# Area of a circle

The area of a circle is the amount of space within its boundary line or circumference.

## Circle area from circle radius

where

• A = area of circle
• π = pi

Given a circle with radius 7cm (centimeters), its area A = π x 7² = 153.9cm² (square centimeters) to 1 d.p. (one decimal place).

To square the radius r on your calculator you'll need to use x². Press the pi sign (π) followed by the multiply sign (x), enter 7, then press x² followed by the equals sign (=). 153.93804 should be displayed which is 153.9 to 1 d.p. You may need to access x² and π via a shift or 2nd function button.

When the circle's radius is 13m (meters), the area of the circle A = π x 13² = 530.93m² (square meters) to 2 d.p. (two decimal places).

On your calculator, enter pi sign (π), multiply sign (x), 13, x², equals sign (=) to give 530.9291585 = 530.93 to 2 d.p.

## Circle area from circle diameter

• A = area of circle
• π = pi

Let's say the circle's diameter d = 4.5 cm. Area A = π (4.5 / 2)² = 15.9cm² to 1 d.p.

Use your calculator's left and right bracket signs

The key strokes are left bracket sign, 4.5, divide sign (÷), 2, right bracket sign, x², multiply sign (x), pi sign (π), equals sign (=), giving 15.90431281 which is 15.9 to 1 d.p.

What about the area of a circle with diameter d = 16m?

Area of circle A   =  π  (16 / 2)²  =  201.06m² to 2 d.p.

On your calculator, enter in order left bracket sign, 16, divide sign (÷), 2, right bracket sign, x², multiply sign (x), pi sign (π), equals sign (=). The display should show 201.0619298 = 201.06 to 2 d.p.

## Circle area from circle circumference

• A = area of circle
• π = pi
• C = circumference of circle

If the circumference of a circle C is 29cm, then its area A = π [29 / (2π)]² = 66.9cm² to 1 d.p.

Enter 29 on your calculator, then divide sign (÷), left bracket sign, 2, multiply sign (x), pi sign (π), right bracket sign, equals sign (=), x², multiply sign, pi sign (π), equals sign (=). 66.92465357 should be displayed which is 66.9 to 1 d.p.

Next consider a circle with circumference C = 1.5m.

Area of circle A   =  π [1.5 / (2π)]² = 0.18m² to 2 d.p.

Enter 1.5 instead of 29, otherwise use the same key strokes on your calculator to give 0.179049311 = 0.18 to 2 d.p.

## Now use some of your own figures to practice calculating the area of a circle

Do the same for the radius, diameter and circumference of a circle and you'll soon get to grips with circle calculations!

For more on the circle click: