A ring sizer is a piece of plastic with marked circles. You can reproduce the important part with paper and a ruler, as long as you measure the right dimension.
Method 1 — the ring you already own
- Take a ring that fits the intended finger.
- Measure the inside diameter in millimetres — the clear space, not the outside edge.
- Match to a chart. 16.5 mm is roughly US 6; 17.3 mm is US 7; 18.1 mm is US 8.
Measuring the outside adds the band thickness (often 1.5–2 mm) and gives a size that is one to two sizes too large.
Method 2 — the paper strip
- Cut a strip about 10 mm wide and 100 mm long.
- Wrap it around the finger base, passing over the knuckle, and pull it snug.
- Mark the overlap and measure the length in millimetres — this is the circumference.
- Divide by 3.1416 for the diameter, or use the circumference column in our chart.
The three mistakes
- Measuring cold. Fingers shrink in the morning and in winter. Measure at the end of the day, at room temperature.
- Ignoring the knuckle. If the knuckle is wider than the base, the ring must pass the knuckle. Size to the larger measurement.
- Forgetting band width. A 6 mm band needs half a size more than a 2 mm band. If you are ordering a wide band, add it now, not after.
If you are between sizes
Go up. A slightly loose ring can be fitted with sizing beads; a tight ring cannot be worn comfortably and stretching damages the setting.
When to get a sizer
For any custom order over $500, ask us to post a plastic sizer — it is free and it removes the guesswork. For a surprise proposal, use a ring she already wears on that finger, or choose a slightly larger size and plan a resize. Full size guide →
The people who cut the stones and set the rings. We publish what we can verify on our own bench, and link to the certificate or the guide when a number needs a source.
