Area Calculator

Area of a Circle Calculator: calculate circle area from radius, diameter, or circumference, and reverse the formula to find a missing circle value from the area.

The area of a circle is the space inside the circle. Use this calculator to find the area from the radius, diameter, or circumference, or reverse the same formulae to find a missing circle value from the area.

The standard circle area formula is A = pi r². If you know the diameter instead, use A = pi d² / 4. If you know the circumference, use A = C² / 4pi.

Initial Data

Area of a circle formula

$$A= \pi \times r^{2}$$

Tip: Choose any one input value to calculate the others. You can enter plain numbers, fractions, or pi expressions such as pi, 1/2 pi, 1 1/2pi, and 2pi.

Radius Diameter Circumference

Radius (r)

Area (A)

Area of a Circle Calculator FAQs

What is the formula for the area of a circle?
The formula for the area of a circle is A = pi r^2, where r is the radius. This is the standard circle area formula used in geometry, school maths, and practical measurements.

How do I calculate the area of a circle from the radius?
Square the radius and multiply the result by pi. For example, if the radius is 10 cm, the area is pi x 10^2 = 314.159 cm^2 approximately.

How do I find the area of a circle from the diameter?
First divide the diameter by 2 to get the radius, then use A = pi r^2. You can also use the direct formula A = pi d^2 / 4.

Can I calculate the area of a circle from the circumference?
Yes. If you know the circumference C, you can use A = C^2 / 4pi. This is useful when the diameter or radius is not given directly.

Can I find radius, diameter, or circumference from the area?
Yes. Switch the input and result fields, enter the area, and the calculator can return the radius, diameter, or circumference using the corresponding inverse circle formula.

Example: what is the area of a circle with radius 10 cm?
If the radius is 10 cm, the area is about 314.159 cm^2. The exact form is 100pi cm^2.