Ring measurement: circumference is not diameter
The numbers 17 and 54 can describe similar rings using different geometric measurements. Identifying what you measured matters more than entering a seemingly precise number.
Choose a measurable dimension
Tiffany describes using flexible tape or string for circumference, or measuring the inside diameter of a fitting ring. Do not measure the outer edge or assume a finger diameter is a precise circle.
References: Tiffany
Use the matching chart column
Circumference equals π × diameter geometrically, but published chart columns are rounded independently. Converting a rounded diameter into circumference before another lookup can incorrectly jump a size. Match the measured column directly.
An example that catches unit mistakes
The reference pairs inside diameter 17.3 mm and circumference 54.4 mm with US 7. Entering 17.3 as circumference is a measurement-type error, not a very small ring size.
A measured circumference of 54.5 mm lies between US 7 and US 7.5. The tool shows the larger US 7.5 and explains that choice. It is an estimate, not remote proof of fit.
Measure again without pulling the tape tight and check the knuckle. Band width and shape affect comfort; try a size with the seller when uncertain, especially before irreversible engraving.