Pipe Volume, Water Weight and Head Pressure

How many gallons are sitting in that pipe, what does the water weigh, and how much pressure does a column of it make? Three classic trade calculations, with every formula shown so you can do them on scratch paper.

1. Pipe and height

The formulas

Gallons in a pipe: 0.0408 × d² × L

With d in inches and L in feet. Where 0.0408 comes from: the cross-section is π/4 × d² square inches. Divide by 144 to get square feet, multiply by the length for cubic feet, and multiply by 7.48 gallons per cubic foot. π/4 ÷ 144 × 7.48 = 0.0408.

Example: 100 ft of 2-inch pipe holds 0.0408 × 2² × 100 = 16.3 gallons.

Weight of the water: gallons × 8.34 lb

A gallon of water weighs about 8.34 pounds. Those 16.3 gallons weigh about 136 lb, on top of the pipe itself. That's one reason support intervals matter.

Static head pressure: feet × 0.433 psi

Every foot of water column adds 0.433 psi at the bottom. Going the other way, 1 psi lifts water about 2.31 ft (1 ÷ 0.433). The pressure depends only on height, not on the pipe size or length.

Keep studying

More free tools: offset calculator · hanger spacing · all tools

Study aid — confirm with the current 248 CMR and your inspector. These are standard physics approximations for water at ordinary temperatures, not code requirements. PressureTest is independent and not affiliated with the Commonwealth of Massachusetts, the Board of State Examiners of Plumbers & Gas Fitters or PSI.