US and UK half sizes differ because the two systems move up in steps of different sizes: a US half size is a bigger jump in circumference than a UK half letter. That means a UK size and a US size often fall between each other's steps, so a straight swap from one label to the other can leave a ring slightly tight or slightly loose.
In short: convert using a millimetre measurement, not by matching labels, and tell your jeweller which system you measured in.
How the two ladders are built
Think of each system as a ladder against the same wall. The UK ladder uses letters, from A upwards, with a half letter between each. The US ladder uses numbers with half and quarter steps. Both ladders are measuring the same thing, the inside circumference of a ring, but the rungs are spaced differently.
The UK rungs are closer together. So moving up one UK half size adds a smaller amount of circumference than moving up one US half size. Over a few sizes the rungs drift in and out of line, which is why some UK letters sit very close to a US half size and others land roughly a quarter size away.
Why this matters for a bespoke ring
For a ring you might return to a shop, being a fraction out is an inconvenience. For a bespoke ring, especially one set with stones all the way round, it can be harder to change once the ring is finished. Full eternity bands and some patterned designs have little or no room to be resized, so getting the size right before making matters more.
US buyers are also used to quarter sizes being common, while UK sizing tends to talk in whole and half letters. If you are an American buyer and your finger sits at a quarter size, say so. A bespoke maker can work to the exact measurement rather than rounding to the nearest UK letter.
Photo: Anthony Shkraba via Pexels.
Converting without losing a size
- Start with millimetres. Ask for, or measure, the inside circumference. That number means the same thing everywhere.
- Use one chart. Read the millimetre figure across a single chart, such as the one on our ring sizing page, rather than going US to UK and back again.
- Say which system you measured in. "US 6.25 from a jeweller's sizer" is more useful than "about a UK M".
- Mention the band width. A broad band can feel snugger than a slim sizer, which can tip a borderline size either way.
- Check the knuckle. If your knuckle is wider than the base of your finger, the right size is the one that passes the knuckle comfortably and does not spin too much once on.
When a jeweller's sizer helps
Paper strips and printed sizers are fine for a first estimate, but a set of metal or plastic sizing rings with a band like the one you are ordering gives a better feel for fit. If you are measuring in the US with American sizers, keep the US reading and let your jeweller convert it from the measurement. That avoids two people each rounding in a different direction.
If you want the common matches in one place, our US to UK ring size conversion guide lists them, and the sizing page has the fuller table.
Frequently asked questions
Why do US and UK half sizes not match?
The two systems step up by different amounts of circumference. UK half letters are smaller steps than US half sizes, so the labels drift in and out of line as sizes go up.
Is a US 6.5 the same as a UK half size?
A US 6.5 sits close to a UK letter but not always exactly on one. Convert from the inside circumference in millimetres to get the closest match.
Can a bespoke ring be made in a quarter size?
A bespoke ring is made to a measurement, so it can usually be made to a quarter size or an exact millimetre figure. Tell your jeweller the measurement and the system you used.
What is the safest way to convert a ring size?
Measure the inside circumference in millimetres, read it across one chart, and tell your jeweller which system and method you used.