How to use this calculator
Calculate circle metrics from radius or diameter: area, circumference, diameter, and radius.
How the calculation works
- Enter radius or diameter.
- The missing measurement is derived.
- Area and circumference are calculated with pi.
Formula
Area = πr². Circumference = 2πr.
Example
A circle with radius 1 has area π and circumference 2π.
Understanding your result
Use the output as a practical starting point, then review it before using it in an important workflow.
Assumptions and limitations
- Radius and diameter cannot be negative.
- Results are rounded for display.
Frequently asked questions
Does this require an account?
No. The core tool works without creating a Tool2Do account.
Is my input included in share links or analytics?
No. Tool2Do share actions use the tool page URL and safe metadata, not private values or uploaded files.