Water pipe sizing calculator by fixture units

A water pipe has to deliver enough flow at the busiest moment of the day without the water moving so fast that it is noisy or wears the pipe. Sizing by fixture units, the method in Appendix E of the International Plumbing Code (IPC), does it in four steps:

1. Fixture units. Give each fixture a load value in water supply fixture units (WSFU) from the code's table and add them up for every fixture the pipe feeds.
2. Peak demand. Convert the WSFU total to gallons per minute with the demand table, which is built on Hunter's curve: the chance that many fixtures run at once drops as the number of fixtures grows.
3. Steady flows. Add the full flow of anything that runs for long stretches, such as hose bibbs and irrigation.
4. Pipe size. Pick the smallest pipe whose inside diameter keeps the velocity under the limit for that material and water temperature.

Velocity is flow divided by the inside area of the pipe. In plumbing units: velocity (ft/s) = gpm ÷ (2.448 × inside diameter²), with the diameter in inches. The calculator uses the real inside diameters of Type K, L and M copper, PEX and Schedule 40 PVC, not the nominal size.

A full design also checks pressure: what is left after the meter, backflow preventer, height and friction in the pipe has to be enough for the farthest fixture. The calculator shows the pressure lost to height (0.43 psi a foot) and leaves the friction check to you for long runs.

These are the private, residential load values from IPC Table E103.3(2), the total (hot plus cold) column the calculator uses. A bathroom group (toilet, lavatory, and tub or shower in one room) counts as a single entry, which is lower than its parts added up.

Use the total column for a pipe that carries both hot and cold demand, such as the water service and the main before the water heater. For a cold-only or hot-only branch, use that column instead and enter it as other fixture units. Public fixtures (offices, restaurants, schools) carry higher values in the same table.

The IPC demand table, Table E103.3(3), has two columns: one for buildings mostly on flush-tank toilets and one for buildings mostly on flushometer valves, which pull a lot of water in a short burst. A few rows:

The calculator reads every row of the table and goes in a straight line between rows, so 14.3 WSFU comes out at 17.15 gpm. The flushometer column starts at 5 WSFU; below that the calculator uses 15 gpm. Above 1,000 WSFU the two columns meet.

The IPC leaves the design velocity to the pipe maker's recommendations. For copper, the usual limits are 8 ft/s for cold water, 5 ft/s for hot water under 140°F and 2 ft/s for hot water over 140°F. Faster water is noisy and wears copper from the inside through erosion and corrosion. Velocity at bends and in small tubes runs higher than in the straight pipe, which is one more reason to stay under the limit.

PEX has thicker walls than copper for the same nominal size, so it carries less. 3/4 in PEX (0.681 in inside) carries 9.1 gpm at 8 ft/s against 12.1 for 3/4 in Type L copper. That is why a PEX main is often a size up from the copper it replaces. PEX, CPVC and PVC makers publish their own velocity limits; switch on "Set my own velocity limit" and use theirs.

A two-bath house, water service in Type L copper. Two full bathrooms with tank toilets (2 × 3.6 = 7.2), a kitchen sink, dishwasher and washing machine (3 × 1.4 = 4.2), a powder room lavatory (0.7) and toilet (2.2). Total 14.3 WSFU.

Demand: 14.3 sits between 14 (17.0 gpm) and 15 (17.5 gpm), so 17.0 + 0.3 × 0.5 = 17.15 gpm. At 8 ft/s, 3/4 in Type L carries only 12.1 gpm (17.15 gpm would run 11.4 ft/s). 1 in Type L runs 17.15 ÷ (2.448 × 1.025²) = 6.7 ft/s, which is under 8. Answer: 1 in. With the top fixture 10 ft above the meter, 4.3 psi goes to height.

The same house with two hose bibbs. 17.15 + 2 × 5 = 27.15 gpm. 1 in Type L would run 10.6 ft/s, so it goes to 1-1/4 in (6.9 ft/s).

The IPC's own example. Appendix E converts 120 WSFU on flush tanks to 48 gpm, then adds two hose bibbs at 5 gpm each for 58 gpm. In Type L copper at 8 ft/s, 1-1/2 in carries 44.4 gpm, too little; 2 in runs 58 ÷ (2.448 × 1.985²) = 6.0 ft/s. Answer: 2 in.

A hot water branch. One bathroom group and a kitchen sink, 5.0 WSFU, 9.4 gpm. Cold, 3/4 in Type L runs 6.2 ft/s and works. Hot at 5 ft/s, 3/4 in is too fast and the answer is 1 in at 3.7 ft/s. In practice the hot side only carries part of the load: use the hot column of the fixture table for a hot-only branch.

The IPC sets the water service at no less than 3/4 in (Section 603.1), so the calculator never suggests a smaller service even when velocity would allow it. Fixture supply lines have their own minimum sizes in the code, in Table 604.5.

Places on the Uniform Plumbing Code (UPC) use their own fixture unit values and sizing tables, which can give a different answer for the same house. Some jurisdictions also change the IPC tables when they adopt them. Check the edition in force where you work.

What the calculator leaves out:

• Friction. A long run loses pressure to friction even at a safe velocity. Check the pressure left at the farthest fixture against the minimum it needs, using the pipe maker's friction tables.
• Meters, softeners and backflow preventers. Each has a pressure loss from its maker; on a low-pressure street they can decide the size.
• Fire sprinklers. A combined domestic and fire sprinkler service is sized by the sprinkler design, not by fixture units.
• Large and commercial buildings. Past 2 in, or with public fixtures and flushometers throughout, size it with the full Appendix E method or an engineer.

Once the size is set, build the estimate in Koira with the pipe, fittings and labor on their own lines. For rates, see the plumbing labor rate calculator.