To calculate the square footage of a rectangle or square, get the length and width of the area in feet and multiply the values.

area = length * width

To get the square footage of a circle, multiply Pi (3.1415) by half the diameter raised to the power of 2.

area = 3.1415 * (diameter / 2) ** 2

The above formulas are easy enough to use with a pocket calculator, to make things quicker, I made this square footage calculator that can translate a variety of different length units into square feet.

Another handy feature of this tool is cost calculation, which will tell you how much the total cost of the area to rent given a price per N foot. Let's say you want to rent 400 ft^{2} for $2 per foot – the total cost will be $800.