What this calculator does
This Ring Size Calculator converts the inner diameter of a ring (measured in millimeters) into its inner circumference and an approximate US ring size. It's handy when you have a ring that already fits and want to find the matching size, or when you've measured a finger's diameter with a caliper or a sized mandrel.
How to use it
Measure the inside diameter of an existing ring straight across, edge to edge, in millimeters. Enter that value and the calculator returns the circumference (\(\pi \times \text{diameter}\)) and the estimated US size. For best accuracy, measure a few times and average, and measure at the end of the day when fingers are largest.
The formula explained
A circle's circumference equals π times its diameter, so $$C = \pi \times d$$ US ring sizing follows a near-linear scale where size 0 corresponds to roughly 36.5 mm of inner circumference and each whole size adds about 2.55 mm. That gives $$\text{US Size} = \dfrac{C - 36.5}{2.55}$$
Worked example
Suppose a ring has an inner diameter of 16.5 mm. The circumference is \(\pi \times 16.5 \approx 51.84\) mm. Plugging in: $$\frac{51.84 - 36.5}{2.55} \approx 6.02$$ so this is approximately a US size 6.
FAQ
Is this exact? It's a close approximation. Different jewelers and standards (US, UK, EU, Japan) round slightly differently, so treat the result as a guide.
Diameter or circumference? Enter the inner diameter. The tool computes circumference for you.
What if I only have circumference? Divide your circumference by \(\pi\) to get the diameter, then enter it — or simply note that the size formula uses circumference directly.
