In US units, choose the pipe material and nominal size, then enter the length in feet; the calculator looks up the published inside diameter. In Metric, type the inside diameter in millimetres and the length in metres. Add the flow of the faucet at the end of the run and it also estimates the seconds it takes to empty that pipe, roughly your wait for hot water when the line has gone cold.
The pipe volume formula
A pipe is a long cylinder, so the water it holds is the area of the bore times the length. The calculator works from the inside diameter, never the nominal size: a "3/4-inch" copper pipe is not 3/4 inch across inside, and any difference grows once it is squared.
gallons = 3.1416 × (inside diameter ÷ 2)² × length in inches ÷ 231
There are 231 cubic inches in a US gallon. Metric is tidier: radius in metres, squared, times 3.1416 and the length in metres gives cubic metres; multiply by 1,000 for litres. The wait time is volume divided by flow, times 60 for seconds.
Worked example: 40 feet of 3/4-inch type L copper
The calculator opens on type L copper, 3/4 inch nominal, 40 feet long, with a faucet running at 1.5 gpm.
- Radius: 0.785 inch inside diameter ÷ 2 = 0.3925 inch.
- Bore area: 3.1416 × 0.3925 × 0.3925 = 0.484 square inches.
- Volume: 0.484 × 480 inches = 232.3 cubic inches.
- Gallons: 232.3 ÷ 231 = 1.01 gallons (3.81 litres).
- Wait: 1.01 ÷ 1.5 × 60 = about 40 seconds.
Those are the figures the tool shows. Press Metric and it starts from a 20 mm bore, 12 metres and a 6 litre per minute tap: 3.77 litres and about 38 seconds.
Pipe volume chart
The same inside diameters the calculator uses, worked out per 100 feet of straight pipe and per metre:
| Pipe | Nominal size | Inside diameter | Gallons per 100 ft | Litres per metre |
|---|---|---|---|---|
| Copper type L | 1/2 in | 0.545 in | 1.21 | 0.151 |
| Copper type L | 3/4 in | 0.785 in | 2.51 | 0.312 |
| Copper type L | 1 in | 1.025 in | 4.29 | 0.532 |
| Copper type M | 1/2 in | 0.569 in | 1.32 | 0.164 |
| Copper type M | 3/4 in | 0.811 in | 2.68 | 0.333 |
| PEX (SDR 9) | 1/2 in | 0.475 in | 0.92 | 0.114 |
| PEX (SDR 9) | 3/4 in | 0.671 in | 1.84 | 0.228 |
| PEX (SDR 9) | 1 in | 0.875 in | 3.12 | 0.388 |
| PVC schedule 40 | 1 in | 1.049 in | 4.49 | 0.558 |
| PVC schedule 40 | 2 in | 2.067 in | 17.43 | 2.165 |
PEX holds noticeably less than copper of the same nominal size because its thicker wall eats into the bore. Type M copper, with a thinner wall than type L, holds slightly more.
What the number is good for
The headline figure is the water standing in that run, which comes up in several jobs.
- Hot water waits. A gallon in the line and a 1.5 gpm faucet means around 40 seconds of waiting and a gallon down the drain each time. Measure the real faucet flow with the flow rate calculator rather than guessing.
- Dosing a heating system. Inhibitor and cleaner are dosed by system volume. The pipework is one part of that, with the radiators and boiler making up the rest; the guide to radiator flushing explains where inhibitor fits in.
- Draining down. The volume tells you how big a container to have ready when you empty pipes before leaving a house unheated; the guide to protecting pipes from freezing covers when that is worth doing.
Limits, and when to call a professional
The calculator counts straight pipe only. Elbows, valves, the water heater itself and any radiators all add volume on top.
The wait time is a minimum. The first hot water spends its heat warming cold pipe walls, so the real wait is longer, especially in winter or along uninsulated copper.
If unsure of the bore, measure it. Old galvanised steel narrows as it corrodes, and metric copper is sized by its outside: 15 mm copper is about 13.6 mm inside.
This is not a pipe sizing tool: it tells you how much a pipe holds, not how much it can carry. Sizing pipe for new work depends on pressure, run length and the fixtures served, and codes set the method, so that belongs with a plumber or designer. Bring one in too if the hot water wait has suddenly got much longer or the water never gets properly hot, which can point to a faulty mixing valve or a crossover between hot and cold.
Where the figures come from
The inside diameters are published nominal dimensions: copper tube types L and M from the US standard for copper water tube, PEX at SDR 9 in copper tube sizes, and schedule 40 PVC pressure pipe. Real pipe varies slightly within tolerance. The 231 cubic inches and 3.785 litres per US gallon are defined values.
FAQ
How do you find the volume of a pipe?
Halve the inside diameter for the radius, square it, then multiply by 3.1416 and by the length in the same units. For gallons from inches, divide the cubic inches by 231. Forty feet of 3/4-inch type L copper comes to 1.01 gallons.
How many gallons are in 100 feet of 3/4-inch pipe?
About 2.51 gallons (9.5 litres) in type L copper, 2.68 in type M copper, 1.84 in PEX and 2.77 in schedule 40 PVC.
How much water is in a metre of 15 mm copper pipe?
With a 13.6 mm bore, about 0.145 litres per metre, so a 10 metre run holds roughly 1.45 litres.
Does a smaller pipe shorten the wait for hot water?
Yes, at the same flow. Half-inch type L copper holds 1.21 gallons per 100 feet against 2.51 for 3/4-inch, so the same run empties in about half the time. A smaller pipe also carries less, so don't downsize a line feeding several fixtures without advice.