How Ring Sizer calculates your size

Diameter, circumference and rounding

Inner diameter is the distance across the inside of the band through its center. Inner circumference is the distance around that opening. For a circular ring, circumference equals diameter multiplied by π.

Our US approximation is diameter in millimeters = 11.63 + (0.8128 × US size). Measured diameters convert back to the nearest US half size. EU output rounds circumference to a whole millimeter. Displayed diameters and circumferences round to one decimal place after the calculation.

These formulas describe our implementation. Retailers may use slightly different tables or rounding. ISO 8653:2016 defines ring-size measurement and designation; referencing it does not certify an at-home measurement.

Which reference ranges are supported?

• The nominal chart covers US 3 through 15, including every half size.
• The UK lookup covers our listed labels from H to Z+1, mapped to US 4 through 13. The converter accepts those labels exactly; it does not guess an unlisted letter.
• The Japanese lookup uses the nearest entry in the app's reference table: sizes 1–27, at diameters 13.0–21.5 mm.
• A dash means there is no supported lookup for that measurement. We do not repeat a boundary label outside its range.
• India is estimated as circumference in millimeters minus 40, rounded to a whole size with a minimum of 1. Confirm the result against the retailer's chart.

This website labels the Japanese reference separately. A country name alone does not guarantee that every retailer follows the same chart. Use the retailer's measurements when available, especially when shopping across regions.

How does band width change the result?

Thin-band mode adds nothing and is the reference for size conversion. Other settings add a small amount to diameter before converting: medium +0.15 mm, wide +0.35 mm, and extra wide +0.55 mm. These are the app's fit adjustments, not a change to the measured finger or a universal jewelry standard.

GIA's sizing guidance explains why band width and getting the ring over the knuckle matter. A jeweler can check the intended fit with sizing gauges.

What affects an at-home measurement?

Paper and string can stretch or tighten; a ruler must cross the ring's center; printed circles depend on print scale. Repeat measurements and use the finger and band width you intend to wear. A comparison with a ring that already fits provides useful context, but an app or chart cannot guarantee comfort.

The iPhone app uses a table of known screen densities and the device's display scale. An unknown device identifier uses an estimate. Calibration is one part of the measurement; alignment, finger pressure, and fit still affect the result. The web calculator converts measurements you enter and does not automatically measure your screen.