Quick answer: a pipe is a long cylinder, so V = π × (inside diameter ÷ 2)² × length. The field shortcut, with diameter in inches: gallons per foot = ID² × 0.0408. Use the inside diameter — and remember that nominal pipe sizes are labels, not measurements.
Gallons per foot for every standard pipe size
Schedule 40 pipe, the most common in domestic plumbing and irrigation. Find your size, multiply by your run length, done:
| Nominal size | Actual inside ⌀ | US gal per ft | Gal per 100 ft | Liters per m |
|---|---|---|---|---|
| 1/2 in | 0.622 in | 0.0158 | 1.58 | 0.196 |
| 3/4 in | 0.824 in | 0.0277 | 2.77 | 0.344 |
| 1 in | 1.049 in | 0.0449 | 4.49 | 0.558 |
| 1¼ in | 1.380 in | 0.0777 | 7.77 | 0.965 |
| 1½ in | 1.610 in | 0.1058 | 10.58 | 1.314 |
| 2 in | 2.067 in | 0.1743 | 17.43 | 2.165 |
| 2½ in | 2.469 in | 0.2487 | 24.87 | 3.089 |
| 3 in | 3.068 in | 0.3840 | 38.40 | 4.769 |
| 4 in | 4.026 in | 0.6613 | 66.13 | 8.213 |
| 6 in | 6.065 in | 1.5009 | 150.1 | 18.64 |
| 8 in | 7.981 in | 2.5988 | 259.9 | 32.27 |
| 10 in | 10.020 in | 4.0963 | 409.6 | 50.87 |
| 12 in | 11.938 in | 5.8147 | 581.5 | 72.22 |
Inside diameters vary with schedule — Schedule 80 has thicker walls and therefore a smaller bore, holding roughly 10–15% less than the figures above at small sizes. Copper tube types K, L and M differ too. When it matters, measure the actual bore or check the manufacturer's spec.
Where the 0.0408 shortcut comes from
gallons per foot = π/4 × d²(in) × 12(in) ÷ 231(in³ per gal) = d² × 0.0408
Worked example — 100 ft of 2-inch Schedule 40:
- Actual inside diameter: 2.067 in
- Per foot: 2.067² × 0.0408 = 4.2725 × 0.0408 = 0.1743 gal
- Over 100 ft: 17.4 gallons
Because the diameter is squared, capacity climbs fast with size. Stepping from 1-inch to 2-inch pipe does not double the water it holds — it nearly quadruples it, from 4.5 to 17.4 gallons per hundred feet. That same squared relationship is why a modest increase in pipe size makes such a large difference to flow and pressure drop.
What plumbers and well owners use this for
Purging a line before sampling
Water sitting in a pipe picks up whatever the pipe is made of. Lead and copper sampling protocols call for flushing a set number of pipe volumes first. A 150 ft service line of 3/4-inch pipe holds 150 × 0.0277 ≈ 4.2 gallons; at a 5 gpm tap that is about 50 seconds per volume, so a three-volume purge runs two and a half minutes.
Shock chlorinating a well system
Dose is set by total system volume — the water in the well casing plus everything in the distribution piping. Miss the pipe volume and you under-chlorinate the very lines you are trying to disinfect. The well volume calculator handles the casing side of that sum.
Sizing an expansion tank or antifreeze charge
Hydronic heating loops need to know their own water content. A 200 ft loop of 1-inch pipe holds about 9 gallons; adding a 50% glycol mix to that means roughly 4.5 gallons of concentrate. Expansion tank sizing works from the same total.
Common mistakes & pro tips
- Using the nominal size as the diameter. "2-inch pipe" has a 2.067 in bore in Schedule 40 — close here, but 1-inch pipe measures 1.049, and 6-inch measures 6.065. Always use the actual inside diameter.
- Using the outside diameter. That gives you the wall as well as the water. For pipe material volume and weight, use the tube volume calculator.
- Forgetting the vertical runs. A service line's total volume includes risers and drops, not just the horizontal distance on the plan.
- Assuming a full pipe. Drain and sewer lines run partly full by design. For a partially filled horizontal pipe, use the fill-depth mode on the tank volume calculator — a pipe is just a small horizontal cylinder.
- Pro tip — pressure testing. Water is nearly incompressible, so the volume here is also roughly what you must add to bring a line up to test pressure, plus a little for pipe expansion.
Frequently asked questions
How do I calculate the volume of water in a pipe?
Use V = π × (inside diameter ÷ 2)² × length. For gallons per foot with the diameter in inches, the shortcut is gallons per foot = ID² × 0.0408. A 2-inch Schedule 40 pipe has a 2.067 in inside diameter, giving 0.174 gallons per foot.
How many gallons are in 100 feet of 1 inch pipe?
Schedule 40 1-inch PVC has an inside diameter of 1.049 in, holding 0.045 gallons per foot — so 100 feet holds about 4.5 gallons. Nominal 1-inch pipe is neither 1 inch outside nor inside, which is why the actual inside diameter matters.
Do I use the inside or outside diameter?
Inside diameter, always — water flows through the bore, not the wall. If you want the volume of the pipe material itself, for weight or for a concrete pour, use the tube volume calculator instead, which subtracts the bore from the outer cylinder.
Why is 2 inch pipe not actually 2 inches?
Nominal Pipe Size is a legacy label, not a measurement. It originated when pipe walls were much thicker and the bore was roughly the nominal size. Modern 2-inch Schedule 40 pipe measures 2.375 in outside and 2.067 in inside. Above 14 inches, the nominal size does equal the outside diameter.
How much water do I need to flush a pipe run?
At minimum one pipe volume, though most guidance calls for two or three volumes to be confident the line is clear. For a 200 ft run of 3/4-inch pipe holding 0.0277 gallons per foot, one volume is 5.5 gallons, so plan on 11 to 17 gallons.
How long does it take to purge a line?
Divide the pipe volume by the flow rate. A 100 ft run of 1-inch pipe holds 4.5 gallons; at a typical 5 gallons per minute tap flow, one full volume takes about 54 seconds. This is the calculation behind "run the tap for two minutes before sampling" advice.