Ring Size Measurement Estimator
Estimate a US half-size reference from inside circumference or diameter in millimetres.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
For a circumference input the model estimates US size from fixed 2.55 mm steps; a diameter input is first converted using π. Snug/loose settings shift the estimate by a quarter size before rounding. This is a simplified reference, not a jewelry standard; use a jeweler’s ring stick or the maker’s guide for fit.
Choose the calculation that matches your task
Use this estimator when you have a measured circumference or diameter and want the model’s US size with a fit adjustment. To compare regional labels from a size you already know, use the approximate converter instead. Neither model guarantees fit for a specific ring.
Worked example
calculated with the default values below
What do you have Inside circumference (mm) · Measurement (mm) 54 mm · Preferred fit Regular US ring size 7.0 · Inside diameter 17.2 mm · Inside circumference 54.0 mm · Model range indicator Standard range
General background references
Disclaimer
These calculators give general estimates. Sizing, pricing, availability and standards vary by brand, retailer and country, so treat the output as a starting point and confirm it against the seller's or authority's own figures.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
C = input if circumference, else π × diameter; US = round(2(3 + (C − 44.2)÷2.55 + a))÷2
- C
- Inside circumference (mm)
- d
- Inside diameter (mm)
- a
- Fit shift: −0.25/0/+0.25
- US
- Estimated US half size
C is inside circumference in mm; a = −0.25 snug, 0 regular, +0.25 loose. The 2.55 mm step is a simplified US reference; confirm physical fit.
Frequently asked questions
How does the measurement method affect ring size?
If you enter inside diameter, the model multiplies it by π to estimate circumference; if you enter circumference, it uses that value directly. It maps circumference to US half sizes in 2.55 mm steps. Snug/loose settings shift the estimate by 0.25 before rounding.