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Lesson · Grade 7 · Mathematics

Circumference from Diameter or Radius

  • Grade 7
  • Mathematics
  • 1 min 6 s
  • British Columbia
  • BC7-MA-48

A circle with a 4 cm radius has its circumference traced, and copies of its diameter bent around the edge show that just over 3 fit, which is pi, about 3.14, so C = π × d. Two radii make the 8 cm diameter, and 3.14 × 8 gives a circumference of about 25.12 cm, ending on the recap C = π × d = 2 × π × r.

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In one sentence

The question this lesson answers

How far is it all the way around this circle?

Its radius, from the centre to the edge, is 4 centimetres.

Transcript

What the lesson says

How far is it all the way around this circle?

Its radius, from the centre to the edge, is 4 centimetres.

The distance around a circle is called its circumference.

The diameter goes straight across the circle, through the centre.

Just over three diameters fit around the edge of any circle.

That number is called pi, and it is about 3.14.

So the circumference is pi times the diameter.

Here we know the radius, and two radii make one diameter.

So the diameter is 2 times 4, which is 8 centimetres.

Now multiply the diameter by pi, which means 3.14 times 8.

So the circumference is about 25.12 centimetres.

Remember, the circumference is pi times the diameter, or 2 times pi times the radius.

Curriculum

Where this fits

British Columbia, Grade 7

Mathematics, Geometry and measurement: circumference and area of circlesBC7-MA-48

Previous: Where pi comes fromNext: Diameter from circumference

The outcomes and strands follow the British Columbia curriculum.

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