Learn the vocabulary related to finding diameter. Four terms relate to diameter: area, circumference, radius and pi. Area is the measurement of the inside of a two-dimensional shape. Circumference is the distance around the outside of the circle. Radius is half the distance across the circle. Pi ( or π) is a mathematical constant with an approximate value of 3.14.
If given no values for your circle, use a ruler to measure across the center point of the circle from edge to edge to find the diameter.
Use radius (r) by multiplying the value by two to find diameter. Example, if r= 3, calculate diameter 3 x 2 = 6.
Circumference (C) is used to determine diameter by dividing the value by pi (π). If circumference is 9, you would divide by pi (3.14). Example: 9 / 3.14 = 2.87
Determining diameter from area is mathematically the most complex. Take the square root after calculating the following: area multiplied by four, divided by pi.