Ring inner diameter

Ring inner diameter is the distance straight across the inside of a ring, from one inner edge to the other. It is the easy way to size from a ring you already own, with a ruler: multiply it by pi (3.1416) and you get the ring circumference that ring sizes are built on. A 1.73 cm diameter is a 5.44 cm circumference, a US 7.

How to measure

1. Take a ring that fits the finger you want to size.
2. Lay it flat on a ruler.
3. Measure across the inside, edge to edge, through the center, not over the metal.
4. Read to the millimeter: one whole US size is 0.032 in of diameter, about 0.8 mm.

On a size chart

The Rings template lists the inner diameter beside the inner circumference for each US size:

Each US size adds about 0.08 cm, which is why a ruler with millimeter marks, read carefully, is enough.

In Ruban Size

Find my size for rings has a field for it, "Or the inner diameter of a ring that fits you", with the help "A ring that fits you: measure across the inside, edge to edge, and enter it as the diameter." A diameter of 1.73 cm is said back as "Diameter 1.73 cm: a finger of 5.4 cm." and gets a 7 on the template. On a chart, a header that says diameter, or values under 3 cm, are read as diameters and compared as circumferences: no finger is that thin.