Diameter of a circle
The diameter of a circle is the "width" of the circle. It is twice the radius. The circle circumference is the diameter (d) multiplied by pi. The area of a circle is 1/4*pi*d2.

Sometimes there is referred to the inside or the outside diameter, in the case of a steel bar for example. The wall thickness equals half times the difference between the outside and the inside diameter.

Exercises
1. The diameter of a circle is 8. How much is the radius?
2. What is the diameter of a circle with radius 20?
3. The surface area of a circle equals 227. What is the diameter?
4. What is the diameter of a circle with circumference 628?
5. A steel bar has outside diameter 23, and the wall thickness is 2. Calculate the inside diameter.
Solutions
1. Radius it half the diameter; it is 4.
2. 40
3. Multily it by 4, divide it by pi and take the square root; 17
4. The diameter is the circumference divided by pi; 200.
5. The inside diameter is the outside diameter minus two times the wall thickness; 23 - 2*2 = 19.

You might want to have a look at our other pages about geometry, volumes and shapes!