Circle Area Calculator
Calculate the area of a circle from its radius or diameter. Also shows circumference and diameter.
Last updated:
Input
Enter a radius or diameter to compute the circle area, circumference, and diameter.
Result
Area
78.539816
Circumference
31.415927
Diameter
10
Lengths share the input unit, and the area uses that unit squared.
How it works
- The area is computed as A = π × r², where r is the radius.
- When you enter a diameter d, the radius is taken as r = d ÷ 2.
- The circumference is C = 2 × π × r and the diameter is d = 2 × r.
- Pi is treated as approximately 3.141592653589793.
- Lengths use the same unit as your input, and the area uses that unit squared. For example, entering cm gives an area in cm².
Frequently asked questions
- What formula does this calculator use for a circle's area?
- The area is A = π × r², where r is the radius and π is taken as about 3.141592653589793. For example, a radius of 5 cm gives A = π × 5², which is about 78.54 cm².
- Can I use the diameter instead of the radius?
- Yes. If you enter the diameter d, it is converted to the radius using r = d ÷ 2 before applying A = π × r². The circumference C = 2 × π × r is shown at the same time.
- What unit will the resulting area be in?
- The area uses the square of your input's length unit. Enter the radius in cm and the area comes out in cm²; enter it in m and the area is in m².
Reviews
Tell us what you think of this calculator.
Write a review
Found an incorrect result or a problem? Turn this on to report it.
Rating