What is the Weight of Water?

All drainage systems are designed as if the piping is full of water, even though the pipe will rarely, if ever, be full.

Reason being, if unaccounted for, the weight of the water in a piping system can negatively impact its durability and effectiveness. Among the primary concerns for engineers and architects are:

1. Piping Clamps or Supports

The total weight (with water), material and diameter of a pipe determine the type and number of clamps or supports needed.

For example, a 6-in. Schedule 40 PVC pipe is going to weigh much more full of water than it will empty, which puts more pressure on the structural strength of the piping material between supports. Because PVC is a flexible material, and a Schedule 40 wall is relatively thin, the pipe has the potential to sag between supports.

Knowing the total pipe weight when full can help engineers determine the appropriate number of supports needed and the associated spacing.

2. Head Pressure 

Head pressure is the velocity and force with which water flows through a pipe. The greater the head pressure, the more force is placed on bends and turns throughout the piping system. Head pressure is based on a number of factors, including pipe size, material type, pitch and length of the system.

The weight of water is also a key factor, as the weight of the fluid itself has a compounding effect. The longer and higher the system, the more pressure is created by the water’s own weight. This pressure is greatest in the lower portions of the system.

Green Roof Design

Another area where the weight of water comes into play is on green roofs. Some of these systems are designed to retain water on the roof for insulation and other purposes. The weight of that water requires additional supports be built into the roof top itself to ensure its structural integrity. 


Water Weight Conversion Tables

We’ve put together the following tables to help calculate the weight of water and the resultant stress on your piping system.

Water Weight by Unit of Measure 

Cubic Feet of Water

62.428 lbs.


8.345 lbs.

1 Ounce of Water

28.35 g


2.205 lbs. or 1,000 g 

Cubic Meter

2,204.5 lbs.

1 pprn

1 lb. per million lbs.

1 ppm

8.34 lbs. per million gallons of water

1 pprn

1 mg per liter of water


Gallons and Weight of Water per 1 Foot of Pipe

The following data is based on Schedule 40 pipe. Volume and weight may vary based on the Type and Schedule of pipe. Also weight given is for the WATER ONLY, and does NOT included the weight of the pipe.

Pipe Size (inches)

Gallons of Water in 1 Foot of Pipe

Weight of Water in 1 Foot of Pipe (pounds)

NOTE: Does NOT include the weight of the pipe





















































Conversion Factors



To Obtain

Acres 43,560 Square Feet
Atmospheres 33.90 Feet of Water
Centimeters 0.3937 Inches
Cubic Feet 7.48052 Gallons
Cubic Feet 28.32 Liters
Cubic Feet/second 449 Gallons/min.
Cubic Meters 35.31 Cubic Feet
Cubic Meters 264.2 Gallons
Cubic Meters 10^3 Liters
Cubic Yards  27 Cubic Feet
Cubic Yards 202.0 Gallons
Feet 30.48 Centimeters
Feet 0.3048 Meters
Feet of Water 62.43 Lbs./sq.ft.
Feet of Water 0.434 PSI (lbs./sq.in.)
Gallons 3,785  Cubic Centimeters
Gallons 0.1337 Cubic Feet
Gallons 3.785 Liters
Gallons Water 8.3453 Pounds of Water
Gallons/minute 2.228 x 10^-3 Cubic Feet/sec.
Gallons/minute 1,440 Gallons/day
Gallons/minute 0.06308 Liters/sec.
Gallons/day 6.944 x 10^-4 Gallons/min.
Gallons/day/sq.ft. 1.604 Inches/day
Grams 2.205 x 10^-3 Pounds
Grams/liter 1,000 Parts/million
Hectares 2.471 Acres
Horsepower 33,000 Foot-Lbs./min.
Horsepower 0.7457 Kilowatts
Inches 2.540 Centimeters
Inches/day 0.6234 Gallons/day/sq.ft.
Kilograms 2.205 Lbs.
Kilowatts 1.341 Horsepower
Kilowatt-hours 2.655 x 10^6 Foot-lbs.
Liters 10^3 Cubic Centimeters
Liters 0.03531 Cubic Feet
Liters 0.2642 Gallons
Meters 3.281 Feet
Milligrams/liter 1 Parts/million
Million Gallons/day 1.54723 Cubic ft./sec.
Parts/million 8.345 Lbs./million gal.
Pounds 453.5024  Grams
Pounds of Water 0.1198 Gallons
PSI (lbs/sq. in.) 2.31 Feet of Water
Square Feet 2.296 x 10^-5 Acres
Temperature (°C) + 17.78 1.8 Temp (°F)
Temperature (°F) - 32 5/9 Temp (°C) 



Ask the Roof Drain Wizard

Have any questions about roof drains or water flow? Ask the Hy-Tech Drain Wizard. Call him at 800-635-0384 or email drains@hy-techroof.com. 



Photo Credit: Henningklevjer