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Landscape Irrigation Formulas

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Quick Index by Formula Type:

Metric to U.S. System Conversions
U.S. To Metric Conversions
Area and Distance Conversions
Water Pressure Conversions
Flow and Water Volume Conversions
Pump Calculations
Miscellaneous Irrigation Formulas
Friction loss calculator

Metric to U.S. System Conversions, Calculations, Equations, and Formulas:

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U.S. to Metric Conversions, Formulas, Calculations, and Equations:

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Area and Distance Conversions, Equations, Calculations, and Formulas:

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Water Pressure Equations, Conversions, Formulas, and Calculations:

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Flow and Water Volume Formulas, Conversions, Calculations, and Equations:

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Pump Calculations, Conversions, Equations, and Formulas:

The following formulas assume 55% pump efficiency (the standard assumption).

Note: Horsepower is Brake horse power for an electric motor. Do not use for fuel-powered pump engines!

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Miscellaneous Irrigation Formulas, Conversions, Equations, and Calculations:

Sprinkler Precipitation Rate

Note: "head spacing" is the distance between sprinkler heads. L = length between heads in the row, W = width between rows. Therefore "head spacing " = L x W. Use the GPM value for a full circle sprinkler. So if your sprinklers are all half circles, you need to double the GPM value so it is equivalent to a full circle value. Precipitation rate result will be in inches per hour. The common abbreviation for precipitation rate is "ppt".

Precipitation rate for square sprinkler spacing:

(GPM of full circle sprinkler x 96.3) / head spacing in feet = precipitation rate


Precipitation rate for triangular sprinkler spacing:

(GPM of full circle sprinkler x 96.3) / (head spacing in feet x 0.866) = precipitation rate


Example: A irrigation system with sprinklers arranged in rows. The rows are 30 feet apart and the sprinklers are 25 feet apart in the row. The sprinkler heads are arranged in a triangle pattern. Each sprinkler head uses 6 GPM. The calculations for precipitation rate would use the formula above for triangular spacing:

(6 x 96.3) / (30 x 25) x 0.866 = ppt

multiply the numbers in the parenthesis to get-
577.8 / 750 x 0.866 = ppt

do multiplication before division-
577.8 / 649.5 = ppt

finally do the division-
0.89 inches per hour = ppt

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Friction Loss Calculators

Here's a Java powered friction loss calculator for your Java enabled browser. It works with the latest versions of both Netscape and Internet Explorer. Because it is true Java you don't need to be online to use it!

Friction Loss Calculator for PVC Pipe

  1. Select pipe type.
  2. Select pipe size in inches.
  3. Enter GPM flow through this section of pipe.
  4. Enter the pipe length.
  5. Click on calculate.

This program is Freeware and may not be sold.
Java programming by Dan Rasmussen, methodology by Jess Stryker, ©1999.

Windows Friction Loss Spreadsheet Calculator

There is also a stand-alone Win95 friction loss calculator created by Steve Granger. Use the value of 1 for both the Specific Gravity and Viscosity of water when using this calculator. Under the heading Material, "Plastic" is PVC plastic. The current version doesn't work for polyethylene or PEX pipe. The windows calculator will calculate friction losses for PVC plastic, galvanized steel, steel, cast iron, concrete, and copper (drawn tube). Download the calculator from Many thanks to Steve for creating this calculator and providing it free for everyone to use.

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