Pipe Volume Calculator

The Formula

A pipe is just a long cylinder, so its volume is π × radius² × length. The catch is which radius to use: the water fills the inner diameter (ID), which is always smaller than the nominal size on the label. A "3/4-inch" Type L copper pipe actually has a 0.785-inch inner diameter, while 3/4-inch PEX is only 0.671 inches inside.

Working in inches and feet: gallons = π × (ID ÷ 2)² × length × 12 ÷ 231, since there are 231 cubic inches in a U.S. gallon. For 100 feet of 3/4-inch copper: π × 0.3925² × 1,200 ÷ 231 ≈ 2.5 gallons. The calculator above uses the true inner diameter for whichever material you select.

Gallons per 100 Feet by Pipe Size

Notice how quickly volume grows with size — doubling the diameter quadruples the volume. A 2-inch copper main holds over 13 times the water of a 1/2-inch branch line of the same length.

Why Pipe Volume Matters

Draining the system. Whether you're winterizing a cabin, repairing a section of line, or adding antifreeze to an irrigation loop, you need to know how much water is coming out. Total up each run's volume and you know what size bucket — or how many — to have ready, and how much air an air-compressor blowout has to displace.

Hot-water wait time. Every drop of cooled-off water sitting between the water heater and your faucet has to be pushed out before hot water arrives. A 50-foot run of 3/4-inch copper holds about 1.26 gallons; at a typical 2.5 GPM shower flow, that's roughly 30 seconds of running water down the drain — every single time. Over a year, a family can easily waste over a thousand gallons this way.

This is why long hot-water runs are often piped in 1/2-inch instead of 3/4-inch where flow allows: half the diameter class means less than half the stored volume, so hot water arrives almost twice as fast. It's also the math behind recirculation pumps, which keep a slow loop of hot water moving through the lines so the faucet is hot within seconds. If your wait time is annoying, calculate the volume of your longest run first — it tells you whether a recirculation system, a point-of-use heater, or simply re-piping the branch line is the right fix. Sizing a new heater too? See our water heater calculator.

Copper vs PEX: Same Label, Different Inside

Copper and PEX use the same nominal sizes but very different wall thicknesses. PEX walls are thick plastic, so 3/4-inch PEX has a 0.671-inch ID versus 0.785 inches for Type L copper — about 27% less cross-sectional area. That means PEX holds less water (faster hot-water delivery, less to drain) but also flows less at the same pressure, which is why plumbers sometimes upsize PEX trunk lines by one nominal size compared with the copper they replace.

Within copper itself, wall thickness varies by type: Type M (thin wall, common indoors) has a slightly larger ID than Type L (standard), which is larger than Type K (thick wall, underground service). This calculator uses Type L, the most common choice for residential supply lines — Type M results will run a few percent higher.