# How to Find the Area of a Circle Using the Diameter

## Finding the Area of a Circle Using the Diameter

The area of a circle is found using the formula: In this formula, d is the diameter of the circle. The image below shows what we mean by diameter: ## Interactive Widget

Use this interactive widget to draw a circle and then calculate its area from the diameter. Start by clicking in the shaded area.

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Turn your phone on its side to use this widget.

## How to Find the Area of a Circle Using the Diameter

Finding the area of a circle using the diameter is easy.

### Question

What is the area of a circle with a diameter of 10 cm, as shown below? # 1

Area = πd2⁄4

Don't forget: π is pi (≈ 3.14) and d2 = d × d (d squared) and / means ÷.

# 2

Substitute the diameter into the formula. In our example, d = 10.

Area = π × 102⁄4

Area = π × 10 × 10 ÷ 4

Area = π × 100 ÷ 4

Area = π × 25

Area = 3.14 × 25

Area = 78.5 cm2

The area of the circle with a diameter of 10 cm is 78.5 cm2.

## Slider

The slider below shows another real example of how to find the area of a circle using the diameter.

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## How to Find the Area of a Circle Using the Radius

The area of a circle can be found using the radius rather than the diameter.

The area of a circle, using the radius, is found using the formula: In the formula, r is the radius of the circle. The image below shows what we mean by radius: 