How to calculate pipe volume (gallons per foot)
How much water does a run of pipe actually hold? It is a one-line calculation — and it is the number behind purging, chlorinating, and every fill-time estimate you make.
Learning how to calculate pipe volume is one of those small skills that saves you constant guesswork. Whether you are flushing air from a fresh line, chlorinating a new water main, dosing a treatment, or estimating how long a system takes to fill, it all starts with how many gallons the pipe holds. The math is simple geometry — a pipe is just a long cylinder — and once you have the per-foot figure, the rest is multiplication. Results here are estimates for guidance, not a substitute for local code.
Open the pipe volume calculator →
The gallons-per-foot formula
Gallons per foot = 0.0408 × (inside diameter in inches)²
Then multiply by the run length in feet:
Total gallons = gallons per foot × length (ft)
That constant, 0.0408, already folds in the geometry (the area of a circle) and the conversion from cubic inches to gallons, so you do not have to. Use the inside diameter, not the nominal pipe size, for an accurate number — the difference matters on smaller pipe.
Gallons per foot for common pipe sizes
| Pipe size | Approx. gallons per foot |
|---|---|
| 1/2″ | 0.010 |
| 3/4″ | 0.023 |
| 1″ | 0.041 |
| 1-1/2″ | 0.092 |
| 2″ | 0.163 |
| 4″ | 0.653 |
These are close approximations for common inside diameters; exact values shift a little with pipe schedule and material. For anything precise, plug your real ID into the pipe volume calculator.
A worked example
Say you have 150 feet of 1-inch line. From the table, 1-inch pipe holds about 0.041 gallons per foot, so 0.041 × 150 = about 6.2 gallons. That is how much water you need to displace to fully purge the run, or how much chlorine solution the line will hold when you dose it. Simple, and reliable enough to plan around.
What pipe volume is used for
The number shows up all over real work: purging air from a new run, chlorinating and then flushing a potable line, estimating fill and drain times, dosing treatment chemicals, and sizing expansion tanks. Pair it with the water flow rate calculator to turn volume into time, and with the water line sizing guide when you are planning the run itself. For the underlying cylinder geometry, any reference on volume shows the same formula.
Skip the math
PlumbCalc Pro turns diameter and length into gallons instantly — free, in your browser.
Open PlumbCalc Pro ▶ Subscribe on YouTubeHow to calculate pipe volume — FAQ
How do you calculate the volume of water in a pipe?
Gallons per foot = 0.0408 × inside diameter squared, then multiply by length. A 1-inch pipe holds about 0.041 gallons per foot.
How many gallons per foot is a 3/4 inch pipe?
About 0.023 gallons per foot; a half-inch holds about 0.010 and a two-inch about 0.163.
Why do I need pipe volume?
For purging, chlorinating, dosing, and fill-time estimates — it tells you how much water a run holds.
Is PlumbCalc Pro free?
Yes — the core calculators are free with no signup; an optional one-time PRO purchase unlocks advanced tools.