In short: pipe pressure drop is the friction loss along a pipe run, calculated directly in pressure units with the Darcy-Weisbach equation. 10 GPM through 100 ft of 1-inch PVC pipe drops about 3.14 psi (21.6 kPa). Diameter has by far the largest effect — undersizing a pipe by one size can multiply the drop several times over. Use the calculator above for your exact pipe and flow.
These figures are planning aids based on published hydraulic formulas. For code-required pipe sizing, confirm with a licensed engineer.
How to calculate pipe pressure drop
The Darcy-Weisbach equation gives pressure drop directly, without first converting to head:
The friction factor itself depends on the Reynolds number and the pipe's relative roughness. This calculator finds it with the Swamee-Jain approximation for turbulent flow, and the simple f = 64/Re relationship for laminar flow:
Worked example. 10 GPM through 100 ft of 1-inch (ID) PVC pipe (ε = 0.0015 mm), water at 20°C: velocity works out to 1.245 m/s, giving a Reynolds number of about 31,514 (turbulent) and a friction factor of 0.0233. Plugging into the equation above gives ΔP ≈ 21,623 Pa, which converts to 3.14 psi (21.6 kPa) — the calculator's default result.
Pressure drop and head loss are the same thing. Divide this pressure drop by density and gravity and you get the same head loss in feet that the friction loss calculator would return for identical inputs — this tool just reports the answer directly in pressure units instead of head, and always uses the physics-based Darcy-Weisbach method rather than the empirical Hazen-Williams shortcut.
Why diameter, roughness and flow change the answer
Diameter dominates. Pressure drop falls roughly with diameter to the fifth power, once the effect on both velocity and the L/D term is combined — a single pipe size increase can cut pressure drop several times over.
Roughness sets a turbulence penalty. A rougher pipe wall increases the friction factor for the same Reynolds number, so switching from smooth PVC to old, corroded cast iron can multiply pressure drop several times over at identical flow and size.
Flow rate compounds through velocity. Since pressure drop depends on velocity squared, doubling the flow rate roughly triples to quadruples the pressure drop, depending on how the friction factor itself shifts with the higher Reynolds number.
Reference: pressure drop per 100 ft (PVC, ε = 0.0015 mm)
Pressure drop in psi per 100 ft of straight PVC pipe, water at 20°C. Every value matches what the calculator returns for the same inputs.
| ID | 5 GPM | 10 GPM | 20 GPM | 30 GPM | 50 GPM |
|---|---|---|---|---|---|
| 0.75 in | 3.65 | 12.42 | 43.03 | 89.69 | 228.27 |
| 1 in (default) | 0.93 | 3.14 | 10.77 | 22.32 | 56.35 |
| 1.25 in | 0.32 | 1.08 | 3.69 | 7.62 | 19.14 |
| 1.5 in | 0.14 | 0.45 | 1.54 | 3.18 | 7.95 |
| 2 in | 0.03 | 0.12 | 0.39 | 0.80 | 1.99 |
Worked examples
10 GPM through 1″ PVC, 100 ft
3.14 psi
The calculator's default: Reynolds number 31,514 (turbulent), friction factor 0.0233, velocity 4.08 ft/s.
Doubled flow, same pipe
10.77 psi
20 GPM through the same 1″ PVC pipe more than triples the pressure drop, since it depends on velocity squared and a higher friction factor.
Same flow, rougher pipe
3.73 psi
10 GPM through 100 ft of 1″ new steel pipe (ε = 0.045 mm) — about 19% more pressure drop than smooth PVC at identical size and flow.
What this calculator doesn't include
Fittings and valves. Elbows, tees, reducers and valves each add their own loss on top of straight-pipe friction. Use the pressure drop across fitting calculator to estimate those with the K-factor method.
Elevation change. This tool reports friction-driven pressure drop only, not the static pressure gained or lost from a change in height — see the elevation head calculator for that term.
Non-water fluids. The default temperature-based density and viscosity assume water. For other fluids, use the custom roughness field with a density and viscosity appropriate to your fluid in mind, and treat the result as an approximation.
Frequently asked questions
What is pipe pressure drop?
Pipe pressure drop is the loss of pressure that occurs as fluid flows through a pipe due to friction against the pipe wall. The Darcy-Weisbach equation calculates it directly in pressure units from flow velocity, pipe geometry, fluid density and a friction factor that depends on the Reynolds number and pipe roughness.
How is this different from a head loss calculation?
Pressure drop and head loss describe the same physical loss, just in different units — pressure drop in psi, kPa or Pa, and head loss in feet or meters. They convert into each other through the fluid's density: divide pressure drop by density and gravity to get head loss, or multiply head loss by density and gravity to get pressure drop. This calculator reports both.
What is the Darcy-Weisbach friction factor?
The friction factor (f) measures how much a pipe resists flow due to its roughness and the flow's turbulence. This calculator finds it from the Reynolds number: for laminar flow (Reynolds number under 2,300) it uses f = 64/Re; for turbulent flow it uses the Swamee-Jain approximation, which closely matches the Colebrook-White equation without requiring iteration.
Why does pipe roughness matter for pressure drop?
Rougher pipe walls create more turbulence right at the surface, which increases the friction factor and therefore the pressure drop for the same flow and pipe size. Smooth materials like PVC or copper have a roughness around 0.0015 mm; corroded or older metal pipe can be 100 times rougher or more, substantially raising pressure drop.
Does water temperature affect pressure drop?
Yes, though usually modestly for typical water temperatures. Warmer water is both less dense and less viscous, which shifts the Reynolds number and friction factor slightly and changes the resulting pressure drop by a small amount. This calculator uses your entered temperature to compute density and viscosity exactly rather than assuming a fixed value.
Does this include losses from fittings and valves?
No. This tool calculates straight-pipe pressure drop only. Elbows, tees, valves and other fittings add their own losses — see the pressure drop across fitting calculator to estimate those separately using the K-factor method.
Sources & references
- Darcy, H. (1857). Recherches Expérimentales Relatives au Mouvement de l'Eau dans les Tuyaux. Mallet-Bachelier.
- Weisbach, J. (1845). Lehrbuch der Ingenieur- und Maschinen-Mechanik. Vieweg.
- Swamee, P. K., & Jain, A. K. (1976). Explicit equations for pipe-flow problems. Journal of the Hydraulics Division, ASCE, 102(5), 657–664.
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)