Gallons in a Pipe by Size: Copper, PEX, PVC and CPVC

Why nominal size won't give you gallons

The math is simple once you have the inside diameter (ID):

Gallons per foot = π ÷ 4 × ID² × 12 ÷ 231 = 0.0408 × ID² (ID in inches)

The 231 is the number of cubic inches in a U.S. gallon, an exact definition in NIST Handbook 44, Appendix C. Multiply gallons per foot by the length of the run.

Here is the same nominal 3/4-inch size in six materials:

Type M copper holds 46% more water per foot than PEX of the same nominal size. Treating 0.75 inch as the ID gives 2.29 gallons per 100 feet, which is wrong for every material on the list.

Where the IDs come from:

• Copper K, L and M: the nominal dimensions in the Copper Development Association's Copper Tube Handbook, Tables 14.2a–c.
• PEX: Table 7.11 of the Plastics Pipe Institute's PEX design guide, which takes its average OD and wall from ASTM F876.
• PVC Schedule 40 and CPVC CTS: the "Average I.D." columns in Spears Manufacturing's dimensions sheet.

Plastic pipe IDs are averages, so an individual length can differ slightly. We compute every volume from the ID instead of reusing the publishers' own volume columns, because a few of those don't follow from their IDs. CDA prints 0.571 gallons per foot for 4-inch Type L, while its own 3.905-inch ID gives 0.622 (9% more), and its 5/8-inch Type L row repeats the Type K figure. PPI's printed PEX volumes differ from its IDs by up to 8.6%: 1.0 gallon per 100 feet printed for 1/2-inch tubing vs. 0.92 computed.

Copper tube, Types K, L and M

Gallons per 100 feet, computed from CDA's inside diameters. Divide by 100 for gallons per foot.

The walls get thinner from K to L to M, so for the same nominal size Type M always holds the most.

PEX, PVC Schedule 40 and CPVC CTS

Gallons per 100 feet, computed from the PPI (PEX) and Spears (PVC, CPVC) inside diameters. A dash means the size isn't in that source's table.

PEX and CPVC CTS share copper's outside diameters but have thicker walls, so they hold less than copper of the same size. PVC Schedule 40 is made to the larger iron pipe size (IPS) outside diameters, so it holds more than PEX or CPVC CTS of the same nominal size. The pipe volume calculator also covers PVC and CPVC Schedule 80 and Schedule 40 steel, and adds up runs of mixed sizes.

These tables count pipe only. Fittings, valves, fixtures and the water heater are extra; a tank heater holds far more than the piping to it.

Hot water wait time: volume ÷ flow

Seconds to hot water (minimum) = gallons in the run ÷ gpm × 60

• 40 feet of 3/4-inch PEX holds 0.7348 gallons. At 1.5 gpm, that's a 29.4-second minimum wait.
• 60 feet of 1/2-inch Type L to a shower holds 0.7271 gallons. At 2.5 gpm, which is the federal maximum for showerheads made after January 1, 1994 (measured at 80 psi, per 10 CFR 430.32), that's 17.5 seconds.
• A trunk-and-branch run of 35 feet of 3/4-inch Type L plus 15 feet of 1/2-inch Type L to a lavatory holds 1.0617 gallons. At 2.2 gpm, the federal maximum for lavatory faucets at 60 psi, that's 29.0 seconds; at 1.5 gpm it's 42.5 seconds.

Those are floors, not forecasts. The math assumes the cold water leaves as one slug and the first hot water arrives hot. In a real house, the pipe walls and fittings soak up heat, so the actual wait is longer. Federal fixture limits are ceilings, and a lower real flow means a longer wait. EPA's WaterSense program gives a field check: at the fixture farthest from the heater, run hot water into a bucket until the temperature rises 10 °F, then measure the water you collected (WaterSense hot water delivery sheet).

The same WaterSense sheet suggests a design target of 0.5 gallons or less between the water heater and each fixture. How much pipe holds 0.5 gallons:

Codes control length rather than volume. The model International Plumbing Code (2024 IPC, Section 607.2, free to read on ICC Digital Codes) limits the developed length of hot or tempered water piping from the source of hot water to the fixtures to 50 feet, and it counts recirculating and heat-traced piping as sources. That's a code limit, while the 0.5-gallon figure is a WaterSense target, not a code requirement. Fifty feet of 1/2-inch PEX holds 0.46 gallons; fifty feet of 3/4-inch Type L holds 1.26.

States adopt the model codes in different editions, with their own amendments. The same 50-foot rule appears in Georgia's code (the 2018 IPC with Georgia amendments), Florida's (the 2023 Florida Building Code, Plumbing, built on the 2021 IPC) and New York's (the 2020 state plumbing code, built on the 2018 IPC), as shown on UpCodes (Georgia, Florida, New York). Some states build on IAPMO's Uniform Plumbing Code (UPC) instead; California's 2022 plumbing code, for example, adopts the 2021 UPC with amendments (UpCodes), and IAPMO posts the UPC free to read online. Check which code and edition your jurisdiction has adopted.

Flushing and disinfection volumes

Minutes to move one pipe volume = gallons in the run ÷ gpm

Take 300 feet of 2-inch PVC Schedule 40. It holds 51.29 gallons, so at 20 gpm one full pipe volume takes 2.56 minutes and three volumes take 7.69 minutes. The water is moving at only 1.95 feet per second at that flow; if a job calls for a minimum flushing velocity, check the flow it takes in the pipe flow velocity calculator; the water velocity guide covers the published limits.

Disinfection works the same way: the pipe volume is how much solution the system holds. Section 610.1 of the 2024 IPC requires new potable water systems to be purged and disinfected before use, following the method the health authority or water purveyor prescribes. Without a prescribed method, it points to AWWA C651 or C652 or to its own procedure:

• flush until dirty water no longer appears at the outlets;
• fill with a solution of at least 50 ppm chlorine and let it stand 24 hours, or at least 200 ppm and let it stand 3 hours;
• flush with clean water until the chlorine is purged;
• repeat where a bacteriological examination shows contamination remains.

Adopted versions of this section differ too. Georgia's and New York's apply it to repaired systems as well as new ones; Florida's requires disinfection of a new system where the authority having jurisdiction requires it; and New York's replaces the procedure with the state Department of Health's regulations (same UpCodes pages as above).

For a repipe with 150 feet of 3/4-inch PEX and 250 feet of 1/2-inch PEX, the piping holds 2.76 + 2.30 = 5.06 gallons. That doesn't include the water heater or fixtures; add them if they're part of the system being treated.

Our calculator also estimates the bleach for a target ppm, but only roughly: it treats the percentage on the label as available chlorine and ignores the bleach's density. Confirm the actual concentration with a chlorine test kit. This is general information, not code advice. The health authority, the water purveyor or the local building department decides how a system is flushed and disinfected. The model IPC and UPC are free to read online at ICC Digital Codes and IAPMO.

Bottom line

Look up the inside diameter for the exact material and type, square it, multiply by 0.0408 and by the length, and you have the gallons. Divide by the flow for the minimum wait or the flush time. The pipe volume calculator does this for mixed runs, shows which source each ID came from, and converts to liters. When a long wait is the complaint, the run's volume is the number to show the customer, and a bigger water heater won't change it (water heater sizing for pros).