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# Landscape Irrigation Formulas

If you don't find what you are looking for here, try using the search engine in the left column.

### 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:

- 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

## 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

- Select pipe type.
- Select pipe size in inches.
- Enter GPM flow through this section of pipe.
- Enter the pipe length.
- Click on calculate.

- Friction loss calculations are for standard PVC pipe.
- Cl 160 PVC pipe is not commonly available in 1/2" size, therefore the calculator assumes that Cl 315 pipe will be substituted.
- Cl 200 PVC pipe is not commonly available in 1/2" size, therefore the calculator assumes that Cl 315 pipe will be substituted.
- Cl 160 PVC pipe is not commonly available in 3/4" size, therefore the calculator assumes that Cl 200 pipe will be substituted.
- This calculator uses a contracted form of the Hazen-Williams formula which is the industry standard for calculating friction loss.
- A conservative C coefficient of 140 is used for PVC pipe (many authorities use 150 which is risky as it leaves no margin for error).
- Velocities over 7 feet per second should not be allowed in PVC pipe.
- Velocities over 5 feet per second should be avoided, especially with pressures higher than 80 PSI, or in pipe larger than 2".

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 http://www.IrrigationTutorials.com/LineSize.exe. Many thanks to Steve for creating this calculator and providing it free for everyone to use.

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