Pipe Flow Calculator
Calculate water flow in gallons per minute for a pipe size and material, either at a design velocity or from the pressure available over the run, with friction loss.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Flow rate
- 7.54 gpm
- 3/4 in Copper, type L, 0.785 in inside diameter
- Velocity
- 5 ft per second
- The design speed you chose
- Friction loss
- 7.6 psi per 100 ft
- Hazen-Williams at C = 130, a head loss of 17.6 ft per 100 ft of pipe
- Loss over the run
- 4.56 psi
- Over 60 ft of developed length, fittings included
- Planning estimate
- Size from fixture units
- Flow is one check. A licensed plumber sizes the supply from water supply fixture units (IPC Appendix E) and the local inspector governs
Pipe Flow formula
Flow (gpm) = 2.448 x Inside diameter (in)² x Velocity (ft/s)
Velocity (ft/s) = Flow (gpm) / (2.448 x Inside diameter (in)²)
Head loss (ft per 100 ft) = 0.2083 x (100 / C)^1.852 x Flow^1.852 / Diameter^4.8655
psi = Feet of head / 2.31Worked example
A 1 in PEX main runs 90 ft of developed length to a bathroom group. The inside diameter is 0.862 in, so at a 5 ft per second design speed it carries 2.448 x 0.862 x 0.862 x 5 = 9.09 gallons per minute.
Friction at that flow, with a C factor of 150 for PEX, is 12.1 feet of head per 100 ft, which is 5.2 psi per 100 ft. Over the 90 ft run that is 4.7 psi. Start at 60 psi at the meter, subtract 4.7 for friction and 4.3 for a second floor 10 ft up, and about 51 psi reaches the fixtures.
Quick reference
| Size | Copper L at 5 fps | Copper L at 8 fps | PEX at 5 fps | PVC 40 at 5 fps |
|---|---|---|---|---|
| 1/2 in | 3.64 gpm | 5.82 gpm | 2.76 gpm | 4.74 gpm |
| 3/4 in | 7.54 gpm | 12.07 gpm | 5.51 gpm | 8.31 gpm |
| 1 in | 12.86 gpm | 20.58 gpm | 9.09 gpm | 13.47 gpm |
| 1-1/4 in | 19.59 gpm | 31.34 gpm | 13.60 gpm | 23.31 gpm |
| 1-1/2 in | 27.72 gpm | 44.36 gpm | 18.91 gpm | 31.73 gpm |
| 2 in | 48.23 gpm | 77.17 gpm | 32.48 gpm | 52.30 gpm |
Flow, velocity and pipe size
Flow through a pipe is area times speed. In plumbing units that is 2.448 times the inside diameter squared times the velocity in feet per second. Because the diameter is squared, one size up buys far more than it looks: 1 inch type L copper carries 12.86 gallons per minute at 5 feet per second against 7.54 for 3/4 inch, a 70 percent gain for a 30 percent bigger number on the label.
The inside diameter is the number that matters, and it varies by material. Three quarter inch type L copper is 0.785 inches inside, 3/4 inch PEX is 0.671, and 3/4 inch schedule 40 PVC is 0.824. A PEX system sized straight off a copper chart runs about a quarter short on flow at every branch.
Friction, pressure and the run
Pressure is what pushes the water; friction is what eats it. The Hazen-Williams equation gives the loss for a given flow, and the C factor stands in for the roughness of the material: about 130 for copper, 150 for PEX and PVC, and around 100 for aged galvanized steel. Loss climbs almost with the square of the flow, so doubling the demand on a line roughly triples the pressure it burns.
Add the fittings before you judge a run. An elbow on 3/4 inch pipe behaves like about 2 feet of straight pipe and a tee taken through the branch like about 4 feet, so a run with a dozen fittings can be half again its measured length. Then subtract elevation at 0.433 psi per foot of rise. What is left is what arrives at the shower valve, and IPC Table 604.3 wants 20 psi there while it is flowing.
What this calculator leaves out
The fixture unit method that actually sizes a supply system, the water meter and service line loss ahead of the house, the pressure drop across a softener or filter, and the simultaneous demand that makes several fixtures share one main. It also leaves out the recirculation loop on a hot system, the expansion control required on a closed system, and any local amendment that sets a stricter velocity ceiling than the manufacturer does.
Code limits that apply
- IPC 604.6checked
Water distribution sized so the velocity does not exceed the manufacturer's limit
If you miss it: Noisy pipe, water hammer at the quick closing valves and eroded elbows on hot copper
- IPC Table 604.3
Minimum flowing pressure at the fixture: 8 psi at a lavatory, 20 psi at a shower valve
If you miss it: A shower that drops to a trickle when the washer fills is under the table minimum
- IPC 604.8
Static pressure over 80 psi needs an approved pressure reducing valve
If you miss it: Supply hoses, fill valves and the water heater relief valve fail early at 90 psi
- IPC Appendix E
Supply pipe is sized from water supply fixture units, not from a single flow figure
If you miss it: A line sized for one shower starves when the dishwasher and the hose bib run together
- IPC 605.4
Materials and fittings have to be listed for the water they carry, hot or cold
If you miss it: A cold-rated fitting on a hot line fails at the joint under thermal cycling
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Sizing off the nominal number: 3/4 in PEX has 27 percent less area than 3/4 in type L copper and delivers proportionally less.
- Counting only the straight pipe. Elbows and branch tees add equivalent length, often a third of the total on a fitting-heavy run.
- Ignoring elevation. Every 10 ft of rise costs 4.3 psi before friction, which is why the upstairs shower is the one that drops.
- Running hot copper at 8 fps to save a pipe size; the elbows wash out from the inside and pinhole in a few years.
- Old galvanized carries far less than its size suggests. Scale drops the C factor to about 100 and narrows the bore.
- Sizing a hose bib line for 5 gpm and then hanging a pressure washer on it; check the appliance demand, not just the pipe.
Key facts
- Flow rises with the square of the diameter, so 1 in pipe carries about 1.7 times what 3/4 in carries at the same velocity.
- PEX of a given nominal size has a smaller bore than copper: 3/4 in PEX is 0.671 in inside against 0.785 in for type L copper.
- Five feet per second is the usual cold water design limit, 8 the ceiling. Hot copper is held to 4 to 5 because hot water erodes the tube wall at fittings.
- One psi lifts water 2.31 feet, so a fixture 10 ft above the meter starts 4.3 psi down before any friction.
- IPC 604.8 requires a pressure reducing valve where the static supply pressure exceeds 80 psi.
Frequently asked questions
How many gpm can a 3/4 inch pipe carry?
About 7.5 gallons per minute in type L copper at the usual 5 ft per second design speed, and about 5.5 in 3/4 in PEX because the bore is smaller. Pushing either to 8 ft per second gets more flow at the cost of noise and erosion.
What is a safe water velocity in a pipe?
Five feet per second for cold water is the common design limit and 8 is the ceiling. Hot water copper is held to 4 to 5 because heat accelerates erosion at the fittings.
Why does my shower lose pressure when another tap opens?
Two fixtures share the same run, so the flow through it roughly doubles and friction loss roughly triples. The fix is a larger trunk line or a home run manifold, not a bigger shower head.
Does PEX flow less than copper?
At the same nominal size, yes. Three quarter inch PEX is 0.671 in inside against 0.785 in for type L copper, about 27 percent less area, so it carries about 27 percent less at the same velocity.
Sources and references
- Copper Tube Handbook, tube dimensions and flow pressure loss tables (Copper Development Association)
- Chapter 6 Water Supply and Distribution, 2021 IPC (ICC)
Next steps for this project
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.