Circle Calculator
Enter any one measurement of a circle — radius, diameter, circumference or area — and get all the others.
Every circle measurement follows from the radius: diameter d = 2r, circumference C = 2πr and area A = πr².
How the circle calculator works
Every circle measurement follows from the radius: diameter d = 2r, circumference C = 2πr and area A = πr². Given a circumference, r = C ÷ 2π; given an area, r = √(A ÷ π). π is taken as 3.14159265….
Formula: d = 2r; C = 2πr; A = πr²
Worked examples
| Inputs | Area | Note |
|---|---|---|
| radius 5 | 78.539816 | area 78.54 |
| diameter 10 | 78.539816 | the same circle |
| circumference 31.4159 | 78.539684 | radius 5 |
| area 100 | 100 | radius 5.64 |
FAQFrequently asked questions
What is π?
The ratio of any circle's circumference to its diameter, about 3.14159 — the same for every circle.
How do I find the area from the diameter?
Halve it to get the radius, then A = πr². For d = 10, r = 5 and A ≈ 78.5.
What units does the area have?
Square units of whatever you entered: cm → cm², m → m².
How do I find the radius from the circumference?
r = C ÷ 2π.
What is the area of a semicircle or sector?
Half the circle's area, or (angle ÷ 360) × πr² for a sector.
Where these figures come from
- NIST Digital Library of Mathematical Functions — reference definitions for elementary and special functions
- Wolfram MathWorld — definitions and formulas for every topic on this page
- NIST/SEMATECH e-Handbook of Statistical Methods — the statistical formulas (mean, variance, z, confidence intervals, sample size)
- National curriculum in England — Mathematics — the terms and methods taught in UK schools
Last checked: September 2026. Formulas are fixed by mathematics and do not change with tax years or regulations.