⭕ Circle Calculator
Everything about a circle flows from its radius: double it for the diameter, multiply by 2π for the circumference, square it and multiply by π for the area. Three formulas, one input.
Enter the radius in any unit — the answers come back in the same unit (and square units for area). The calculator uses the full precision of π, not 3.14.
How to use this calculator
Type the radius and press Calculate to get the area, circumference and diameter.
- Type the radius into the Radius field, in any unit you like.
- Press Calculate.
- Read the area as the headline result, with circumference and diameter below it.
- Keep your units consistent: a radius in centimetres gives an area in square centimetres.
Frequently asked questions
What does this circle calculator compute?
From a single radius it returns the area (pi r squared), the circumference (2 pi r) and the diameter (2r), using the full precision of pi.
Can you show a worked example?
With radius 5: the diameter is 10, the circumference is about 31.42, and the area is about 78.54 square units.
What if I know the diameter instead?
Halve it to get the radius, then enter that. A diameter of 10 means a radius of 5.
Why use pi instead of 3.14?
The calculator uses the full precision of pi stored in your browser, so results are more accurate than hand calculations with 3.14, especially for large circles.