Ring Size in Millimetres: Diameter vs Circumference
The difference between inside diameter and inside circumference, how to convert between them, and which one jewellers actually use.
4 min read
Two millimetre measurements describe a ring, and mixing them up is the fastest way to order the wrong size.
Inside diameter
The inside diameter is the straight-line distance across the inside of the band, passing through its centre. It is what you measure when you lay an existing ring flat on a ruler. Typical adult sizes run from about 14 mm to 22 mm.
Inside circumference
The inside circumference is the distance around the inside of the band. It is what you measure when you wrap string around your finger. Typical adult sizes run from about 44 mm to 70 mm.
Converting between them
The relationship is simple geometry:
- Circumference = diameter × 3.1416
- Diameter = circumference ÷ 3.1416
So a 17.3 mm diameter is a 54.4 mm circumference, and a 60 mm circumference is a 19.1 mm diameter.
Which one do jewellers use
It depends on the market. European and ISO sizing quotes circumference. German sizing quotes diameter. American and British sizing quotes neither directly, using a code instead. That is why product listings from different countries appear to contradict each other when in fact they agree.
When a listing says "size 54", it almost certainly means 54 mm circumference. When it says "17.3", it almost certainly means diameter, because no adult ring has a 17 mm circumference.
Inches
Some American listings give inches. Divide millimetres by 25.4. A 54 mm circumference is 2.126 inches; a 17.3 mm diameter is 0.681 inches. Inches are rarely precise enough at three decimal places, so millimetres remain the better unit for ordering.
Measuring accurately
Measure diameter across the widest interior point and take the average of two or three readings. A ring tilted a few degrees on the ruler reads larger than it is. If you have digital callipers, measure the inside edge to inside edge; that is the figure jewellers expect.
For circumference, use a strip of paper rather than string, because string stretches. Mark the overlap with a fine pen and read against a steel ruler.
Half a millimetre matters
One US size is 0.81 mm of diameter, or 2.55 mm of circumference. A half size is about 0.4 mm of diameter. That means a measurement error of half a millimetre moves you a full half size, which is the difference between a ring that spins and one that fits. Measure carefully, measure twice, and use the converter to cross-check the result in a second unit.