Single-dimensional measurement units like centimeters can be converted into two-dimensional units like square centimeters through common geometric shapes' area formulas. A circle's diameter, one of its defining and longest linear measurements, is a line segment that extends from one point on its circumference, through the circle's center and to another point on its circumference. With the diameter, you can find a circle's area with the equation 1/4 * diameter^2 * pi, where pi is a math constant that approximately equals 3.142, and convert measurements in centimeters to square centimeters.
Measure the diameter in centimeters. For this example, let the diameter measure 10 cm.
Multiply the diameter's length to itself to square it — 10 cm multiplied by 10 cm results in 100 cm^2.
Multiply the squared diameter by pi — 100 cm^2 multiplied by pi equals approximately 314.2 cm^2.
Divide the product from the last step by 4 to calculate the circle's area — 314.2 cm^2 divided by 4 equals 78.55 cm^2.
TL;DR (Too Long; Didn't Read)
If the diameter is given in units other than centimeters, then convert the measurement with a conversion program (see Resources).