Pipe Volume Calculator (Water in a Pipe)
To find the water volume in a pipe, use: gallons = 0.0408 × ID² × L, where ID is the inside diameter in inches and L is the pipe length in feet. The geometry is Volume = π × r² × L; the 0.0408 constant bakes in π/4 and the 231 in³-per-gallon conversion. Example: 100 ft of 3/4" Schedule 40 (0.824" ID) holds about 2.8 gallons. Always calculate off inside diameter, not nominal size.
Run it free on PlumbCalc Pro → Enter size and length, get gallons and liters instantly. No signup, no callbacks.
The formula (and why ID, not nominal)
Water volume in a round pipe is a cylinder: V = π × (ID/2)² × L.
For plumbing, convert straight to gallons: gallons = 0.0408 × ID² × L (ID in inches, L in feet). A quick field version is gallons = ID² × L ÷ 24.5.
Nominal size is a label, not a measurement. A "3/4-inch" pipe is not 0.75" inside. Copper Type L 3/4" is about 0.785" ID; Schedule 40 3/4" is 0.824" ID; PEX 3/4" is roughly 0.681" ID. Use the true ID for the material on the job or your number runs high.
Water volume per foot by pipe size
These use Schedule 40 inside diameters, the safe default for a first pass. Multiply gal/ft by your run length.
| Nominal size | Inside dia (Sch 40) | Gallons per foot | Gallons per 100 ft |
|---|---|---|---|
| 1/2" | 0.622" | 0.0158 | 1.58 |
| 3/4" | 0.824" | 0.0277 | 2.77 |
| 1" | 1.049" | 0.0449 | 4.49 |
| 1-1/4" | 1.380" | 0.0777 | 7.77 |
| 1-1/2" | 1.610" | 0.1058 | 10.58 |
| 2" | 2.067" | 0.1743 | 17.43 |
| 3" | 3.068" | 0.3841 | 38.41 |
| 4" | 4.026" | 0.6613 | 66.13 |
Rule of thumb worth taping to the truck: a 3/4" line carries roughly 2.8 gallons for every 100 feet. That's the number that tells you how long to flush a new run or how much chlorine to dose for disinfection.
Worked example
Say you're purging a 250 ft run of 1" Schedule 40. From the table, 1" holds 0.0449 gal/ft. So 0.0449 × 250 = 11.2 gallons of standing water. Size your flush time and disinfection dose off that, not a guess.
Sizing check: don't just fill it, flow it
Volume tells you what's sitting in the pipe. It won't tell you if the pipe is big enough to move the water. For supply lines, keep design velocity in the 5–8 ft/s range—go higher and you get erosion, water hammer, and noise. To size for a flow rate, solve d = √(4Q / (π × v)), where Q is flow and v is velocity in consistent units. PlumbCalc Pro runs both the volume and the velocity check so you don't oversize or starve a branch.
Why this calculator
Built by a tradesman, not a marketer. The defaults are the safe ones, the IDs are real, and the answer loads without a form or a "talk to sales" wall. Open the free pipe volume calculator →
Frequently asked questions
How do I calculate the volume of water in a pipe?
Use gallons = 0.0408 × ID² × L, with inside diameter in inches and length in feet. For example, 50 ft of 1" Sch 40 pipe (1.049" ID) holds 0.0408 × 1.100 × 50 = about 2.2 gallons. For liters, multiply gallons by 3.785.
Should I use nominal size or inside diameter?
Always inside diameter (ID). Nominal size is just a name. A "1-inch" pipe can have an ID from about 0.85" (PEX) to 1.05" (copper/Sch 40), which changes volume by 30% or more. Look up the true ID for your material and schedule.
How much water is in 100 feet of 3/4" pipe?
About 2.8 gallons for 3/4" Schedule 40 (0.824" ID). Copper Type L 3/4" (0.785" ID) holds roughly 2.5 gallons per 100 ft. Use these for flushing, freeze-drain, and disinfection dosing.
What's the difference between pipe volume and pipe flow rate?
Volume is the standing water the pipe holds (gallons). Flow rate is how fast water moves through it (GPM), which depends on diameter, pressure, and velocity. Keep supply velocity at 5–8 ft/s. PlumbCalc Pro handles both so you can fill and flow-check in one place.