How to use the pipe volume calculator
- Name a run of pipe and enter its inside diameter in inches and its length in feet.
- Use Add a run if you have more than one size or section.
- Use the inside measurement of the pipe, not the outside size or the nominal name.
- Read the gallons, liters, cubic feet and gallons per foot for each run and in total.
Formula
Volume of a run = pi x (inside diameter / 2)^2 x length
Cubic inches = volume with diameter in inches and length in feet x 12
Gallons = cubic inches / 231, and liters = gallons x 3.785411784
Gallons per foot = gallons / length in feet
One US gallon is 231 cubic inches. A cubic foot is 1,728 cubic inches.
A 1 inch pipe that is 100 feet long
A pipe with a 1 inch inside diameter and a length of 100 feet holds pi x 0.5 x 0.5 x 1,200 = 942.48 cubic inches, which is 4.08 gallons, or 15.44 liters, or 0.545 cubic feet. That is about 0.0408 gallons per foot.
Adding a 50 foot run of 0.75 inch pipe (1.15 gallons) and a 20 foot run of 2 inch pipe (3.26 gallons) brings the total to 8.49 gallons in 170 feet of pipe.
Inside diameter is what counts
Pipe is sold by a nominal size that does not equal its inside diameter. The inside diameter depends on the material and wall thickness, so a pipe called 1 inch may measure a little more or less on the inside. Use the real inside measurement from the pipe specification or by measuring the open end.
Because the diameter is squared, small errors in it matter a lot. A pipe with a 10% larger inside diameter holds about 21% more water.
Where the volume is useful
Knowing how much water a run holds helps with sizing a drain-down, estimating the water you will waste before hot water arrives, planning a flush or choosing a container to catch what comes out. It also helps with sizing chemical doses for a closed loop.
The result is for a full, straight, round pipe. Fittings, valves, tanks and partly full pipe are not counted. For tanks use the tank volume calculator, for general shapes the volume calculator, for an aquarium the aquarium volume calculator, and for watering time the sprinkler run time calculator.
Assumptions and limits
- Each run is a full, straight pipe with a round inside.
- The diameter you enter is the inside diameter in inches and the length is in feet.
- Fittings, valves and equipment connected to the pipe are not counted.
- A US gallon of 231 cubic inches is used, and liters are converted from that.
Frequently asked questions
How many gallons are in a pipe?
Multiply pi by the square of half the inside diameter and by the length, in the same units, then convert to gallons. A 1 inch pipe 100 feet long holds about 4.08 gallons.
Do I use the outside or inside diameter?
The inside diameter, because that is the space the water fills. Nominal pipe sizes do not match it exactly.
How many gallons per foot does a pipe hold?
It depends on the diameter. A 1 inch inside diameter holds about 0.0408 gallons per foot, and the table shows the figure for each of your runs.
Can I add several pipe sizes together?
Yes. Add a row for each run with its own diameter and length, and the tool totals the volume.