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Lesson · Grade 7 · Mathematics

Area of a Circle from the Radius or the Diameter

  • Grade 7
  • Mathematics
  • 1 min 5 s
  • British Columbia
  • BC7-MA-51

A circle with an 8 cm diameter is used with the rule Area = π × r × r. You use a known radius straight away, but here the diameter is halved first to get a 4 cm radius. The working goes π × 4 × 4 ≈ 3.14 × 16, giving about 50.24 cm², and the video ends on a one-line recap: halve the diameter, then use π × r × r.

Concept: Area of a circle from radius or diameter

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In one sentence

The question this lesson answers

What is the area of this circle?

Its diameter, straight across through the centre, is 8 centimetres.

Transcript

What the lesson says

What is the area of this circle?

Its diameter, straight across through the centre, is 8 centimetres.

The area of a circle is pi times r times r.

The rule uses the radius. When you know the radius, use it straight away.

Here we know the diameter, so halve it first to get the radius.

8 divided by 2 is 4, so the radius is 4 centimetres.

Now put 4 in place of each r. That is pi times 4 times 4.

Pi is about 3.14, and 4 times 4 is 16.

Now multiply. 3.14 times 16 is 50.24.

So the area of the circle is about 50.24 square centimetres.

Remember: halve the diameter to get the radius, then the area is pi times r times r.

Curriculum

Where this fits

British Columbia, Grade 7

Mathematics, Geometry and measurement: circumference and area of circlesBC7-MA-51

Previous: Why the Area of a Circle Is π × r × rNext: Volume Is the Area of the Base Times the Height

The outcomes and strands follow the British Columbia curriculum.

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