Pressure Drop Calculator
Darcy-Weisbach friction arithmetic: the head the pipe steals per hundred metres, returned as pascals a pump must repay.
Pressure Drop Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Straight-run Darcy friction; no fittings
- Factor entered, not solved from Colebrook
In short: Water at 2 m/s through a 100 m run of 100 mm pipe at friction factor 0.02 loses 40,000.000000 Pa — h_f = f·(L/D)·v²/2g = 4.077472 m of head the pump must buy back. The L/D spine is the brutal part: a hundred diameters of pipe multiplies the velocity head by two hundred before the factor even looks at it. Friction is quadratic in velocity, which is why doubling the flow quadruples the bill and why pipe upsizing is the cheapest energy project in industry.
Formula
hᶠ = f·(L/D)·v²/2g ··· Δp = ρ·g·hᶠ = f·(L/D)·ρv²/2
Darcy-Weisbach is friction's universal receipt: the velocity head, times how many diameters long the pipe is, times the factor the Moody diagram assigns to roughness and Reynolds. Everything scales honestly — length linear, velocity squared, diameter inverse — which is why the formula has survived every alternative thrown at it since Darcy's century.
Worked Example
- Get f from the Moody diagram (or Colebrook).
- Enter length, bore, velocity and density.
- Read the loss in pascals and in head.
- Check the L/D spine before blaming the factor.
Defaults: f 0.02, 100 m, 100 mm, 2 m/s, water → 40,000.000000 Pa, 4.077472 m of head. The velocity check: 3 m/s multiplies the loss by 2.25 → 90,000.000000 Pa — the square law, invoiced.
Strengths & Limits Of This Model
Where this engine is strong
- Pascals and head both returned
- The L/D spine shown as the multiplier
Where it stops
- No fitting losses
- No laminar/turbulent classification
Practical Use Cases
Pump selection
the friction share of TDH
Pipe sizing
upsizing vs energy bill
Teaching
quadratic friction
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Pressure Drop Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Where does the friction factor come from?
The Moody diagram — or the Colebrook-White equation behind it — read with the pipe's relative roughness and Reynolds number. Laminar flow is the easy branch: f = 64/Re exactly. Turbulent water in commercial steel typically lands near 0.02, smooth plastics near 0.015, corroded or rough bores higher. This page takes f as given so the loss arithmetic stays visible; the diagram is a chart lookup away.
Why does velocity enter squared?
Because friction is momentum exchange at the wall, and the momentum flux itself grows with v². Doubling the flow quadruples the loss per metre — the single most consequential fact in pipe economics, and the reason a bore one size up can pay for itself in motor kilowatts within a few years on long runs. The square law is the enemy of optimistic sizing.
Why is Fanning sometimes a quarter of Darcy?
Two conventions coexist: the Fanning factor uses hydraulic radius where Darcy uses diameter, and the geometry differs by exactly four. Entering a Fanning value into a Darcy formula understates the loss by 75% — a classic exam trap and an occasional plant surprise. The page's door refuses implausibly small large values and names the suspicion in the refusal text.
What about bends and fittings?
Each fitting adds loss the way a stretch of pipe would — priced as an equivalent length or as a loss coefficient times the velocity head. A short-Radius elbow can cost as much as several metres of straight pipe. This page prices the straight run honestly; the fittings enter the sum as extra L or extra velocity heads, per their tables. Complex headers want the full treatment.
Why does the pipe's own length matter linearly when velocity is squared?
Because each diameter-length of pipe taxes the SAME velocity head: the L/D ratio counts how many such toll booths the flow passes. Ten times the length is ten times the tolls at the same rate each. The spine card prints L/D because it is the multiplier everything else hangs on — 1,000 diameters of pipe is a serious sentence even at gentle velocities.
How does viscosity change the answer?
Twice removed: viscosity sets the Reynolds number that picks f off the Moody chart, and then f does the taxing. Thick fluids push Reynolds down toward the laminar branch where f = 64/Re makes viscosity DIRECTLY the friction — honey pays terrible tolls. The density input here converts the head loss to pascals once f is known; the viscosity's fingerprint is inside f.
Is the loss recoverable?
No — that is the definition: friction converts useful pressure into heat along the wall, every metre, every second. Unlike static head, which a downhill pipe returns, friction is spent once. It is why long pipelines need booster stations and why the return leg of a loop costs what the outbound leg cost. The pump buys it back with kilowatts, forever.
How do I choose between more pump and bigger pipe?
Price both against energy: a bigger pump raises the electric bill every hour of the plant's life; a bigger pipe is a one-time capital spend with a lower running toll forever after. Long runs and high duty cycles favour pipe; short intermittent runs favour the bigger pump. The quadratic law decides most arguments: velocity you pay for twice, diameter once.