Use a measuring tape to measure the outside edges of your yard.
Draw the area of your lawn on a piece of graphing paper.
Use simple conversions, like 1 square equals 1 square foot of yard. If your yard is bigger, you can use 1 square equals 5 or 10 square feet of yard.
Look on the label of your sprinkler head to determine what its radius is. Purchasing one that has a 15 foot radius will make calculating easy.
Use your graph paper to determine where your sprinklers will go.
Lay them out 15 feet apart, if you are using sprinklers with a 15 foot radius. If you are using a sprinkler with a 12 foot radius, place them 12 feet apart.
Look over the graph of your yard to make sure every area of lawn will be covered by your sprinklers.
Count how many sprinklers you have placed on your graph and purchase that many.