Water pipe sizing calculator
Pick the fixtures that run together, the pipe material and the run, and get the supply pipe size that keeps velocity down and leaves enough pressure at the far end. A sizing chart for each material is underneath.
Inputs
Result
Every size for this run
A ✗ marks the limit a size fails. The smallest size with no ✗ is the answer.
| Size | Velocity, ft/s | psi per 100 ft | Loss over run, psi | Left at fixture, psi |
|---|
Pipe sizing chart: PEX
The most flow each size carries at the velocity limit, with the friction it costs at that flow. Change the material or the hot/cold choice above to redraw it.
| Size | Inside diameter, in | Max flow, gpm | psi per 100 ft at that flow |
|---|
Two limits, and the pipe has to pass both
Velocity. Push water too fast and the pipe hammers when a valve snaps shut, hisses in the walls, and in copper slowly erodes the inside of the elbows. The common limits are about 8 ft/s for cold water and 5 ft/s for hot, since hot water erodes copper faster.
Pressure. Friction eats pressure along the run, and lifting water eats more. What reaches the fixture has to be enough to run it. On a long run, or a well system at 40 psi, this is often the limit that decides the size, not velocity.
d is the inside diameter, which is why the material matters. A ¾ in PEX pipe has a bore of 0.681 in against 0.785 in for ¾ in Type L copper, so it carries noticeably less at the same velocity. Nominal size is not bore.
Why PEX often needs to go up a size
PEX has smooth walls (a Hazen-Williams C of 150 against about 140 for copper), but thick ones. The wall eats into the bore, and bore matters far more than smoothness: friction goes with the inside diameter to nearly the fifth power. A ¾ in PEX run loses noticeably more pressure than ¾ in copper at the same flow, so on a long run where copper would just squeak through, PEX will often need the next size up.
Do not add up every fixture
A house with three bathrooms does not run every shower, faucet and toilet at once. Summing them all gives a main far bigger than any house needs. Enter what plausibly runs together on the pipe you are sizing: two showers and a toilet refilling is a realistic morning peak for a family home.
Questions
What size water pipe do I need for a house?
Most houses use a ¾ in or 1 in main with ½ in branches to individual fixtures. A ¾ in copper or PEX line carries about 9 to 12 gpm before velocity gets too high, which covers two showers and a toilet at once. Long runs, low supply pressure, or three or more bathrooms usually justify a 1 in main.
How many gpm can a ¾ inch pipe carry?
At 8 ft/s, the usual cold-water limit, ¾ in Type L copper carries about 12 gpm and ¾ in PEX about 9. For hot water at 5 ft/s those drop to roughly 7.5 and 5.7. The difference between the two is the bore: PEX has thicker walls and a smaller inside diameter.
Is PEX the same size as copper?
Not on the inside. PEX and copper share the same outside diameter, so fittings match, but PEX walls are thicker. A ¾ in PEX pipe has a bore of 0.681 in against 0.785 in for Type L copper, which costs it around a quarter of its capacity at the same velocity.
What is the maximum water velocity in a pipe?
About 8 ft/s for cold water and 5 ft/s for hot is the common guidance, and copper manufacturers recommend staying under those figures to avoid erosion. Lower is quieter. Above them you get water hammer, noise in the walls, and in copper, wear at elbows over time.
How much pressure do I lose going upstairs?
0.433 psi for every foot of height. Lifting water 20 feet to a second-floor bathroom costs about 8.7 psi before any friction, which is why upstairs showers on a weak supply feel feeble.
What is the difference between Type K, L and M copper?
Wall thickness. All three share the same outside diameter; K has the thickest wall and is used underground, L is the usual choice inside buildings, and M is the thinnest and is allowed in some jurisdictions for residential work. Thicker walls mean a smaller bore and slightly less capacity.
More Plumbing calculators
Gas pipe sizing calculator
Schedule 40 steel pipe size for a natural gas load by the longest-length method.
Water heater sizing calculator
Tank size by first hour rating, or tankless capacity from simultaneous flow and temperature rise.