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Ring size converter: find your size in US, UK and European sizing

To find your ring size, measure the inside circumference of your finger in millimeters: 54.4 mm is a US size 7. The converter below works from your finger’s circumference, the inside diameter of a ring that already fits, or a size you already know, and gives US, UK, European and Brazilian sizes.

Measure at the end of the day, when fingers are warm, and make sure the ring clears your knuckle. String and paper methods can be off by half a size or more; for a wedding band, confirm at the jeweler.

How to measure your finger at home

  1. Wrap a strip of paper or thin string around the base of your finger, snug but not tight.
  2. Mark where the end meets the strip and measure that length in millimeters with a ruler: that is the circumference.
  3. Check your knuckle: the ring has to slide over it. If your knuckle is wider than the base of your finger, use the knuckle measurement.
  4. Measure at the end of the day at room temperature. Fingers are smaller in the morning and in the cold.

You can also measure a ring that already fits the same finger: use a ruler to measure the inside diameter, edge to inside edge, and choose “Inside diameter of a ring that fits”.

How the conversion works

  • US: each whole size adds about 0.81 mm to the inside diameter. A size 7 measures 17.3 mm across and 54.4 mm around.
  • UK: letters, with each letter adding 1.25 mm of circumference. A US 7 is about an N½.
  • European (ISO 8653): the size is simply the inside circumference in millimeters.
  • Brazilian and Italian: the circumference in millimeters minus 40.

When your measurement falls between two sizes, the tool points you to the larger one. A slightly loose ring spins; a tight one will not clear your knuckle.

When the measurement can mislead you

Wide bands usually need a half size up, because they cover more skin. Jewelers can also differ slightly in how they size. For a wedding band, or any ring that cannot be resized, use this as a starting point and confirm with a ring sizer at the jeweler.