The square footage of your lawn tells you how much area you have to work with. When you plan your yard's layout, this is vital information so you don't plan for more space than you actually have. Once you have planted your lawn, knowing the square footage tells you how much water and fertilizer you need to use. To compute the square footage of your yard, you need to know the shape of your yard and its dimensions.
Measure the dimensions of your yard. If your yard is rectangular, measure the length and width. If your yard is circular, measure the distance across the yard through the center, also known as the diameter.
Convert your measurement to feet if you have not measured in feet. You can convert yards to feet by multiplying by 3 or convert inches to feet by dividing by 12. For example, if your yard was 5 yards long, you would multiply 5 by 3 to find it is 15 feet long.
Compute the square footage of your yard by using the appropriate volume formula for the shape of your yard. For rectangular yards, multiply the length times the width. For example, if your yard is 10 feet by 15 feet, you would multiply 10 by 15 to get 150 square feet. For circular yards, divide the diameter by two, square the result and then multiply by 3.14. For example, if your yard had a diameter of 10 feet, you would divide 10 by 2 to get 5, square 5 to get 25 and then multiply by 3.14 to get 78.5 square feet.