Pipe Volume Calculator

Pipe Volume Calculator finds the gallons in a pipe run from the true bore of Schedule 40, Schedule 80, copper K, L or M, or PEX, then gives the hot water wait and the water weight.

Pipe Volume Calculator for Real Pipe Bores

The Pipe Volume Calculator finds how much water a pipe run holds, using the true inside diameter of the pipe you pick. It covers Schedule 40 and 80 steel or PVC, copper Type K, L and M, and PEX, plus any custom bore.

Plumbers use it to estimate the hot water wait at a fixture. Hydronic and fire sprinkler installers use it to size a fill or a flush, and service techs use it to dose a cleaning or disinfection flush by the gallon.

In the Pipe Volume Calculator, pick the pipe type and nominal size, then enter the length and a flow rate. Up to two more runs of the same pipe type can be added, so a 3/4 in trunk and a 1/2 in branch add up in one total.

Pipe Volume Formula and the Gallons per Foot Shortcut

A pipe is a cylinder, so the volume of water in a pipe is its inside cross-section times its length. In US units, the area in square inches times the length in inches gives cubic inches, and 231 cubic inches make one US gallon.

$$V=\frac{\pi }{4}\times I{D}^2\times \frac{12\times L}{231}$$

Here ID is the inside diameter in inches and L is the length in feet. The constants reduce to a shortcut that gives gallons per foot of pipe directly.

$$\text{gal per ft}=0.0408\times I{D}^2$$

The Pipe Volume Calculator's default run is 25 ft of 2 in Schedule 40, with a 2.067 in bore. That is 0.1743 gal per foot and 4.36 gal in the run. For flow and velocity through the same bore, the Pipe Flow Calculator picks up where this one stops.

Water operator training, such as American Water College's exam math, uses the same formula in feet. Multiply 0.785 by the diameter in feet squared, then by the length in feet and by 7.48 gallons per cubic foot. Both routes land on 4.36 gal for the default run.

Why Nominal Size Gives the Wrong Pipe Volume

Nominal size is a trade name, not a measurement. A 1/2 in Schedule 40 pipe has a 0.622 in bore, so using 0.5 in as the ID gives 0.0102 gal per foot when the pipe really holds 0.0158, about 55% more. The Pipe Volume Calculator fills in the published bore for every size to avoid that error.

Each pipe family sets its bore a different way. Schedule pipe keeps a fixed outside diameter per ASME B36.10, and PVC Schedule 40 and 80 follow the same OD and wall, so a thicker Schedule 80 wall shrinks the bore. Copper water tube per ASTM B88 has an OD 1/8 in larger than the nominal size, with Type K the thickest wall and Type M the thinnest.

PEX is the smallest bore of the group. The calculator uses Uponor's published SDR 9 inside diameters, such as 0.475 in for 1/2 in and 1.629 in for 2 in, which are based on average wall thickness.

Some PEX charts list minimum-wall bores instead, such as 0.485 in for 1/2 in. That changes the volume by about 4% on small sizes, so match the chart to your maker when the figure matters.

The fourth result card shows how far the same nominal size swings across types. For 2 in, Schedule 40 holds 8.4% more than copper Type L, while 2 in PEX holds only 0.1083 gal per foot.

Hot Water Wait and the WaterSense 0.5 Gallon Limit

Every gallon sitting in the hot line has to run out before hot water arrives. The calculator divides the total volume by your flow rate to get that fill or purge time, and also shows how fast the water moves in the first run.

The default 4.36 gal at 2.5 gpm takes 1 min 45 s to purge, with the water moving only 0.24 ft per second. That slow speed is why oversized hot lines feel so slow at the tap. To time a real fixture, the Gallons Per Minute Calculator turns a bucket fill into its flow rate.

EPA's WaterSense New Home Specification limits a labeled home to 0.5 gallons stored in the piping between the hot water source and any hot water fixture. Its inspection test allows 0.6 gallons at the tap, to cover the extra water that has to run before hot water arrives.

The multi-run fields in the Pipe Volume Calculator make that check quick. A 20 ft run of 3/4 in PEX plus 10 ft of 1/2 in PEX stores about 0.46 gal and passes. The same layout in copper Type L stores about 0.62 gal and fails, only because of the larger copper bore.

Treat the purge time as the shortest possible wait. A cold pipe in an unheated crawl space takes longer, because the pipe wall has to warm up before the water reaching the tap stays hot.

Water Weight for Hangers and Fill Loads

The weight card uses about 8.34 lb per gallon of fresh water. The default run carries 36.3 lb of water, or 1.454 lb per foot of 2 in Schedule 40, which adds to the pipe's own weight on every hanger.

A 20 ft stick of that pipe holds 29.1 lb of water. Friction loss along a long fill line is a separate check, which the Hazen Williams Equation Calculator covers. For the steel or PVC itself, the Pipe Weight Calculator covers the empty pipe, and the Tank Volume Calculator handles the storage tank at the end of the line.

Pipe Volume in Liters and Cubic Meters

Switch the Pipe Volume Calculator to metric to enter millimeters and meters and read liters. With the diameter in meters, the formula gives cubic meters, and multiplying by 1,000 gives liters.

$$V=0.7854\times D^2\times L$$

A 100 mm bore over 30 m holds 0.7854 × 0.1² × 30, or about 0.236 m³, which is 236 liters or about 62 US gallons. For liters per meter, a 50 mm bore gives 0.7854 × 0.05² × 1,000, or 1.96 L per meter.

A quick check is that 1 L per meter matches a bore of about 35.7 mm. Anything wider holds more than a liter for every meter of run.

Pipe Volume by Size and Type

What is the volume of a 1 inch pipe?

It depends on the wall. One inch Schedule 40 has a 1.049 in bore and holds 0.0449 gal per foot, or 4.49 gal per 100 ft, while Schedule 80 holds 0.0374 gal per foot.

One inch copper Type L holds 0.0429 gal per foot. One inch PEX, with a 0.862 in bore, holds only 0.0303 gal per foot, about a third less than Schedule 40.

What is the volume of a 2 in pipe?

Two inch Schedule 40 holds 0.1743 gal per foot, or 17.43 gal per 100 ft. Schedule 80, with a 1.939 in bore, holds 0.1534 gal per foot. The Pipe Volume Calculator's capacity chart lists every Schedule 40 size from 1/2 to 12 in the same way.

Copper runs from 0.1566 gal per foot for Type K to 0.1647 for Type M, with Type L at 0.1608. Two inch PEX holds 0.1083 gal per foot, which is 38% less than Schedule 40.

What is the volume of a 4 in pipe?

Four inch Schedule 40 has a 4.026 in bore and holds 0.6613 gal per foot, or 66.1 gal per 100 ft. Schedule 80 drops to 0.5972 gal per foot because of its 3.826 in bore.

Four inch copper Type L holds 0.6222 gal per foot. At that size, 100 ft of Schedule 40 holds about 552 lb of water, which is worth knowing before filling a long overhead run.