How much water does it take to grow an attractive and healthy lawn? There really is no “right” answer to this question, it all depends factors such as where you live, the type of lawn you have, local climate conditions, lawn maintenance practices, and so on.

Research has shown that on average about half of the water used in a single-family home during the course of a year will be put onto the landscape. Obviously if you live in a wet climate such as the Pacific Northwest you will use less water outdoors and if you live in a hot dry climate like Southern California you will use more. But even in a wet climate, the lawn area is often the single highest user of water in the house.

How to calculate the water requirements for your lawn

If you are an intrepid irrigator and internet user it should be fairly easy to calculate the theoretical irrigation requirement for your lawn. There are two key pieces of information you need to obtain: 1) The area (in square feet) of your lawn; and 2) the evapotranspiration (ET) rate for the irrigation season in your area. Don’t worry, both of these items should be fairly easy to obtain.

Lawn Area
If your lawn isn’t too big you could simply go outside with a tape measure and physically measure the area. Divide the yard into a series of rectangles and triangles and sum up the areas. Recall that the area of a rectangle is the base length ´ height length. The area of a triangle is ½ ´ the base length ´ height length.

If you’re not in the mood to measure your yard you can calculate the lawn area in another way. Start with the total lot size. If you only know the lot size in acres you can convert to square feet by knowing that 1 acre = 43,560 square feet. From the total lot size subtract the footprint of your house and the area of your driveway and sidewalks. If you don’t know these exact areas make an educated guess. Finally, subtract any other areas on your lot that are not irrigated (swimming pools, patios, bare patches, ponds, etc.). The result will be an estimate of the lawn area at your house. This calculation is summarized below.

Total lot area (sf)
- building footprint (sf)
- driveway area (sf)
- sidewalk area (sf)
- all other non-irrigated areas (sf)
-----------------------------------
= Total irrigated area (sf)

Evapotranspiration (ET) Rate
Evapotranspiration or ET is a measurement (usually in inches) of the amount of water required to maximize plant growth. ET can be based on a number of factors including the local temperature, precipitation, cloud cover, solar radiation, and the type of plants you are growing. Most of the ET calculations done in urban settings are for turf grass.

To find the ET rate for your area you will need to do a little searching on the world wide web. Using google or your favorite search engine simply search for “evapotranspiration” followed by your city and state. You should turn up a number of possibilities. Usually ET is calculated by a local university or weather service. It is also used frequently in agriculture.

Once you have a measurement of the annual ET rate in inches you are ready to go!

Calculate the Water Requirement for Your Lawn
Use the following equation to calculate the water requirement for your lawn:

Irrigated area (sf) x ET rate (inches) x 0.6233 = Water requirement in gallons

This calculation will give you a rough estimate of the amount of water your landscape needs over the course of the entire irrigation season.