Pipe Volume Calculator — Gallons in a Pipe
Useful for chlorinating a line, charging a system with glycol, sizing a hydrostatic test — and for the question every homeowner asks. Hot water is slow to arrive because you have to push the cold water already in the pipe out first, and this tells you exactly how much that is and how long it takes.
Work out the volume
Used for the purge time.
Overrides the pickers — use it for an unlisted size, a tank, or a well casing.
Water held
At 2.0 gpm the tap runs 38 seconds before the water standing in this run has been pushed out.
Litres
4.8 L
Time to purge
38 sec
See the breakdown
Geometry only — fittings, valves and the water in the heater are not counted. For a whole system, add the appliance volumes separately.
The formula, explained in plain English
The area of a circle, a length, and one unit conversion.
Bore squared, so it moves fast
Doubling the bore quadruples the volume. Going from 3/4 to 1 inch copper is a 31% wider bore and 71% more water standing in the run — which is why oversizing a hot line makes the wait worse.
Nominal size is not the bore
Nominal 3/4 inch is 0.785 in Type L copper, 0.824 in Schedule 40, and 0.681 in PEX. Every figure here uses the real bore, computed from outside diameter and wall thickness.
PEX wins this one
PEX's small bore is usually a disadvantage — worse velocity and worse friction. For hot-water wait time it is the opposite: a PEX run holds about 25% less water, so it delivers hot sooner.
Weight matters at scale
8.33 lb per gallon is nothing on a fixture branch and a real structural load on a large main — 100 feet of 6-inch holds over half a ton of water before you add the pipe itself.
Worked examples
The slow hot tap, chlorinating a well line, and why upsizing makes the wait worse.
Why the far bathroom is slow
0.0251 gal/ft × 50 = 1.26 gal standing in the pipe
at 2 gpm: 1.26 ÷ 2 = 0.63 min = 38 seconds
Result: 38 seconds of cold water before the hot even starts arriving — and that is at full flow. Insulation does not fix this; it only keeps the standing water warm for longer between draws.
Chlorinating a service line
200 feet of 1-inch Schedule 40 from a well to the house.
× 200 ft = 9.0 gal
= 34.0 L, weighing 75 lb
Result: knowing the exact volume is what lets you dose a disinfection correctly rather than guessing — and it tells you how long to run the tap to confirm the chlorine has reached the far end.
Upsizing the hot line backfires
50 ft of 1" Type L: 2.14 gal → 64 s
50 ft of 3/4" PEX: 0.94 gal → 28 s
Result: the 1-inch line makes the wait 27 seconds longer, and the PEX run makes it 9 seconds shorter. For hot-water branches, smaller bore is the right instinct — the opposite of the rule on cold mains.
Gallons per foot by size
Same nominal size, three materials, three different answers — because the bore differs. Computed from the same code the calculator runs.
| Nominal size | Copper Type L | Schedule 40 | PEX |
|---|---|---|---|
| 1/2" | 0.0121 | 0.0158 | 0.0096 |
| 3/4" | 0.0251 | 0.0277 | 0.0189 |
| 1" | 0.0429 | 0.0449 | 0.0312 |
| 1-1/4" | 0.0653 | 0.0777 | 0.0467 |
| 1-1/2" | 0.0924 | 0.1058 | 0.0652 |
| 2" | 0.1608 | 0.1743 | 0.1115 |
| 2-1/2" | 0.2479 | 0.2487 | — |
| 3" | 0.3539 | 0.3840 | — |
100 feet of Schedule 40
Where the water weight stops being an afterthought and starts being a hanger calculation.
| Size | Bore | Volume | Water weight |
|---|---|---|---|
| 1" | 1.049 in | 4.5 gal | 37 lb |
| 2" | 2.067 in | 17.4 gal | 145 lb |
| 3" | 3.068 in | 38.4 gal | 320 lb |
| 4" | 4.026 in | 66.1 gal | 551 lb |
| 6" | 6.065 in | 150.1 gal | 1,250 lb |
The wait for hot water
3/4-inch Type L copper at 2 gpm — the standing water that has to clear before hot arrives.
| Run length | Standing water | Wait at 2 gpm |
|---|---|---|
| 20 ft | 0.50 gal | 15 sec |
| 40 ft | 1.01 gal | 30 sec |
| 50 ft | 1.26 gal | 38 sec |
| 75 ft | 1.89 gal | 57 sec |
| 100 ft | 2.51 gal | 75 sec over a minute |
Sources & standards: this is geometry, not code — π ÷ 4 × d² × length, converted at 231 cubic inches per US gallon and 8.33 lb per gallon. Pipe dimensions come from ASTM B88 (copper), F876 (PEX) and D1785 (Schedule 40), with every bore computed from outside diameter and wall thickness rather than transcribed. Fittings, valves and appliance volumes are not included.
Frequently asked questions
Common questions about pipe volume, purge time, and why hot water is slow.
How many gallons are in my pipe?
Multiply 0.0408 × the bore squared to get gallons per foot, then multiply by the length. A 50-foot run of 3/4-inch Type L copper (0.785-inch bore) holds 1.26 gallons — 4.8 litres, weighing 10.5 lb. The calculator uses the real bore for each material rather than the nominal size.
Why does hot water take so long to arrive?
Because you have to push the cold water already sitting in the pipe out of the way first. That 1.26 gallons in a 50-foot 3/4-inch run takes 38 seconds to clear at 2 gpm — before the hot water even starts to arrive. It is the reason recirculation loops and smaller-bore home runs exist, and why moving the heater closer beats insulating a long run.
What is the formula for pipe volume?
Area times length, converted to gallons: gal/ft = π ÷ 4 × d² × 12 ÷ 231, which collapses to 0.0408 × d² with d the inside diameter in inches. Cross-check: 1-inch Schedule 40 has a 1.049-inch bore, giving 0.0449 gal/ft against a published 0.0449.
Should I use the nominal size or the actual bore?
Always the bore, and the gap is large. Nominal 3/4-inch is 0.785 inch in Type L copper, 0.824 in Schedule 40, and only 0.681 in PEX. Because volume goes as the square of the bore, that PEX run holds 25% less water than the copper one at the same nominal size — which is one of the few places PEX's small bore is an advantage.
How much does the water in a pipe weigh?
8.33 lb per gallon, so this 50-foot run carries 10.5 lb. That is negligible here, but it stops being negligible on a large main: 100 feet of 6-inch Schedule 40 holds about 150 gallons weighing roughly 1,250 lb, which is a real load on hangers and structure.
What do I need this for besides curiosity?
Chlorinating or shock-treating a line, working out an antifreeze or glycol charge, filling and bleeding a system, estimating flush and purge time, sizing a hydrostatic test, and calculating how much water a repipe wastes on start-up. It is also the honest way to answer a client asking why the hot tap is slow.
Does it work for a tank or an odd-shaped vessel?
For anything round, yes — enter the internal diameter in the custom bore field and the length or height. For a rectangular tank, multiply the three internal dimensions in inches and divide by 231 to get gallons. The custom bore field also lets you handle a pipe size or material this calculator does not list.
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