Missoula County Weed District & Extension

Turfgrass Irrigation Calculator

The Turfgrass Irrigation Calculator is meant to be a tool for home owners to estimate the irrigation requirements for their lawns in Missoula.  This calculator uses estimated evapotranspiration (ET) rates for a typical turfgrass as well as historical climactic data for our region.  This calculator is does not integrate current environmental conditions (temperature, wind, precipitation, RH) in the calculations.  This resource is meant to help guide irrigation decisions; but attention to environmental conditions, site specific soil types, as well as plant growth and development will be critical for establishing and maintaining a healthy lawn.


To use Turfgrass Irrigation Calculator:

Step 1:  Select current month from drop down menu

Step 2:  Select mowing height of grass (typically 2-3” for Kentucky bluegrass lawn)

Step 3:  Select number of days between irrigation cycles (generally 3, but 2 during hot and dry cycles)

Step 4:  Estimate or Measure Sprinkler Precipitation Rate (PPT) or the output of your irrigation system per hour

            Estimate PPT:

                        Step 4a:  Measure distance between sprinkler heads (in ground system)

                        Step 4b:  Measure distance between rows of sprinklers

                        Step 4c:  Select geometric design of sprinkler system

PPT Estimate

            Measure PPT:

Step 4a’:  Place 10 equal size containers (tin can) randomly throughout irrigation zone

Step 4b’:  Run irrigation for 15 minutes

Step 4c’:  Consolidate collected irrigation water into one container

Step 4d’:  Measure depth of water in container (in inches) and divide by 10 to get average output per zone

Step 4e’:  Multiply above (output) by 4 to get Sprinkler Precipitation Rate (inches/hour) and record in calculator table

Measured PPT 1 Measured PPT 2

Step 5:  Click the “Calculate” button to calculate duration required

Step 6:  Repeat for each zone

Mowing Height (3, 2, 1") inches
Days between irrigation events days
Distance between heads feet
Distance between rows feet
Distribution volume of full circle Sprinkler Heads in GPM (if half circle than multiply by two, quarter circle multiply by 4) GPM
Geometric deisgn of irrigation system shape
Sprinkler Precipitation rate (enter value if measured or leave blank if using calculated estimate above) ppt (inches/hour)
Irrigation Rate Calculate minutes