In short: pressure loss is the pressure a pipe "eats" through friction as water moves through it. For 10 GPM through 100 ft of 3/4 inch PVC pipe, that is about 12.4 psi (28.7 ft of head). Loss grows fast with smaller diameter and longer runs, and rises with rougher pipe material — use the calculator above for your own numbers.
This is a planning aid based on the Darcy-Weisbach equation. For code compliance or critical system design, confirm sizing with a licensed engineer.
How to calculate pressure loss in a pipe
Pressure loss from friction is found with the Darcy-Weisbach equation. It combines how fast the water is moving, how long and narrow the pipe is, and how rough the pipe wall is, into a single friction factor and a resulting pressure drop:
The calculator first finds velocity from your flow rate and pipe diameter, then the Reynolds number, which tells it whether flow is laminar or turbulent. For turbulent flow — nearly all real plumbing and irrigation runs — it solves the Swamee-Jain equation for the friction factor using your pipe's roughness. That friction factor plugs into the equation above to give pressure loss in psi and, equivalently, head loss in feet of water.
Worked example. Take 10 GPM through 100 ft of 3/4 inch (0.75 in) inside-diameter PVC pipe at 20 °C. Velocity works out to about 7.26 ft/s (2.21 m/s), giving a Reynolds number near 42,018 — solidly turbulent. Swamee-Jain gives a friction factor of about 0.0219. Plugging that into the Darcy-Weisbach equation gives a head loss of about 28.71 ft, which converts to a pressure loss of about 12.42 psi over the 100 ft run.
This is friction loss only. It does not include the pressure change from elevation gain or loss (use the elevation head calculator) or losses from valves, elbows and tees (use the fitting pressure drop calculator). Add all three for the total pressure available at a fixture.
Why pipe diameter and material change the answer
Velocity is the biggest lever. Halve the pipe diameter at the same flow rate and velocity roughly quadruples (area shrinks with the square of diameter), and since friction loss scales close to velocity squared, pressure loss can rise by a factor of 15 or more. That is why undersized pipe is the most common cause of poor pressure at a fixture, even when the water main is delivering plenty.
Pipe roughness matters once flow is turbulent, which is true for almost every plumbing and irrigation run. A rougher wall — steel that has begun to scale, older galvanized pipe, or aged cast iron — increases the friction factor for the same Reynolds number, so the same flow loses more pressure. Smooth materials like PVC, copper and HDPE keep the friction factor low. Below the laminar threshold (Reynolds number under about 2,300) roughness stops mattering entirely, because the water moves in smooth layers that never touch the wall closely enough to feel its texture — but ordinary water pipes are almost never in that regime.
Check the flow regime with the standalone Reynolds number calculator if you want to explore that boundary on its own.
Pressure loss per 100 ft of 3/4 inch PVC pipe
Friction loss at 20 °C for a range of flow rates through 100 ft of 3/4 inch (0.75 in) inside-diameter PVC pipe — the values this calculator produces for those inputs.
| Flow rate | Velocity | Head loss | Pressure loss |
|---|---|---|---|
| 2 GPM | 1.45 ft/s | 1.71 ft | 0.74 psi |
| 4 GPM | 2.90 ft/s | 5.70 ft | 2.47 psi |
| 6 GPM | 4.36 ft/s | 11.61 ft | 5.03 psi |
| 8 GPM | 5.81 ft/s | 19.31 ft | 8.36 psi |
| 10 GPM (default) | 7.26 ft/s | 28.71 ft | 12.42 psi |
| 15 GPM | 10.89 ft/s | 59.25 ft | 25.64 psi |
| 20 GPM | 14.52 ft/s | 99.43 ft | 43.03 psi |
Worked examples
Baseline: 3/4" PVC, 10 GPM, 100 ft
12.42 psi
Head loss of 28.71 ft over the run. Velocity is about 7.26 ft/s — on the fast side for continuous flow, which is part of why loss is relatively high here.
Upsize to 1" PVC, same flow
3.14 psi
Same 10 GPM and 100 ft, but 1 in inside diameter instead of 3/4 in. Velocity drops to about 4.08 ft/s and pressure loss falls to roughly a quarter of the baseline.
Swap to galvanized steel
21.02 psi
Same 3/4 in diameter, 10 GPM and 100 ft as the baseline, but galvanized steel instead of PVC. Higher roughness alone adds about 70% more loss at identical flow and velocity.
What else affects the result
Fittings and valves. Every elbow, tee, valve and reducer adds its own "minor loss" on top of straight-pipe friction — sometimes more than the pipe itself on a short run with many turns. Add those separately with the fitting pressure drop calculator.
Elevation change. Running uphill adds a static pressure loss independent of friction; running downhill can partly offset friction loss. See the elevation head calculator to add that in.
Pipe age and scale. Roughness values here assume clean, new pipe. Mineral scale, corrosion and biofilm raise effective roughness over years of service, which is why older galvanized and cast-iron systems often lose more pressure than their listed roughness would suggest.
Non-water fluids. This tool assumes water. Fluids with different density or viscosity — glycol mixes, oils, wastewater with solids — will show different pressure loss for the same flow and pipe, because both properties enter the Reynolds number and the Darcy-Weisbach equation directly.
Frequently asked questions
What causes pressure loss in a pipe?
Pressure loss comes from friction between moving water and the pipe wall. As water flows, it drags against the wall and against itself, converting some of its pressure energy into heat. Faster flow, longer pipe, smaller diameter and rougher pipe walls all increase this friction loss.
How much pressure loss is normal for a home water line?
For typical residential branch lines, engineers often target no more than about 4 to 10 psi per 100 feet of pipe, keeping velocity under roughly 8 ft/s. Our example of 10 GPM through 100 ft of 3/4 inch PVC pipe loses about 12.4 psi, which is on the high side — a good reason to check pipe sizing before extending a line.
Does pipe material really change the pressure loss?
Yes, in turbulent flow. Smoother materials like PVC and copper have very low roughness and produce lower friction loss. Rougher materials like galvanized steel or aged cast iron increase the friction factor. In our reference example, switching from smooth PVC to galvanized steel at identical flow raised the head loss from about 28.7 ft to roughly 48.6 ft over 100 ft of pipe.
Does water temperature affect pressure loss?
A little. Warmer water is less viscous, which slightly lowers the friction factor and reduces pressure loss for the same flow rate. The effect is usually a few percent between cold supply and hot tap water — small compared to the effect of pipe diameter or material.
How is pressure loss different from head loss?
They describe the same friction effect in different units. Head loss is expressed in feet or meters of water column; pressure loss is the same quantity expressed in psi, kPa or bar. Multiplying head loss by water's unit weight converts it to pressure loss, so the two numbers always represent one physical loss.
Sources & references
- Swamee, P. K., & Jain, A. K. (1976). Explicit equations for pipe-flow problems. Journal of the Hydraulics Division, ASCE, 102(5), 657–664.
- Crane Co. (2013). Flow of Fluids Through Valves, Fittings, and Pipe (Technical Paper No. 410).
- National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluid Systems.
- See the underlying Darcy-Weisbach equation for the full derivation this calculator is built on.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)