Process & Mechanical
Pipe Pressure Drop
Friction pressure drop for a straight run of pipe using the Darcy-Weisbach equation, with the Swamee-Jain explicit approximation of the Colebrook-White friction factor.
Standard method — stated scope limits apply
Flow & Pipe
Fluid
Default roughness 0.045 mm is typical commercial steel. Use ~0.0015 mm for drawn tubing, ~0.15–0.3 mm for cast iron or older/corroded steel.
Enter flow and pipe data.
About This Calculation
Fluid flowing through a pipe loses pressure to friction against the pipe wall (a function of velocity, pipe roughness, and Reynolds number via the Darcy-Weisbach or Colebrook-White friction factor) and to fittings, valves, and direction changes along the route — sizing a pipe or specifying a pump correctly means totaling BOTH contributions across the full pipe run, not just the straight-pipe friction loss alone, since fittings can easily dominate total pressure drop on a route with many bends and valves even when the straight-pipe sections are short.
Frequently Asked Questions
Why does pressure drop increase so sharply with pipe velocity?
Friction pressure drop scales with velocity SQUARED in the Darcy-Weisbach equation — doubling flow velocity in the same pipe roughly quadruples the friction pressure drop, which is why undersizing a pipe even slightly can produce a disproportionately large pumping energy penalty over the system’s operating life.
What is equivalent length and why use it for fittings instead of a separate calculation?
Equivalent length expresses a fitting’s pressure loss as "however many diameters of straight pipe would cause the same loss," letting fittings simply be added to the actual pipe run length and run through the SAME friction-factor calculation, rather than needing a completely separate loss-coefficient method for every fitting type.
Does pipe roughness matter as much as pipe diameter for pressure drop?
It matters, but less dramatically — roughness affects the friction factor logarithmically (via the Colebrook-White relation) while diameter affects pressure drop to roughly the 5th power in the Darcy-Weisbach equation, meaning a modest diameter increase reduces pressure drop far more than an equivalent percentage change in pipe roughness or condition.
Why does the Reynolds number matter for choosing a friction factor method?
It determines whether flow is laminar (smooth, predictable, friction factor = 64/Re) or turbulent (chaotic, friction factor depends on both Reynolds number and relative roughness via Colebrook-White) — using a laminar-flow formula for turbulent flow, or vice versa, gives a genuinely wrong answer, not just a slightly imprecise one.