Pipe Volume Calculator
Calculate how much water a pipe holds in gallons and cubic feet from its inside diameter and length, with copper, PEX and PVC sizes built in, plus the hot water wait.
By CalcBuilt Editorial TeamUpdated September 11, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Water in the pipe
- 2.51 gal
- 3/4 in copper type L, 0.785 in inside diameter, 100 ft long
- Volume
- 0.34 cu ft
- 581 cu in
- Water per foot of pipe
- 0.0251 gal per ft
- 2.51 gal per 100 ft
- Weight of the water
- 21 lb
- At 8.34 lb per gallon; add the pipe's own weight for hangers
- Wait for hot water
- 75 seconds
- Time to push the cooled water out at 2 gpm; the pipe wall absorbs heat too, so expect 20 to 30% longer. Over a minute is where a demand recirculation pump at the far fixture or 1/2 in home runs from a manifold pay off
Pipe Volume formula
Gallons = pi x (Inside diameter (in) / 2)² x Length (ft) x 12 / 231
Cubic feet = Gallons x 231 / 1,728 (7.48 gallons per cubic foot)
Weight (lb) = Gallons x 8.34
Seconds to clear = Gallons / Flow (gpm) x 60Worked example
A 60 ft run of 1/2 in PEX (0.475 in inside diameter) to a bathroom: pi x (0.475 / 2)² = 0.1772 sq in of bore. Multiply by 60 ft x 12 = 720 in for 127.6 cu in, and divide by 231 to get 0.55 gallons. At a 2 gpm faucet the cold water clears in 0.55 / 2 x 60 = 17 seconds.
A 250 ft run of 2 in Schedule 40 PVC (2.067 in inside diameter) to a barn: pi x (2.067 / 2)² = 3.356 sq in. Times 250 x 12 = 3,000 in is 10,067 cu in, or 10,067 / 231 = 43.6 gallons. That is 10,067 / 1,728 = 5.83 cubic feet and weighs 43.6 x 8.34 = 363 lb, which matters if the line hangs from a ceiling or has to be drained for winter.
Quick reference
| Pipe | Inside diameter | Gallons per 100 ft | Weight of water |
|---|---|---|---|
| 1/2 in copper type L | 0.545 in | 1.21 gal | 10.1 lb |
| 3/4 in copper type L | 0.785 in | 2.51 gal | 21.0 lb |
| 1 in copper type L | 1.025 in | 4.29 gal | 35.7 lb |
| 1/2 in PEX | 0.475 in | 0.92 gal | 7.7 lb |
| 3/4 in PEX | 0.671 in | 1.84 gal | 15.3 lb |
| 1/2 in PVC sch 40 | 0.622 in | 1.58 gal | 13.2 lb |
| 3/4 in PVC sch 40 | 0.824 in | 2.77 gal | 23.1 lb |
| 1 in PVC sch 40 | 1.049 in | 4.49 gal | 37.4 lb |
| 2 in PVC sch 40 | 2.067 in | 17.43 gal | 145.4 lb |
| 4 in PVC sch 40 | 4.026 in | 66.13 gal | 551.5 lb |
Why the volume of a pipe matters
The most common reason to know how much water a pipe holds is the wait for hot water. Every gallon sitting in the line between the water heater and the shower has to be pushed down the drain first, and at 2 gpm that is 30 seconds per gallon. A 3/4 in copper trunk holds 2.5 gal per 100 ft against 1.2 for 1/2 in copper and 0.9 for 1/2 in PEX. A manifold with 1/2 in PEX home runs, or a demand recirculation pump under the farthest sink, gets hot water to a fixture in 15 to 20 seconds. A dedicated return line with a timer or aquastat does the same for a 3/4 in trunk. That loop must be insulated to R-3 (IECC R403.5.3) and cannot run 24 hours a day in new work (R403.5.1).
The same number sizes a recirculation pump, tells you how much glycol a hydronic loop or a winterized irrigation main needs, how much a drained line will release into a basement, and how much weight a long overhead run adds to its hangers. For a hydronic system, add the boiler, radiators and expansion tank to the pipe volume.
Use the inside diameter, not the nominal size
Nominal pipe size is a label, not a measurement. A 3/4 in copper tube is 0.875 in outside, and its bore depends on the wall thickness: 0.785 in for type L and 0.811 in for type M. PEX of the same nominal size has a much thicker wall and a 0.671 in bore, while 3/4 in Schedule 40 PVC is 1.050 in outside and 0.824 in inside. The built-in list uses the ASTM standard inside diameters; pick Custom and enter a measured bore for anything else, such as galvanized steel, CPVC or polyethylene.
The result is water at rest, not flow; sizing a line for fixture units and pressure is the pipe size calculator's job. The first hot water also warms the pipe wall on its way, so add 20 to 30 percent to the wait, more for bare copper in a crawlspace. A full 4 in PVC line is about 7.5 lb per ft on hangers spaced 4 ft apart (IRC Table P2605.1).
What this calculator leaves out
The result is a volume. Not included: pipe insulation, sold in 6 ft sticks by pipe size (1/2 in wall for R-3), the recirculation pump, return line, check valve, timer and hangers. For a hydronic loop add the boiler, emitters and expansion tank before mixing glycol at 30 to 50 percent propylene by volume. Replacing water lines needs a plumbing permit in most jurisdictions, typically $50 to $150.
Mistakes that cost money
- A 3/4 in trunk holds 2.5 gal per 100 ft, 1/2 in PEX 0.9; manifold home runs in 1/2 in cut the wait two thirds.
- A crossover recirculation kit with no return line sends hot water back through the cold pipe, so the cold tap runs warm for a minute.
- The energy code (IECC R403.5.1) does not allow a 24-hour recirculation pump in new work; use a demand button or timer and insulate the loop.
- IECC R403.5.3 requires R-3 on 3/4 in and larger hot lines and on recirculation loops; 3/8 in wall foam is R-2, buy 1/2 in wall.
- Nominal size as the bore overstates 3/4 in PEX by 25 percent and understates type L copper 9 percent; buy glycol by the ID.
- A full 4 in PVC line weighs about 7.5 lb per ft; hang it every 4 ft and PEX every 32 in (IRC Table P2605.1).
Key facts
- One US gallon is 231 cubic inches, and one cubic foot holds 7.48 gallons, about 62.4 lb of water.
- Type L copper has a thicker wall than type M: 3/4 in type L has a 0.785 in inside diameter and 3/4 in type M 0.811 in.
- PEX is sold by copper tube size but has a smaller bore: 3/4 in PEX is 0.671 in inside, so it holds about 27% less water per foot than 3/4 in type L copper.
- Volume rises with the square of the diameter, so doubling the inside diameter holds four times as much water per foot.
- Water weighs 8.34 lb per gallon, so 100 ft of 4 in Schedule 40 PVC holds about 66 gallons and 550 lb of water.
Frequently asked questions
How much water is in 100 ft of 3/4 in pipe?
About 2.51 gallons for 3/4 in type L copper (0.785 in bore), 1.84 gallons for 3/4 in PEX (0.671 in) and 2.77 gallons for 3/4 in Schedule 40 PVC (0.824 in).
How many gallons per foot is 1 in pipe?
About 0.043 gallons per foot for 1 in type L copper (1.025 in inside), 0.030 for 1 in PEX and 0.045 for 1 in Schedule 40 PVC.
Why does hot water take so long to arrive?
Because the cooled water in the pipe has to be pushed out first. A 100 ft run of 3/4 in copper holds 2.51 gallons, which is about 75 seconds at a 2 gpm faucet, and the pipe wall also absorbs heat from the first hot water that passes.
How do I get hot water faster to a far bathroom?
Three fixes, cheapest first. A demand recirculation pump under the farthest sink (button or motion sensor) pushes the cooled water back through the cold line and needs only an outlet; the cold tap runs warm briefly afterward. A dedicated 1/2 in return line to the heater with a timer or aquastat is the clean fix during a remodel, insulated to R-3. A 2.5 gal point-of-use mini tank under the sink covers a lavatory but not a shower.
Sources and references
- Copper Tube Handbook (A4015), tube dimensions and physical characteristics (Copper Development Association)
- Pressure Dimensional Catalog, PVC Schedule 40 and 80 pipe (Charlotte Pipe)
- NIST Handbook 44, Appendix C, General Tables of Units of Measurement (NIST)
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