Pipe Internal Volume

Knowing the system volume is what turns a chemical dose rate into a number of litres to pour in. A 100 ft run of 4 in Schedule 40 steel holds π/4 × (4 in)² × 1200 in ≈ 15 080 in³ ≈ 65.3 US gallons, which is the origin of the plumber's constant "0.653 gallons per foot for 4-inch". Water treaters carry a whole table of these — 0.0408 gal/ft for ½ in, 0.163 for 1 in, 1.47 for 6 in — and estimate a building's loop volume by tallying pipe lengths from the riser diagram.

Use the true inside diameter, not the nominal size, or you will be out by 10–15% before you start: nominal 4 in Schedule 40 is 4.026 in ID, Schedule 80 is 3.826 in, and type L copper is 3.905 in. And remember the pipe is usually the small half of a system — an air handler coil, a buffer tank and a boiler can hold several times what the distribution piping does, which matters enormously when you are calculating how long a glycol charge or a biocide slug takes to circulate.