How to Determine Pump/Well Size & Capacity for Lawn Irrigation

Measure the length and width of your lawn, in feet. If your lawn is not rectangular, divide it up into rectangular sections and measure the length and width of each section. Multiply the length of each section by its width. The result is the area of that section. Add the areas for each section together to determine the total area of your lawn.

Divide the area of your lawn by the area covered by a single sprinkler. The area covered by a single sprinkler will be noted on the instructions or package the sprinkler comes in. The result is the number of sprinklers that you need to cover your lawn.

Multiply the area of your lawn in square feet by 0.0625, which is the amount of water the average mature lawn needs in a single watering expressed in feet. A dimension of 0.0625 feet equals ¾ inch. If your lawn needs more or less than this amount of water, then divide the number of inches by 12 and multiply the area of your lawn by the result. This will give you the volume of water that your lawn needs for a single watering, in cubic feet.

Divide the water volume by 7.48, which is the number of cubic feet per gallon. The result is the number of gallons your lawn needs for a single watering. Your well needs to have enough capacity to deliver this much water before running dry.

Multiply the number of sprinklers that your lawn requires by the flow rate of each sprinkler, in gallons per minute. Most sprinklers use between 3 gallons per minute and 10 gallons per minute. The result is the total number of gallons per minute that your lawn irrigation system needs. This is the rate at which your well and pump need to deliver water.