# Ring Size Calculator

> Ring size calculator: US ring size from finger circumference. Free ring size calculator with the formula shown - runs entirely in your browser, no signup.

- **Canonical URL:** https://boringtoolskit.com/ring-size-calculator/
- **Category:** Converters
- **Evidence level:** authored-methodology
- **Reviewed:** 2026-09-20
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Circumference C = pi x d; US ring sizes step ~0.81 mm in inner diameter (full sizes), roughly 0.4 mm at half sizes. US size 7 is 17.35 mm inner diameter / 54.4 mm inside circumference; EU sizes read the inside circumference in millimeters directly.

## Assumptions

- Manufacturer charts vary; conversion outputs are the standard-mapping midpoint.
- Measurements are body measurements unless the tool labels them as garment measurements.
- Between-size results round per the rule stated in the FAQ.

## Common questions

### Why does my ring size differ between jewelers?

Sizing is measured on tapered gauges that vary by maker, and comfort-fit interiors run a half size larger than flat interiors. Measure circumference and convert, rather than trusting a labeled gauge.

### What if I fall between sizes?

Round the direction that matches use: clothing and shoes generally round up; ring sizes round down (a slightly loose ring still stays on; a tight one has to come off).

## Sources

- [Aggregated manufacturer sizing conventions](https://boringtoolskit.com/methodology/)

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