Circle calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Radius r = 5.
- Diameter = 2r = 10.
- Circumference = 2πr = 31.415927.
- Area = πr² = 78.539816.
| Inputs | Answer |
|---|---|
| You know the: Radius; Radius: 5; Diameter: 10; Circumference: 31.4159; Area: 78.5398 | A circle with radius 5 has circumference 31.415927 and area 78.539816. |
| You know the: Diameter; Radius: 5; Diameter: 12; Circumference: 31.4159; Area: 78.5398 | A circle with radius 6 has circumference 37.699112 and area 113.097336. |
| You know the: Area; Radius: 5; Diameter: 10; Circumference: 31.4159; Area: 100 | A circle with radius 5.641896 has circumference 35.449077 and area 100. |
How to use it
A circle is fixed by its radius r. The diameter is 2r, the circumference is 2πr and the area is πr². If you know the diameter, the circumference or the area instead, the calculator first works back to the radius.
For a circle of radius 5: diameter 10, circumference 31.4159 and area 78.5398. π is used at full floating-point precision.
Frequently asked questions
How do I find the area of a circle from its diameter?
Halve the diameter to get the radius, then use πr².
What is the circumference formula?
C = 2πr, or πd with the diameter d.
How do I find the radius from the circumference?
r = C ÷ 2π.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
About · How we verify this · Editorial policy · Corrections
General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.