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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

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

- Millimeters (mm) x 0.03937 = inches (")(in)
- Centimeters (cm) x 0.3937 = inches (")(in)
- Meters (m) x 39.37 = inches (")(in)
- Meters (m) x 3.281 = feet (')(ft)
- Meters (m) x 1.094 = yards (yds)
- Kilometers (km) x 0.62137 = miles (mi)
- Kilometers (km) x 3280.87 = feet (')(ft)
- Liters (l) x 0.2642 = gallons (U.S.)(gals)
- Liters (l) x 0.0353 = cubic feet
- Bars x 14.5038 = pounds per square inch (PSI)
- Kilograms (kg) x 2.205 = Pounds (P)
- Kilometers (km) x 1093.62 = yards (yds)
- Square centimeters x 0.155 = square inches
- Square meters x 10.76 = square feet
- Square kilometers x 0.386 = square miles
- Cubic centimeters x 0.06102 = cubic inches
- Cubic meters x 35.315 = cubic feet

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

- Inches (")(in) x 25.4 = millimeters (mm)
- Inches (")(in) x 2.54 = centimeters (cm)
- Inches (")(in) x 0.0254 = meters (m)
- Feet (')(ft) x 0.3048 = meters (m)
- Yards (yds) x 0.9144 = meters (m)
- Miles (mi) x 1.6093 = kilometers (km)
- Feet (')(ft) x 0.0003048 = kilometers (km)
- Gallons (gals) x 3.78 = liters (l)
- Cubic feet x 28.316 = liters (l)
- Pounds (P) x 0.4536 = kilograms (kg)
- Square inches x 6.452 = square centimeters
- Square feet x 0.0929 = square meters
- Square miles x 2.59 = square kilometers
- Acres x 4046.85 = square meters
- Cubic inches x 16.39 = cubic centimeters
- Cubic feet x 0.0283 = cubic meters

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

- Acre x 43560 = square feet
- Inches x 0.0833 = feet
- Feet x 12 = inches
- Square mile ("Sections") x 640 = Acres
- Miles x 5280 = feet
- Circumference of circle x 0.3183 = Diameter of the circle
- Diameter of circle x 3.14 = Circumference of circle
- Diameter squared x 0.7854 = Area of circle
- Radius squared x 3.14 = Area of circle
- Miles x 1760 = yards
- Square Miles x 259 = hectares
- Square kilometers x 100 = hectares
- Acres x 40.47 = areas
- Hectares x 100 = areas
- Areas x 100 = square meters
- Yards x 3 = feet
- Acres x 4840 = square yards

## Water Pressure Equations, Conversions, Formulas, and Calculations:

- Feet head (of water) x .433 = Pounds per square inch (PSI)
- Feet head (of water) x 0.3049 = meters head (of water)
- Feet head (of water) x 0.0295 = atmospheres
- Feet head (of water) x 2.988 = kiloPascals
- Meters head (of water) x 3.28 = feet head (of water)
- atmosphere (Atm) x 1.013 = 1 bar (b)
- atmosphere x 14.696 = Pounds per square inch (PSI)
- bars x 1.033 = kilograms per square centimeter
- bars x 14.67 = Pounds per square inch (PSI)
- bars x 100 = kiloPascals
- Kilograms per square centimeter x 14.223 = pounds per square inch (PSI)
- Pounds per square inch (PSI) x 2.31 = feet head (ft. hd.)
- Pounds per square inch (PSI) x 0.0703 = kilograms per square centimeter
- Pounds per square inch (PSI) x 0.06895 = bars
- Pounds per square inch (PSI) x 0.06805 = atmospheres
- Pounds per square inch (PSI) x 2.31 = feet head (of water)
- Pounds per square inch (PSI) x 6.896 = kiloPascals
- kiloPascal x 0.145 = Pounds per square inch (PSI)
- kiloPascal x 0.01 = bars
- kiloPascal x 0.334 = feet head (of water)

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

- U.S. Gallons per minute (gpm) x .1337 = Cubic feet per minute
- Cubic feet per minute x 7.48 = U.S. gallons per minute
- Cubic feet per second x 448.8 = U.S. gallons per minute
- U.S. gallons per minute x 0.00223 = Cubic feet per second
- Acre inches per hour x 453 = U.S. gallons per minute
- British Imperial gallons x 1.201 = U.S. gallons
- U.S. gallons x 0.833 = British Imperial gallons
- Acre feet x 325,850 = U.S. gallons
- Acre inches x 27154 = U.S. gallons
- Velocity in feet per second = (0.408 x GPM) / Inside diameter of pipe in inches, squared
- Q=AV (quantity = area x velocity) ("the basic equation of water flow")

(example: quantity in cubic feet per second = square feet of area x feet per second velocity) - One inch of water depth = 0.62 gallons per square foot of area
- GPM x 226.8 = liters per hour
- cubic feet per minute x 1699 = liters per hour
- acre inches per hour x 1712.3 = liters per minute
- acre feet x 1231.7 = cubic meters
- acre inch x 102.64 = cubic meters
- velocity (feet/second) x 0.3047 = velocity (meters/second)
- velocity (meters/second) x 3.281 = velocity (feet/second)

## 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!

- GPM = (horsepower x 2178) / feet head
- Feet head = (2178 x horsepower) / GPM
- Efficiency of pump = (GPM x feet head) / (horsepower x 3960)
- Horse powers x 745.7 = watts (W)
- Water flow (in cubic meters per second) = 0.55 x pump power (watts) / pressure (pascals)
- Water flow (in liters per second) = 5.43 x pump power (kilowatts) / pressure (bars)

## 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

Text and Images by Jess Stryker unless noted. Copyright © Jess Stryker, 1997-2011. All rights reserved.