Circle Area Calculator

Circle area formula explained

The area of a circle is the amount of flat space inside its curved boundary. The formula is A = π × r², where r is the radius. Because the radius is squared, a circle with twice the radius has four times the area, not twice the area.

Use this calculator when you know the radius of a circular object: a round table, pipe opening, garden bed, tank base, pizza, plate, or circular plot. If you know the diameter instead, divide it by two first.

Worked example: radius 5 ft

Using diameter instead of radius

The diameter crosses the entire circle through the center. The radius is half of that measurement. If a circle has a diameter of 14 cm, the radius is 7 cm, and the area is π × 7² = 153.9380 cm².

Common mistakes

• Entering the diameter in the radius field.
• Forgetting that the output is in square units.
• Mixing units, such as radius in inches and another measurement in centimeters.

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Step-by-step method

1. Measure the radius from the center of the circle to the edge.
2. Square the radius by multiplying it by itself.
3. Multiply the squared radius by pi.
4. Label the answer with square units.

Second example: diameter 18 cm

Practical interpretation

Circle area is often used as an estimate rather than an exact material requirement. For example, a round rug, circular patio, or garden bed may have borders, seams, or irregular edges that require a small allowance. The calculator gives the mathematical area of the ideal circle.

If you are estimating material, calculate the exact area first and then decide whether to add extra based on the job. Paint, fabric, stone, and flooring each have different waste or overlap assumptions.