Ring Size Converter
Computed from the geometry, not a lookup table.
Unlike shoe sizes, this is real geometry
Ring sizes are a straight line. The US scale starts at a diameter of 11.63 mm for size zero and adds 0.8128 mm — exactly one 32nd of an inch — for each full size.
diameter (mm) = 11.63 + 0.8128 × US size
So a US 7 is 17.32 mm across and 54.4 mm around. Those are measurements, and every figure on this page is computed rather than looked up.
The UK system is letters, and it is half-steps
UK and Australian sizes run A, B, C upwards, and each letter is half a US size. That gives the relationship:
UK letter position = (2 × US size) + ½
US 7 lands at position 14.5, which is N½. US 8 is P½. The half-letter sizes are why UK rings are written as "N½" rather than a plain letter.
European sizing is just the circumference
An ISO or European ring size is the inside circumference in whole millimetres. A size 54 ring measures 54 mm around, which makes it the most transparent of the three systems.
Measuring correctly, which matters more than the conversion
A ring that is one size out is noticeably wrong: it either will not pass the knuckle or it turns and slides. Half a size is about 0.4 mm of diameter, which is very little room for error.
- Measure at the end of the day. Fingers are largest then, and swell in heat.
- Be warm. Cold fingers can be a full size smaller. The classic mistake is measuring after coming in from outside.
- Wrap a strip of paper round the base of the finger, mark the overlap, and measure the length — that is the circumference.
- Check the knuckle. If it is much larger than the base, you need a size that passes it, and the ring will be loose below. A jeweller can fit sizing beads for this.
- Measure three times on different days.
Wide bands fit tighter
A band 6 mm or wider sits against more of the finger and feels noticeably snugger than a thin one of the same size. Jewellers commonly advise going up a quarter to a half size for a wide band.
String is a poor method
It stretches and it cuts in, which makes the reading too small. Paper is better. Best of all is a proper ring sizer, or measuring an existing ring that fits the same finger.
Common uses
- Buying from an overseas jeweller using a different system.
- Converting a measured circumference into a size to order.
- Checking a ring you already own by measuring its inside diameter.
- Buying a surprise gift, by borrowing a ring and measuring it.
Questions people ask
What is a US size 7 in UK sizes?
N½. Each UK letter is half a US size, so the position is (2 × 7) + ½ = 14.5, which is N and a half.
How do I measure my ring size at home?
Wrap a strip of paper round the base of the finger, mark where it overlaps, and measure the length. That is the inside circumference.
Does temperature affect ring size?
Considerably. Cold fingers can be a full size smaller. Measure when warm, at the end of the day.
What is a European ring size?
The inside circumference in millimetres. A size 54 ring is 54 mm around.
Should a wide band be a different size?
Usually a quarter to a half size larger. A wide band contacts more of the finger and feels tighter.
Can I use string to measure?
It is unreliable — string stretches and cuts in, giving a reading that is too small. Paper is better.
Worked out in the page
Nothing is sent anywhere. The tables are built into the page and the arithmetic runs in your browser, so it works with no connection.