Circle

circle
A circle is easy to make:
Draw a curve that is "radius" away
from a central point.
And so:
All points are the same distance from the center.

You Can Draw It Yourself

Put a pin in a board, put a loop of string around it, and insert a pencil into the loop. Keep the string stretched and draw the circle!

Drag a point!
© 2015 MathsIsFun.com v0.86

Play With It

Try dragging the point to see how the radius and cirfcumference change.

Radius, Diameter and Circumference

The Radius is the distance from the center outwards.
The Diameter goes straight across the circle, through the center.
The Circumference is the distance once around the circle.
And here is the really cool thing:
When we divide the circumference by the diameter we get 3.141592654...
which is the number π (Pi)
So when the diameter is 1, the circumference is 3.141592654...
We can say:
Circumference = π × Diameter

Example: You walk around a circle which has a diameter of 100m, how far have you walked?

Distance walked = Circumference = π × 100m
314m (to the nearest m)
Also note that the Diameter is twice the Radius:
Diameter = 2 × Radius
And so this is also true:
Circumference = 2 × π × Radius

Remembering

The length of the words may help you remember:
  • Radius is the shortest word
  • Diameter is longer (and is 2 × Radius)
  • Circumference is the longest (and is π × Diameter)

Definition

The circle is a plane shape (two dimensional):
And the definition of a circle is:
plane
The set of all points on a plane that are a fixed distance from a center.
A circle has about 80% of the area of a similar-width square.
The actual value is (π/4) = 0.785398... = 78.5398...%