Pipe Volume Calculator
Enter the inside diameter and the length of the run to get volume in gallons and liters, the weight of the water it holds, and how long it takes to fill or flush at a given flow rate. Useful for purge times, chemical dosing, and hydronic fill volumes.
Pipe Run
Pick a size or enter your own inside diameter.
Used for fill and flush time.
Volume
1.51 gal
Volume (metric)
5.7 L
Volume
0.202 ft³
Weight of water
12.6 lb
Fill or flush time
0.4 min
At 4 GPM
Gallons per foot
0.0251
Where The Number Matters
Disinfection is the most common use: chlorination dosing is specified in parts per million of system volume, so getting the volume wrong means either an ineffective flush or an unnecessary one. Hydronic glycol charging works the same way — the percentage is meaningless without the total loop volume.
The fill time output is the one dispatchers care about. A long service line at a modest flow rate can tie up a technician for the better part of an hour, and that is time that should be in the job estimate rather than a surprise on the schedule.
Measure The Inside, Not The Label
Pipe volume scales with the square of the diameter, so a small error compounds fast. Going from a nominal three-quarter inch assumption to the actual PEX inside diameter changes the volume by more than a quarter. When you are not certain, measure the pipe or look up the schedule.
Frequently Asked Questions
- What is the formula for pipe volume?
- Volume equals pi times the radius squared times the length, using the inside diameter. In US units, gallons equal 0.0034 times the inside diameter in inches squared times the length in feet — the same calculation with the unit conversion folded in.
- Should I use nominal or actual inside diameter?
- Always the actual inside diameter. Nominal sizes are labels, not measurements: half-inch copper type L has an inside diameter near 0.545 inches, and half-inch PEX is closer to 0.475. On a long run the difference is significant.
- How much does the water weigh?
- About 8.34 pounds per gallon at room temperature. On long horizontal runs and on rooftop or attic piping, that weight drives hanger spacing and structural support decisions.
- What is this used for day to day?
- Purge and flush times, chemical and antifreeze dosing on hydronic systems, chlorination volumes on well and service line disinfection, and estimating how long a fill will tie up a technician.
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