54 ring has a 54 mm inner circumference, with nothing to convert.

To measure a finger, cut a strip of paper about 5 mm wide, wrap it round the base of the finger, mark where it overlaps, and measure the length to the mark. That length is the circumference and the tool above will do the rest. Use paper rather than string, which stretches, and rather than thread, which digs in and reads small.

To measure a ring you already own, measure the inner diameter across the widest point inside the band, and enter that instead. This is usually more reliable than measuring a finger, because a ring that already fits has settled every question about knuckles and comfort. If you can borrow one, do.

What not to do: measure around the knuckle and use that, which gives a ring that spins and falls off; measure over a cold finger in the morning; or take a size from a printable paper ring-sizer, since those depend on the printer scaling the page exactly and most do not.

Your finger is not a fixed size

One US size is 2.55 mm of circumference — which is only 0.81 mm of diameter. The scale is very fine, and the thing it is measuring is not stable at that resolution.

A finger commonly changes by around 2.5 mm of circumference across a day. That is essentially a full size, and more than the half-size step most shops stock. Heat swells fingers, cold shrinks them, salt and alcohol swell them, exercise swells them, and the difference between first thing in the morning and the end of a warm evening is easily a whole size on the same hand.

The consequence is that a single measurement is a snapshot, not a fact. Measure at least three times, on different days, in the evening, and use the largest reading — a ring that is very slightly loose is wearable and a ring that will not come off is a problem. Longer-term changes matter too: pregnancy, weight change, and some medications and conditions all move ring size, sometimes permanently.

The two hands also differ. The dominant hand is usually slightly larger, so measure the finger the ring will actually be worn on.

The knuckle and the band

A ring has to satisfy two conditions that can conflict: it must pass over the knuckle, and it must sit comfortably at the base of the finger without turning. Where the knuckle is much larger than the base, there is no size that does both well, and the answer is a compromise rather than a measurement.

The usual approach is to size to the knuckle — a ring that will not go on is useless — and then deal with the looseness at the base with sizing beads, a small internal bar, or a slightly heavier band that resists spinning. A jeweller can fit any of those; a converter cannot tell you that you need one.

Band width changes the fit too, and this one is predictable. A wide band sits against more of the finger and grips more, so it feels tighter at the same nominal size. The trade allowance is a quarter size for a band over about 6 mm and a half size over about 8 mm — which the calculator applies if you give it the width. A comfort-fit band, rounded on the inside, needs less allowance than a flat one of the same width.

Buying as a surprise

If the ring is a surprise, everything above applies and you cannot ask. A few approaches work better than guessing.

Borrow a ring they already wear on the right finger and measure its inner diameter. This is by far the most reliable method, and it carries all the knuckle information with it. Do check which finger it is worn on — a ring from the right hand is often half a size different from the left.

Trace the inside of a ring on paper if you cannot take it away, or press it into soap or clay. A traced circle measured across gives a diameter good to a quarter size or so, which is enough to order and adjust.

Ask someone who would know, or buy something resizable. Most plain bands can be sized up or down by a size or two without difficulty. Rings that resist resizing are worth knowing about before you buy: full eternity bands, anything with stones set all the way round, tension settings, and rings in tungsten, titanium or ceramic, which generally cannot be resized at all and have to be replaced.

And if in doubt, order slightly large. A ring can be made smaller far more easily than a finger can, and a ring that will not go on cannot even be tried.

Related calculators

Other sizing systems that do not line up:

More in Conversion, or browse all calculators.

Sources and methodology

One of these matters more than the others. ISO 8653 defines the international ring size as the inner circumference in millimetres, which is why the European size is simply that number and why it is the only figure here that is not a convention layered over a convention. The US and British scales are national practice rather than international standards, which is precisely why they do not align — and why this page derives the US relation and checks it against published circumferences rather than copying a chart whose rounding it cannot see.

• ISO 8653:2016 — Jewellery: ring sizes, definition, measurement and designationInternational Organization for Standardization · verified 2026-09-07 · That the international ring size is defined as the inner circumference in millimetres, which is why the European size is that number and why it is the only unambiguous figure on this page
• Assay Office and British hallmarking guidance on the alphabetical ring scaleGoldsmiths’ Company Assay Office, London · verified 2026-09-07 · The British alphabetical scale from A upwards with half sizes, and its use across the UK, Ireland and Australia
• NIST Handbook 44, Appendix C — the international inch as exactly 25.4 mmNational Institute of Standards and Technology, US · verified 2026-09-07 · The inch-to-millimetre factor underlying the US scale, which is defined on an inch-based progression of inner diameter