Circumference of a circle
You learn to work out the circumference of a circle.
How to work it out
- Multiply radius by 2 for diameter.
- Multiply diameter by π (approximately 3.14).
A worked example
A circle has radius 5 cm. What is the circumference?
- The diameter is 2 × 5 = 10 cm.
- The circumference is 10 × 3.14 ≈ 31.4 cm.
Another example
A circle has diameter 8 m. What is the circumference?
Circumference is diameter × π: 8 × 3.14 ≈ 25.12 m.
Watch out
You forget the times 2: with radius 5 you work out 5 × 3.14 = 15.7.
π goes with the diameter, straight across the middle. That is 2 radii.
You multiply the radius by itself, as you do for the area.
The circumference is the edge: times 2 and times π. The area is radius times radius, times π.
Tips
- Circumference = diameter × π, or circumference = 2 × radius × π.
Why it works
The circumference is the distance all the way round. If you know the radius (from the middle to the edge), you multiply by 2 to get the diameter, then by π (about 3.14) to get round the circle.