# Circle Calculator

> Circle calculator: solve circumference, area, diameter, and radius from any one known value instantly. Free online tool, private, runs locally, no signup.

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

## How it works

Diameter = 2r; circumference = 2 x pi x r; area = pi x r^2.

## Worked example

A circle of radius 5: diameter 10, circumference 31.4159, area 78.5398.

## Sources

- [Calculation methodology and verification standard](/methodology/)

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