Transparent by design

Measurement methodology and accuracy

RingSizeCalculator estimates jewelry sizes. It does not guarantee fit or replace a physical fitting. Calculator results, the searchable chart, and printable guides all use the same versioned dataset: v1.0.0, reviewed October 6, 2026.

Our references

• GIA international ring size chart: reference diameters for US 3–13 in half sizes and UK F–Z½ including half-letter steps. The UK values follow GIA’s convention; other jewelers may differ by a half-letter or more.
• Tiffany US size guide: cross-check for US diameters. GIA lists US 9 as 19.0 mm, while Tiffany lists 18.9 mm. We consistently use GIA’s 19.0 mm, rather than mixing reference tables.
• BlueStone ring measuring guide, v1: numeric India sizes 6–30, using its diameter column. These values agree with the corresponding CaratLane diameter guide. Indian brand conventions differ; this is not a universal India standard. Measurements below 14.6 mm show “< 6,” without extrapolation.
• 4°C BRIDAL ring size guide: Japanese numbered sizes, using its diameter column. We do not treat GIA’s combined Asia column or Tiffany’s brand equivalents as a universal Japan standard.

One physical measurement, several conventions

The internal reference is inner diameter in millimeters. Circumference equals diameter × Math.PI; a circumference input is divided by Math.PI. One centimeter is 10 mm, and one inch is exactly 25.4 mm. We retain precision during calculation and round only when displaying values.

Diameter and circumference display to 2 decimal places in mm/cm or 4 in inches. Those digits help adjustment and comparison; they are not a claim of physical measurement precision. Source diameters themselves are rounded to 0.1 mm. Because we recalculate circumference, it may differ from a source’s independently rounded circumference column.

We find the nearest reference diameter independently in each system. Exact midpoint ties select the lower entry for deterministic display, not as fit advice. When a measurement falls between US half-size entries, both neighboring sizes are shown. EU / ISO is the nearest integer circumference label; half EU sizes are not offered. Conversions are approximate, not necessarily reversible.

Supported input ranges

Physical measurement tools cover 14.1–22.2 mm inside diameter (about 44.30–69.74 mm circumference), corresponding to our US 3–13 chart. The known-size converter only offers labels whose reference diameter lies within that range: US 3–13, UK F–Z½ including half letters, EU 45–69, India 6–29, and Japan 5–28. We do not invent missing sizes or silently clamp out-of-range input.

Screen calibration

• Device / assisted setup: the browser cannot reliably reveal an exact device model or physical size. We show its reported full-screen dimensions for context, require user confirmation, and direct users to their manufacturer’s physical diagonal specification. No inferred device database match is made.
• Screen diagonal: CSS pixels per mm = √(screen.width² + screen.height²) ÷ (diagonal in inches × 25.4). This uses the full screen, never the browser window. It assumes accurate browser-reported aspect ratio and consistent scaling. It remains an estimate until physically verified.
• Standard ID-1 card: the outline represents 85.60 × 53.98 mm. In portrait orientation its outer width is 53.98 × pixels-per-mm. Adjust both edges against a real standard-size card. Only the scale is saved; no card details or images are collected.

Uncalibrated circles are explicitly illustrations. Card calibration enables a physical-scale estimate, but still depends on careful alignment. Local calibration records the reported screen dimensions, pixel ratio, and visual zoom. Detectable changes request recalibration. Identically reported monitors can still have different physical dimensions: manually verify after moving monitors or changing devices. Reset calibration at any time in the calibration dialog.

Printing and fit limitations

Print on A4 or US Letter at 100% / Actual size, with browser headers/footers disabled. Do not use Fit to page. Circle outside diameters use physical millimeter units. Verify the 50 mm line with a real ruler before using any printout. Printer drivers may change scale despite the requested settings.

A paper strip can distort or compress skin. Knuckle size, ring width, shape, and comfort change fit. Repeat measurements and consult a jeweler. Do not automatically size up. Oura and other smart rings require their model-specific sizing kits; their labels are not standard jewelry equivalents.

Report a reference or calculation issue so it can be checked against the source.