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.
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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.
If your yard is not a perfect rectangle, you may need to divide it into portions, calculate the square footage of each and add these together. For example, you may have a family room or other part of your home that juts out into the yard, giving you an L-shaped lawn (imagine the sides of the L are quite thick). In that case, you would divide the L into two rectangles, determine the square footage of each and add them together to get your yard's total square footage.
Similarly, if your yard has a portion within the grass that is not part of your lawn, simply calculate the area of that portion and subtract it from the total. For example, if you have a rectangular yard measuring 100 square feet except a square in the middle for your tree that covers 20 square feet, you would subtract 20 from 100 to find your lawn covers 80 square feet.