How to calculate a circle
- Choose which value you have: radius, diameter, circumference or area.
- Enter its value.
- If you want the arc and the sector, also enter the angle in degrees.
- See the other values of the circle and tap “Copy result” to take them with you.
Circle formulas
With the radius r: diameter = 2r, circumference = 2πr and area = πr². The other way round: r = diameter ÷ 2, r = circumference ÷ 2π and r = √(area ÷ π). The number π is approximately 3.14159.
For a sector with angle θ (in degrees): arc = 2πr × θ ÷ 360, sector area = πr² × θ ÷ 360 and chord = 2r × sin(θ ÷ 2). Example: a circle with radius 5 has a diameter of 10, a circumference of 31.416 and an area of 78.540. With an angle of 90° the arc is 7.854 long and the sector has an area of 19.635.
Good to know
- Units are not converted: the result stays in the same unit you entered (the area, in that unit squared).
- The sector angle goes from 0 to 360 degrees.
- The browser's full value of π is used and the result is only rounded when displayed.
Frequently asked questions
How do I calculate the area of a circle?
Multiply π by the radius squared. With a radius of 5, the area is π × 25 = 78.54.
How do I find the radius if I know the area?
Choose “Area” as the value: the tool calculates the radius as the square root of (area ÷ π).
What is the difference between circumference and perimeter?
They are the same thing: the circumference is the perimeter of the circle, 2πr.
What is a circular sector?
It is the part of the circle between two radii. With the angle you see its area and the length of its arc.
Is my data saved?
No. Everything is calculated in your browser.