If you have a ring that already fits the finger you're sizing for, this is one of the most reliable home methods — there's no risk of pulling a string too tight or too loose. All you need is a ruler with millimeter markings.
Lay the ring flat on a table with a ruler placed directly underneath it, or beside it, so you can read the scale clearly.
Measure straight across the inside edge of the ring — from one inner wall to the opposite inner wall, through the center. This is the inside diameter, not the outside width of the band.
Enter that number below to see it converted across every major sizing system.
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EU / Ø mm—
A few things that affect accuracy
Make sure you're measuring the inside of the band, not the outside — the metal itself adds a millimeter or two to the outside width.
Wide bands (6mm+) tend to fit more snugly than a thin band at the same labeled size, so if this ring is unusually wide, you may want to size slightly differently for a new, thinner ring.
If the ring is oval or slightly warped from wear, measure at a couple of different angles and average the results.
Don't have a ring to measure? Try the string or paper method instead.
This gives a strong estimate based on standard sizing tables. For an engagement ring or anything hard to resize, confirming with a jeweler's metal ring gauge is worth the extra step.