The formula
A pipe is a cylinder lying on its side, so the volume is the cylinder formula with the radius in feet:
cubic feet = π × (inside diameter in ft ÷ 2)² × length (ft)- With the diameter given in inches, that is
π × (d ÷ 24)² × length— the 24 is 2 to get the radius, and 12 to get inches into feet. gallons = cubic feet × 7.48052water weight (lb) = gallons × 8.34
One cubic foot is 7.48052 US gallons, and a gallon of water weighs about 8.34 pounds at room temperature. Those two constants are the whole conversion.
Use the inside diameter, not the nominal size
This is the single biggest source of error. A "3/4 inch" pipe does not have a 3/4 inch bore — the nominal size is a label, not a measurement. Wall thickness differs by material and schedule, and the difference is not small: 3/4 in copper Type L has a bore near 0.785 in, while 3/4 in PEX is closer to 0.67 in because the plastic walls are thicker.
| Nominal size | Sch 40 ID used here | Gallons per 100 ft | Water weight |
|---|---|---|---|
| 1/2" | 0.622" | 1.58 gal | 13.2 lb |
| 3/4" | 0.824" | 2.77 gal | 23.1 lb |
| 1" | 1.049" | 4.49 gal | 37.4 lb |
| 1-1/4" | 1.38" | 7.77 gal | 64.8 lb |
| 1-1/2" | 1.61" | 10.58 gal | 88.2 lb |
| 2" | 2.067" | 17.43 gal | 145.4 lb |
| 3" | 3.068" | 38.40 gal | 320.3 lb |
| 4" | 4.026" | 66.13 gal | 551.5 lb |
If your material is PEX, copper Type M, CPVC or steel, look up the actual bore for that product — the numbers above are steel Schedule 40.
Three worked examples
- 100 ft of 1 inch Schedule 40. π × (1.049 ÷ 24)² × 100 = 0.600 cu ft = 4.49 gallons, weighing about 37.4 lb. That is the whole contents of a 100 ft branch — far less than most people expect.
- 100 ft of 3/4 inch. 0.370 cu ft = 2.77 gallons, about 23 lb of water.
- 100 ft of 2 inch. 2.330 cu ft = 17.43 gallons, about 145 lb. Note that doubling the nominal size from 1 inch to 2 inches takes the volume up by nearly four times, because volume goes with the square of the diameter.
The per-foot figure is often what you actually want: 1 inch Sch 40 holds about 0.045 gallons per foot, so a 40 ft run is roughly 1.8 gallons.
Why the number matters
Pipe volume is not an academic figure — it shows up in several real jobs:
- Time to hot water. A 3/4 in line at 2.77 gal per 100 ft means a 60 ft run holds about 1.7 gallons that have to be pushed out before hot water arrives. At 2 GPM that is under a minute of pure waiting, per draw.
- Recirculation and purge time. Knowing the volume tells you how long a loop takes to turn over.
- Freeze protection and drain-down. The volume tells you how much water has to go somewhere when you blow out a line or open a drain.
- Chemical dosing. Disinfection, glycol and cleaning concentrations are all calculated against the actual water volume of the system, not against pipe length.
- Expansion tank sizing. Thermal expansion acts on the water volume in the system.
- Weight. A 2 inch line full of water weighs about 145 lb per 100 ft before you add the pipe itself. That matters for hangers and for what a section weighs when you cut it.
Common mistakes
- Using the nominal size as the bore. 3/4 in pipe is not 0.75 in inside.
- Mixing up radius and diameter. The formula needs radius; using the diameter squares the error.
- Forgetting to convert inches to feet. The 24 in the shortcut is doing two conversions at once.
- Using imperial gallons. 7.48052 is US gallons. An imperial gallon is about 20% larger.
- Assuming PEX flows like copper. Thicker walls mean a smaller bore at the same nominal size, which raises velocity as well as reducing volume.
- Ignoring the water weight. It frequently exceeds the weight of the pipe itself.
How this connects to flow and velocity
Volume, flow rate and velocity are three views of the same pipe, and it is worth being able to move between them:
- Volume is how much the line holds — this page. It depends on bore and length only.
- Flow rate (GPM) is how much moves per minute — what a fixture or pump demands. It is the volume delivered divided by time.
- Velocity (ft/s) is how fast it moves — flow divided by the cross-sectional area. It is the number that decides whether a line is noisy and durable.
So the same 3/4 inch line holds 2.77 gallons per 100 ft regardless of use, but it carries 8.3 GPM at 5 ft/s and about 13 GPM at 8 ft/s. Volume tells you what is in the pipe; the other two tell you what the pipe can do. Sizing usually runs the other way round from this page: start with the flow the fixtures demand, then pick a bore that keeps velocity inside the band, then check the volume if you need it for dosing, purge time or freeze protection.
Measuring and protecting what is in the pipe
Pipe volume shows up when you are dosing, purging or protecting a line from freezing. These four are what those jobs actually need.
- Digital caliper — You cannot calculate pipe volume from a nominal size. A caliper gives you the real bore, and the volume scales with its square — so the measurement matters more than you would expect.
- Pipe insulation foam tube — The first line of defence for a line whose volume you just calculated. Sized by pipe OD and wall thickness, so measure before ordering.
- Self-regulating heat tape — For lines that can still freeze despite insulation. Choose by the length of pipe to protect, which is the same length the volume calculation used.
- Graduated measuring bucket — For the other half of the job: timing a fill to check flow, or measuring a dose against the volume you calculated.
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Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on the equipment manufacturer, the page says so instead of guessing one.
- Copper Development Association — copper tube dimensions, wall thickness and installation practice.
- ASHRAE — hydronic system volume, piping and water properties references.
- NIST — weights and measures: US gallon to cubic foot conversion (7.48052).
Figures, formulas and reference tables last verified: 21 September 2026.