Standard US ring sizes. Measure a ring that fits you, or your finger, and pick the closest row.
How to measure at home
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With a ring that fits
Lay it on a ruler and measure the inside, edge to edge, in millimetres. Compare it with the diameter column.
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With a string or a strip of paper
Wrap it once around the base of your finger, snug but not tight, and mark where it meets. Lay it flat, measure the length in millimetres and compare it with the circumference column.
Fingers are a little larger in the evening and when it is warm, so measure twice. Between two sizes, choose the larger one.