Pipe Volume Calculator
Enter the inside diameter and the length of the run. You get the gallons it holds, which is what you need for flushing, dosing, antifreeze or working out how long the hot water takes to arrive.
Take this list to the store
We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Use the inside diameter
Pipe is named by a nominal size that is neither the inside nor the outside measurement. Half-inch copper has a 0.545 in bore; half-inch PEX only 0.475. The table below lists the real figures.
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Measure the whole run
Follow the pipe, including the vertical drops and the loops around obstructions. On a hot water line this is what decides how long you stand at the tap.
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Add the runs together
Several identical branches? Enter how many and they are totalled. For a mix of sizes, work each size out separately and add the gallons.
Water held by common pipe
Gallons per 100 ft, using typical inside diameters.
| Pipe | Inside diameter | Gallons per 100 ft |
|---|---|---|
| 1/2 in PEX | 0.475 in | 0.92 gal |
| 1/2 in copper type L | 0.545 in | 1.21 gal |
| 3/4 in PEX | 0.671 in | 1.84 gal |
| 3/4 in copper type L | 0.785 in | 2.51 gal |
| 1 in PEX | 0.862 in | 3.03 gal |
| 1 in copper type L | 1.025 in | 4.29 gal |
| 1/2 in PVC schedule 40 | 0.602 in | 1.48 gal |
| 3/4 in PVC schedule 40 | 0.804 in | 2.64 gal |
| 1 in PVC schedule 40 | 1.029 in | 4.32 gal |
| 2 in PVC schedule 40 | 2.067 in | 17.4 gal |
Why pipe volume matters
- Waiting for hot water. Every gallon sitting in the line is a gallon that runs cold first. 60 ft of 3/4 in copper holds about 1.5 gallons — at 2 gallons a minute that is 45 seconds of waiting.
- Shocking a well or system. Chlorine doses are per gallon of standing water, pipe included.
- Winterizing. Antifreeze has to fill the volume, not the length.
- Freeze risk. A long run of small pipe in an unheated space holds little water and chills fast.
Nominal size is not the bore
Copper, PEX and PVC all use nominal sizing, and none of them measure what they are called. PEX has thick walls, so a half-inch PEX line holds a quarter less water than half-inch copper of the same length.
That difference shows up as a faster hot water arrival on PEX, which is one reason manifold systems with small home-run lines feel quicker.
Cutting the wait for hot water
Three things shorten it: smaller pipe on the branch to each fixture, a shorter route, or a recirculation loop that keeps hot water near the tap.
Work out the volume of the run first. If it is under about half a gallon, the wait is already short and a pump will not gain you much.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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radius = inside diameter ÷ 2 ÷ 12Inches to feet, so everything is in the same unit. -
cubic feet = π × radius² × length × runsThe volume of a cylinder. -
gallons = cubic feet × 7.48US gallons in a cubic foot. -
pounds = gallons × 8.34The weight of the water in the line.
Common questions
How much water is in 100 feet of 3/4 inch pipe?
About 2.5 gallons in copper type L, or 1.84 gallons in PEX, because PEX has a smaller bore.
How do I calculate the volume of a pipe?
π × radius² × length. With the radius in feet and the length in feet you get cubic feet; multiply by 7.48 for gallons.
How many gallons per foot in 1/2 inch pipe?
About 0.0121 gallons per foot in copper, 0.0092 in PEX. Roughly a gallon per 100 ft either way.
Why does my hot water take so long?
Because the water already in the pipe has to come out first. Work out the volume of the run, divide by your flow rate, and that is the wait.
Does pipe volume affect water pressure?
Not directly — pressure comes from the supply. But a smaller bore means more friction loss over a long run, which shows up as weaker flow at the far tap.
How much does the water in my pipes weigh?
8.34 lb per gallon. It matters when pipe is hung from joists over a long horizontal run.
Which inside diameter should I use for PVC?
Schedule 40 figures are in the table. Schedule 80 has thicker walls and a smaller bore, so it holds slightly less.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- ASTM pipe dimension standards (B88 copper, F876 PEX, D1785 PVC) — inside diameters for each pipe type and schedule
- NIST Special Publication 811 — cubic foot to gallon and gallon to liter conversions
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 27, 2026.