Area of a Circle Calculator

Last updated: 15 Sep 2026 | Author: Brij | Review By: Irshad

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Calculate the area, radius, or diameter of a circle by entering any known circle measurement for geometry, design, and planning.

Circle Calculator

Radius (r):

Diameter (d):

Area (A):

An Area of a Circle Calculator helps you find the area, radius, or diameter of a circle quickly. Enter any known value, such as the radius, diameter, or area, to calculate the remaining circle measurements.

This calculator is useful for geometry, construction, landscaping, design, flooring, paving, and other projects involving circular shapes.

What Is the Area of a Circle?

The area of a circle is the amount of space inside its boundary. It is measured in square units, such as square centimetres, square metres, square inches, or square feet.

The area depends on the radius of the circle, which is the distance from the centre of the circle to its edge.

How to Use the Area of a Circle Calculator

Enter one known circle measurement:

Radius
Diameter
Area

The calculator uses the entered value to calculate the other measurements.

For example:

Enter the radius to calculate the diameter and area.
Enter the diameter to calculate the radius and area.
Enter the area to calculate the radius and diameter.

Area of a Circle Formula

To calculate the area of a circle, multiply pi by the radius squared.

$\text{Area} = \pi \times \text{Radius}^{2}$

Radius and Diameter Formula

The diameter is twice the radius.

$\text{Diameter} = 2 \times \text{Radius}$

To calculate the radius from the diameter, divide the diameter by 2.

$\text{Radius} = \frac{\text{Diameter}}{2}$

Calculate Radius from Area

If you know the area, divide it by pi and find the square root.

$\text{Radius} = \sqrt{\frac{\text{Area}}{\pi}}$

Calculate Diameter from Area

If you know the area, first calculate the radius, then multiply it by 2.

$\text{Diameter} = 2 \times \sqrt{\frac{\text{Area}}{\pi}}$

Example: Calculate Area from Radius

Suppose the radius of a circle is 5 units.

$\text{Area} = \pi \times 5^{2}$

$\text{Area} = \pi \times 25 = 78.54\text{ square units}$

The area of the circle is approximately 78.54 square units.

Example: Calculate Area from Diameter

Suppose the diameter of a circle is 10 units.

First, calculate the radius.

$\text{Radius} = \frac{10}{2} = 5\text{ units}$

Then calculate the area.

$\text{Area} = \pi \times 5^{2} = 78.54\text{ square units}$

The area of the circle is approximately 78.54 square units.

Example: Calculate Radius from Area

Suppose the area of a circle is 78.54 square units.

$\text{Radius} = \sqrt{\frac{78.54}{\pi}} = 5\text{ units}$

The radius of the circle is approximately 5 units.

Important Notes

Use the same measurement unit for radius and diameter.
Area is always expressed in square units.
Radius is half of the diameter.
Round results only when your project allows for it.
Use precise measurements for construction, fabrication, or material estimates.

Frequently Asked Questions

What is the formula for the area of a circle?

$\text{Area} = \pi \times \text{Radius}^{2}$

What is the difference between radius and diameter?

The radius is the distance from the centre of a circle to its edge. The diameter is the distance across the circle through its centre.

$\text{Diameter} = 2 \times \text{Radius}$

Can I calculate the area using the diameter?

Yes. Divide the diameter by 2 to find the radius, then use the area formula.

Can I calculate the radius from the area?

Yes. Divide the area by pi and find the square root.

$\text{Radius} = \sqrt{\frac{\text{Area}}{\pi}}$

What units should I use?

You can use any unit, such as centimetres, metres, inches, or feet. The calculated area will be shown in the corresponding square unit.

Why is pi used in circle calculations?

Pi is a mathematical constant that describes the relationship between a circle’s circumference and its diameter. Its approximate value is 3.14159.

Is the calculated area exact?

The formula is exact, but displayed results may be rounded because pi is often shown as a decimal.

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