Pipe Volume Calculator

What a pipe holds

A pipe is a cylinder, so the water inside it is the area of the bore multiplied by the length of the run. Nothing about the outside of the pipe matters, and the wall thickness only matters because it changes the bore. That single idea answers most of the questions people bring to this page: how much water sits in a irrigation line before the sprinklers charge, how many gallons a boiler loop carries, and how much antifreeze a cabin needs before the first freeze.

The size of the answer surprises people. A 100 foot run of 2 inch Schedule 40 PVC holds 17.43 gallons, close to the volume of a large trash can. The same 100 feet in 3/4 inch copper holds 2.51 gallons, roughly a seventh as much, because capacity grows with the square of the diameter. Going from 1 inch to 2 inch pipe does not double the water, it quadruples it.

This page is about the water sitting in a pipe, not the water in a tank or a rectangular container. If you are filling a glass box with gravel and a filter, the aquarium volume calculator handles that shape, including the displacement from substrate and rock. A pipe run is a different question: one bore, one length, and often a flow rate that decides how fast it fills or drains.

The formula in inches and gallons

The US liquid gallon is defined as 231 cubic inches, so every volume can be worked in inches and converted at the end. The area of a circle is 0.7854 times the diameter squared, the length has to be in inches as well, and the result is cubic inches. Divide by 231 and you have gallons.

Doing that once per foot gives a shortcut worth memorizing: gallons per foot = 0.0408 x inside diameter squared. The 0.0408 is what you get from 0.7854 x 12 / 231, so it already carries the foot to inch conversion and the gallon conversion. For a 2 inch pipe with a real bore of 2.067 inches: 2.067 x 2.067 = 4.272 square inches of area factor, and 0.0408 x 4.272 = 0.1743 gallons per foot. Multiply by 100 and you get 17.43 gallons.

Liters work the same way once you have gallons, at 3.78541 liters per gallon, so 17.43 x 3.785 = 65.99 liters. Cubic feet come from dividing gallons by 7.48052, which puts that 100 foot run at 2.33 cubic feet. Weight follows the density of water, about 8.34 pounds per gallon at room temperature, so 17.43 x 8.34 = 145.4 pounds of water.

Inside diameter is the number that matters

Hardware stores sell pipe by nominal size, and the nominal size is a label rather than a measurement. A 3/4 inch copper tube measures 0.875 inches across the outside and 0.785 inches inside when it is Type L. The same nominal size in Schedule 40 PVC has an outside diameter of 1.050 and a bore of 0.824, which is 10 percent more water per foot. Schedule 80 PVC walls are thicker again, so its bore at the same nominal size is smaller than Schedule 40.

Copper adds its own wrinkle: Type M, Type L and Type K share outside diameters but carry different wall thicknesses. Type M is the thinnest and holds the most water at a given nominal size, Type K the least. Dimensions for all three types are tabulated in the Copper Tube Handbook from the Copper Development Association, and PVC dimensions come from ASTM D1785, the standard the pipe is printed with.

If you are measuring a pipe you already own, skip the tables. A caliper or a ruler across the open end gives the bore directly, and a tape wrapped around the outside gives the circumference, which divides by 3.1416 to give the outside diameter. Subtract twice the wall thickness and you have the bore.

Worked example: 60 feet of 3/4 inch copper

A cabin water line runs 60 feet of 3/4 inch Type L copper from the pressure tank to the kitchen. The bore is 0.785 inches, and 0.785 x 0.785 = 0.616. Applying the shortcut, 0.0408 x 0.616 = 0.0251 gallons per foot. Sixty feet of that is 0.0251 x 60 = 1.51 gallons, or 1.509 x 3.785 = 5.71 liters. Filled with water, the run adds 1.51 x 8.34 = 12.6 pounds to the structure.

That number matters when the line has to be drained for winter. Opening the low-point drain leaves about a gallon and a half of water in the pipe, plus whatever sits in the water heater, the pressure tank and the fixtures. A 40 gallon heater adds 40 gallons, which is why draining the heater is a separate step from draining the lines.

It also matters for hot water wait time. If the heater holds 1.51 gallons in the line between it and the tap, and the tap runs at 1.5 gallons per minute, the cold slug clears in about a minute. Long runs in big houses are the reason a recirculation loop pays for itself in convenience, not in water savings alone.

Worked example: 100 feet of 2 inch PVC, plus fill time

An irrigation main is 100 feet of 2 inch Schedule 40 PVC. The bore is 2.067 inches, so 2.067 x 2.067 = 4.272 and 0.0408 x 4.272 = 0.1743 gallons per foot. The run holds 0.1743 x 100 = 17.43 gallons, which is 17.43 x 8.34 = 145.4 pounds of water and 17.43 x 3.785 = 65.99 liters.

Fill time divides that volume by the flow going in. At 10 gallons per minute the main charges in 17.43 / 10 = 1.74 minutes, or about 1 minute 45 seconds. Anyone who has waited at a hose end knows this pause: the water has to fill the pipe before it reaches the last head, and a bigger main means a longer wait even though it delivers more water once charged.

The same arithmetic runs backwards for draining. A 17.43 gallon main emptied through a 5 gallon per minute drain valve takes 17.43 / 5 = 3.49 minutes, and the last of it slows to a trickle as the head pressure falls. That is why gravity drains are given a long time in winterizing instructions.

Fill, drain and flush time

Flow rate is the only other input the calculator needs, and it is usually the least certain one. A showerhead sold in the US after January 1, 1994 is capped at 2.5 gallons per minute at 80 psi under 10 CFR 430.32(p), which makes it a useful yardstick: a 2.5 gpm flow fills a 17.43 gallon main in about 7 minutes. An outdoor spigot or a sump pump moves several times that, and a garden hose at full open can empty a rain barrel in minutes.

Real flow also drops as pressure drops. A pump rated at 20 gpm at zero head delivers much less at 40 psi, and a long run of small pipe adds friction loss that cuts flow further. Treat the fill time as a floor, not a promise, and add time for the last stretch when the pipe is nearly full.

Flushing a line after a repair uses the same table in reverse. A 5 gpm flush for three minutes pushes 15 gallons through, which is enough to clear a 100 foot run of 2 inch main once over, but not enough to clear a 300 foot run twice.

Winterizing: how much antifreeze the line takes

Antifreeze for a water line is sized by displacement, not by dilution. The goal is to push the remaining water out of the low spots and leave a slug of protected fluid behind, so the amount needed is the pipe volume plus a margin for traps, fittings and the fact that some water always stays behind. A working rule is 1.1 times the calculated volume.

For the 60 foot 3/4 inch copper run above, 1.51 x 1.1 = 1.66 gallons, so two one gallon jugs of RV antifreeze cover the line with a little to spare. The 100 foot 2 inch PVC main at 17.43 gallons is a different job: 17.43 x 1.1 = 19.17 gallons, which is why big irrigation mains are blown out with air instead of filled with antifreeze.

Air blowing is measured by volume too. A compressor that delivers 4 cubic feet per minute at 50 psi pushes 4 x 7.48 = 29.9 gallons of air per minute, which sweeps a 17.43 gallon main in well under a minute once the water starts moving. Run each zone until nothing but mist comes out, then repeat, because water pools at the low points and the first pass mostly moves it around.

Where the estimate drifts

Fittings add a little water that the straight-run formula misses. A handful of elbows, tees and couplings in a house run might add a few percent, not a doubling, so the per-foot answer stays useful. Valve bodies, filters, water hammer arrestors and pressure tanks are a different story: each one can hold more than the pipe around it, and a filter housing is often a quart on its own.

Pipes that are not full change the answer completely. A drain line running by gravity is partly air, sometimes less than half full, so the volume of water it moves is well below the calculated capacity. A heating loop sealed under pressure is the opposite case, full to the top with no air pocket, and it also expands when it heats, which is what an expansion tank is for.

Scale and sediment shrink the bore over time, especially in hard water areas, and a 3/4 inch line that has run for twenty years may carry noticeably less than its nominal capacity. That loss shows up as lower flow at the tap rather than as a volume error you can see, which is why old galvanized pipe gets replaced instead of cleaned.