Field calculator
Pipe volume calculator (gallons of water)
Knowing how much water a run of pipe holds is the starting point for sizing a flush, mixing a glycol charge, dosing chlorination, or estimating the water to drain down a system. The volume is the area of the bore times the length: pi divided by four, times the inside diameter squared, times the length, converted to gallons at 7.48 gallons per cubic foot. Enter the inside diameter in inches and the length in feet. Use the actual inside diameter for the pipe material and schedule, not the nominal size, because the nominal call-out is not the real bore and the difference adds up over a long run. For a whole-system volume, add the fittings, the water heater or storage tank, and any vessels or coils on the loop.
Result
Water volume held in a pipe: V = pi/4 × diameter squared × length, converted to gallons (1 cubic foot = 7.48 gallons). Enter the inside diameter in inches and the length in feet. Use the actual inside diameter for the pipe material and schedule, since the nominal size is not the bore. This is handy for sizing a flush, a glycol charge, chlorination dosing, or the water to drain down. For a full system add the fittings, the heater or tank, and any vessels.
Worked example
How much water does a 100 ft run of 2 in pipe hold (for a flush, glycol charge, or drain-down)?
- Inside diameter2 in
- Length100 ft
- Diameter in feet = 2 ÷ 12 = 0.1667 ft. Volume = π÷4 × 0.1667² × 100 = 2.18 cu ft.
- Gallons = 2.18 × 7.48 = 16.3 gallons.
About 16.3 gallons. Use the pipe's ACTUAL inside diameter (not the nominal size) for an accurate charge or flush volume.
anvilfield.com/calculators/pipe-volume-capacity-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Pipe volume FAQ
What is a water supply fixture unit?
A water supply fixture unit, or WSFU, is a dimensionless number assigned to a fixture that represents its load on the water system, folding together how much it draws, how long, and how often. Totaling WSFU and reading the demand curve gives a probable peak flow in gpm. Verify the values against the adopted code.
How do you size water supply pipe?
Total the WSFU for each segment, convert to a probable demand in gpm off the Hunter curve, then build a pressure budget by subtracting elevation, meter, backflow, and fixture residual from the source pressure. Size each segment to carry its demand under both the leftover friction budget and the velocity limit. The adopted code controls.
What velocity is too high for water pipe?
Common design limits are about 8 ft per second for cold water and 5 ft per second for hot, with hot lower because it erodes copper faster. Above 140 degrees F, drop copper to 2 to 3 ft per second. Past these limits you get erosion corrosion, pinholes at the elbows, noise, and water hammer.
Why not size water pipe for every fixture running at once?
Because that moment never happens, and sizing for it builds oversized pipe that then sits stagnant. Not every fixture draws at the same second, and the odds fall as the building grows. That diversity is built into the demand curve, which flattens as fixture units climb, so the curve, not the summed flow, is the demand.
How much pressure do I lose going up a building?
About 0.43 psi for every foot of height, the static head, with a precise figure of 0.433 psi per foot. A fixture 40 ft up has lost roughly 17 psi to elevation before any flow starts. On a high-rise the elevation alone can outrun the street pressure, which is why tall buildings need pressure zones and booster pumps.