Ring Size Converter

A ring size is a label for how big the hole in a ring is. Each country uses a different label, but they all describe the same thing: the distance around the inside of the ring. This converter changes that distance into each system's label.

What Ring Size Conversion Measures

It converts labels, not fingers. A US 7, a UK N½ and an EU 54 all describe a ring with an inner circumference of about 54 mm. The tool does not tell you how a ring will feel on your hand.

The Formula

The international standard ISO 8653 defines ring size as the circumference of the largest cylinder that fits inside the ring, written in millimeters and rounded to the nearest millimeter¹. The other systems are fixed steps from a starting point.

EU/ISO size = inner circumference in mm

Inner circumference = inner diameter x 3.14159

US/Canada size = (inner diameter in mm - 11.633) / 0.8128

UK/Australia: each letter adds 1.25 mm, with size A at 37.5 mm

Japan size = 3 x (inner diameter in mm - 13) + 1

The UK letters come from British Standard 6820 (1987), withdrawn in 1993², though the letter scale is still used in the UK, Ireland and Australia³. In the US, each whole size adds 0.032 inch to the inner diameter³. A Japanese size 1 is 13 mm across, and each size adds one third of a millimeter⁴.

How This Calculator Computes Your Result

Choose what you have: a circumference or diameter in mm or inches, or a size in any system. Everything is turned into inner circumference in mm first, and nothing is rounded until the end. EU/ISO sizes round to the nearest whole mm, US sizes to the nearest quarter, UK sizes to the nearest half letter and Japan sizes to the nearest whole number. "Round up" gives the next size when you land between two. UK sizes are shown for A to Z and Japan for 1 to 27. Beyond that the tool says so and does not guess.

What Your Result Means

The table shows the same ring in every system, calculated with the formulas above.

One US size is about 2.55 mm of circumference. One UK letter is 1.25 mm, one EU size is 1 mm and one Japanese size is about 1 mm. So one US size spans about two UK letters. Charts from different jewelers can place a US size half a UK letter away from this table.

How to Measure Correctly

Jewelers agree on a few habits. They are trade convention, not part of any standard.

• Wrap a strip of paper or a thin string around the base of the finger, mark where it overlaps, lay it flat and measure the length in mm⁵. That length is the circumference.

• Or measure the inside diameter of a ring that already fits, across the widest point.

• Measure toward the middle or end of the day, when the finger is at a normal, comfortable temperature. Fingers shrink in the cold and expand in the heat⁵.

• If your knuckle is clearly bigger than the base of the finger, measure both and choose a size in between⁵.

• Wide bands fit more tightly. One major jeweler suggests going up at least half a size for bands over 4 mm⁵.

• If you are between sizes, choose the larger one⁵.

Why Your Finger Size Changes

Your hand is not the same size all day. In a study of 36 healthy adults, hand volume fell 2.7 percent from 8 a.m. to 2 p.m., fell another 1.7 percent by 8 p.m., and rose 4.3 percent overnight⁶. It measured the whole hand, not the finger, and found volume highest in the morning, which does not match the common tip to measure late in the day. The safe habit is to measure on two or three occasions. For scale, one US size is about 5 percent of a 52 mm ring.

Why Ring Size Systems Do Not Match Exactly

The systems grew up separately. The first British standard (1945) used 1/64 inch steps of diameter, and a metric version replaced it in 1987³. In the US, a 1921 Bureau of Standards circular noted that there are many similar but not identical standards³. Some Italian, Spanish and Swiss charts use circumference minus 40 mm, so their size 14 is ISO 54³. Some Japanese charts use the circumference directly, with sizes like 41 or 42⁴. If a chart does not name its system, check it against a ring you know.