Three ways, from most to least exact. Sizes are US; half sizes are fine.

1. A ring that already fits. Lay it on a ruler and read the inside width in millimeters, then match it in the chart.
2. Our printable sizer. Print the ring-size page at 100 percent and slide a ring over the circles.
3. A strip of paper. Wrap it snug where the ring will sit, mark the overlap, measure the length in millimeters.

US ring size to inside diameter

US size	Inside width	Around
4	14.9 mm	46.8 mm
5	15.7 mm	49.3 mm
6	16.5 mm	51.9 mm
7	17.3 mm	54.4 mm
8	18.1 mm	57.0 mm
9	19.0 mm	59.5 mm
10	19.8 mm	62.1 mm
11	20.6 mm	64.6 mm
12	21.4 mm	67.2 mm
13	22.2 mm	69.7 mm

Between two sizes? Take the larger one for a band wider than 3 mm, the smaller one for a thin band. Wrong anyway? Exchange it within 30 days, no restocking fee.

Full guide with the printable sizer