How much water is in the pipes?

How do I calculate the volume of water in a pipe?

Use the inside diameter: π × (ID ÷ 2)² × length, all in inches, gives cubic inches; divide by 231 for gallons. 100 feet of ¾ inch Type L copper, 0.785 inches inside, holds π × 0.3925² × 1,200 ÷ 231 = 2.51 gallons.

Why does the pipe type matter?

Pipes of the same nominal size have different inside diameters. ¾ inch Type L copper is 0.785 inches inside, Type M 0.811, PEX about 0.681 and Schedule 40 PVC 0.824 — so the same length holds noticeably different amounts of water.

How much antifreeze do I need for a heating system?

Add up the water in every pipe run, then add the boiler, radiators and expansion tank from their spec sheets. Mix the glycol to the strength the maker calls for — often 30 to 50% — and multiply the total volume by that share for the gallons of concentrate.

How long does it take for hot water to reach a faucet?

The water sitting in the pipe must be pushed out first. 50 feet of ½ inch PEX holds about 0.48 gallons; at 1.5 gallons per minute that takes about 20 seconds before hot water arrives.

How much does the water in a pipe weigh?

Water weighs about 8.34 pounds per gallon, so 2.5 gallons in a pipe run adds about 21 pounds — worth knowing when you hang long runs of large pipe.

How many gallons are in one foot of pipe?

¾ inch Type L copper holds 0.025 gallon per foot, and ½ inch PEX 0.010 gallon per foot. Multiply by the length for the total: 100 feet of 2 inch Schedule 40 PVC holds 17.4 gallons.

The formula and its units

A pipe is a cylinder, so volume = π × (inside diameter ÷ 2)² × length. Work in inches throughout, with the length converted from feet (multiply by 12), then divide the cubic inches by 231 to get US gallons, since 1 gallon is defined as 231 cubic inches. For liters, multiply gallons by 3.785411784. The pipe wall plays no part; only the empty space inside counts, which is why the inside diameter, not the nominal size, goes into the formula.

A worked example with two runs

A house has 60 ft of ¾ inch PEX to a bathroom and 40 ft of ½ inch PEX on to a sink. With inside diameters of 0.681 and 0.486 inches, the volumes are 1.13 and 0.39 gallons. Together that is 1.52 gallons, or 5.8 liters.

Waiting for hot water

The water standing in the line between the heater and the faucet has to be drawn off before warm water arrives. For 50 ft of ¾ inch Type L copper (1.26 gallons) at a 2 gallon per minute tap, that is about 38 seconds, assuming the water does not mix as it moves. A shorter or narrower run holds less water to waste.

Common mistakes

Using the nominal size as the diameter overstates the volume in PEX, where the inside diameter is well under the nominal size. Forgetting the unit steps is the other source of errors: cubic inches, cubic feet and gallons are three different numbers, so convert the length to inches before squaring anything. Also, the calculator counts straight pipe only; fittings, valves and tanks add some volume, so add the figures from their data sheets.

Sources: the gallon (231 in³) and liter conversions from NIST Handbook 44 and NIST SP 811; inside diameters of copper tube from ASTM B88 dimensions, of PEX from ASTM F876 dimensions and of PVC from Schedule 40 pipe dimensions (ASTM D1785). Related: pipe size calculator, tank volume calculator, expansion tank calculator and water heater calculator.