US Size: The standard American ring size number (e.g., 5, 7, 10). This is the most commonly used system at U.S. and Canadian jewelers.
UK Size: The British ring size letter (e.g., J, N, R). Used in the UK, Australia, and some Commonwealth countries.
EU Size: The European ring size, which equals the inner circumference in millimeters rounded to a whole number.
Matched Diameter: The standard diameter and circumference closest to your measurement. The tool snaps to the nearest whole US size.
How This Calculator Works
You enter the inner diameter or circumference of a ring (or your finger) in millimeters. If you enter circumference, the tool divides by π to get diameter. It then finds the closest match in a standard ring-size table and returns the US, UK, EU, and Japanese equivalents. The table covers the most common range (US 5–10).
Quick Questions
How do I measure my ring size at home?
Wrap a thin strip of paper or string snugly around the base of your finger, mark where it overlaps, then measure the length in millimeters. Enter that as the circumference. Alternatively, measure the inner diameter of a ring that fits well.
Should I round up or down between sizes?
Most jewelers recommend rounding up, especially for wider bands (over 6 mm) or if you have larger knuckles. A slightly loose ring is easier to resize than one that's too tight.
Does finger size change throughout the day?
Yes — fingers are typically smaller in the morning and when cold, and larger in the evening or when warm. For the most accurate measurement, measure at room temperature in the afternoon.
Is this accurate enough for an engagement ring?
This tool gives a reliable starting point, but for a high-value purchase like an engagement ring, it's best to have a jeweler measure the finger in person with a professional ring mandrel.