Measuring an existing ring
A ring that already fits well is one of the best pieces of evidence you have — but only if it fits the same finger, is still round, and has a similar band to the ring you plan to buy.
Before you measure: is this ring good evidence?
- Same finger, same hand. A ring worn on the right middle finger tells you little about the left ring finger.
- Fits comfortably. It slides over the knuckle with slight resistance and does not spin freely or leave a deep mark.
- Round. Thin bands bend easily. If it looks oval, the reading is unreliable.
- Similar band. A 2 mm band and an 8 mm band in the same size feel different.
Measuring the inside diameter
- Use a millimetre ruler or, better, calipers. Lay the ring flat.
- Measure straight across the inside edge to inside edge, through the centre. Do not include the metal.
- Rotate the ring a quarter turn and measure again. If the two readings differ by more than about half a millimetre, the ring is probably not round.
- Enter the diameter in the ring size calculator with “Inside diameter of a ring” selected.
A diameter is roughly 14–24 mm for adults; a circumference is roughly 44–74 mm. If you enter one when we expect the other, the calculator flags a likely mix-up rather than returning a wrong size.
Why a 0.5 mm error matters
On our canonical table, one full US size is about 0.81 mm of inside diameter. A half-millimetre reading error is therefore more than half a size. That is why we ask for a second reading and why a ruler reading is treated as weaker evidence than a physical sizer or jeweller.