Pressure drop calculator: friction loss in a straight pipe

This pressure drop calculator uses the Darcy-Weisbach equation from the KSB pump lexicon and the friction factor formulas of the U.S. EPA’s EPANET 2.2. Example: 20 m³/h of water through 100 m of steel pipe with an 80 mm bore loses 1.64 m of head – a pressure drop of 0.161 bar.

Example: Flow rate Q 20 m³/h · Inside diameter d 80 mm · Pipe length L 100 m · Pipe material (roughness of new pipe): steel (0.046 mm) · Kinematic viscosity ν 1 mm²/s · Density ρ 1,000 kg/m³ → Pressure drop: Pressure drop 0.161 bar (1.64 m head). Source: KSB Centrifugal Pump Lexicon, “Head loss”: H_v = λ · (L/d) · v²/(2g), v = 4Q/(πd²), Re = v·d/ν (water 20 °C: ν = 1.00 · 10⁻⁶ m²/s), laminar λ = 64/Re, valves and fittings H_v = ζ · v²/(2g); 5 % smaller bore → up to 30 % more head loss (read 09.10.2026). Updated: 9 October 2026.

Flow rate Q

m³/h

Inside diameter d

the real bore, not the nominal size

Pipe length L

Pipe material (roughness of new pipe)

Kinematic viscosity ν

mm²/s

water at 20 °C: 1.0

Density ρ

kg/m³

Sum of loss coefficients Σζ (optional)

bends, valves, inlet, outlet; 0 = pipe only

My ToolboxYour inputs are saved in this browser only.

Result

Pressure drop: Pressure drop 0.161 bar (1.64 m head)
Details: Quantity Value
Flow velocity v = 4Q/(πd²) 1.11 m/s
Reynolds number Re = v·d/ν 88,419
Flow regime turbulent (Re > 4000)
Roughness k 0.0457 mm
Friction factor λ 0.0210
Head loss pipe λ·(L/d)·v²/2g 1.64 m
Total head loss 1.64 m
Pressure drop Δp = ρ·g·H 0.161 bar = 16.06 kPa
Pipe head loss per 100 m 1.64 m; Calculated for a full pipe and steady flow with the roughness of new pipe; old, scaled or dirty pipes lose more.

Quantity	Value
Flow velocity v = 4Q/(πd²)	1.11 m/s
Reynolds number Re = v·d/ν	88,419
Flow regime	turbulent (Re > 4000)
Roughness k	0.0457 mm
Friction factor λ	0.0210
Head loss pipe λ·(L/d)·v²/2g	1.64 m
Total head loss	1.64 m
Pressure drop Δp = ρ·g·H	0.161 bar = 16.06 kPa
Pipe head loss per 100 m	1.64 m

How it is calculated

Head loss = λ × (L ÷ d) × v² ÷ 2g. This is the Darcy-Weisbach equation as KSB gives it for a straight pipe with a circular cross-section. v is the flow velocity 4Q ÷ (π d²), L the length, d the inside diameter and g = 9.80665 m/s². Multiply the head loss by density and g and you have the pressure drop: Δp = ρ · g · H.

Where the friction factor λ comes from

λ depends on the Reynolds number Re = v·d/ν and on the relative roughness k/d. The calculator uses the same three formulas as the EPA’s EPANET 2.2 software: below Re 2000 the laminar value 64/Re, above Re 4000 the Swamee-Jain approximation of the Colebrook-White equation, f = 0.25/[log(k/3.7d + 5.74/Re^0.9)]², and in between a cubic interpolation. The roughness values for new pipe come from EPANET Table 3.2 (given there in thousandths of a foot): steel 0.15, cast iron 0.85, galvanized iron 0.5, plastic 0.005, concrete 1.0 to 10.

Examples (water, ν = 1.0 mm²/s, 1000 kg/m³)

Flow	Bore	Length	Pipe	λ	Pressure drop
20 m³/h	80 mm	100 m	steel	0.0210	0.161 bar
10 m³/h	50 mm	50 m	steel	0.0229	0.229 bar
5 m³/h	40 mm	30 m	plastic	0.0215	0.098 bar
50 m³/h	100 mm	200 m	cast iron	0.0261	0.818 bar

A thick oil with ν = 100 mm²/s and 900 kg/m³ flows laminar: 2 m³/h through 20 m of 50 mm pipe gives Re ≈ 141, λ = 0.4524 and 0.065 bar.

Bore, roughness and fittings

KSB warns that the inside diameter enters the equation almost to the 5th power: a 5 % smaller bore raises the head loss by up to 30 %. In the first example, 76 mm instead of 80 mm gives 0.207 bar instead of 0.161 bar. Older steel pipe with k = 0.3 mm gives 0.225 bar. Bends and valves are added as Σζ · v²/2g; EPANET Table 3.3 lists for example 0.9 for a short-radius elbow, 0.2 for an open gate valve, 0.5 for an entrance and 1.0 for an exit. With Σζ = 3.5 the first example reaches 1.86 m and 0.182 bar.

The flow rate for a given pipe and velocity is worked out by the pipe flow rate calculator, the flow regime alone by the Reynolds number calculator, and the power the pump needs for the total head by the pump power calculator.

Frequently asked questions

How do I calculate pressure drop in a pipe?

Find the velocity v = 4Q/(πd²), the Reynolds number and the friction factor λ, then H = λ · (L/d) · v²/2g and Δp = ρ · g · H. For 20 m³/h in 100 m of 80 mm steel pipe that is 1.64 m or 0.161 bar.

Which friction factor formula does the calculator use?

The formulas of EPANET 2.2: 64/Re for laminar flow, the Swamee-Jain equation for turbulent flow above Re 4000 and an interpolation in between.

What roughness should I enter?

For new pipe the EPANET values: steel 0.046 mm, cast iron 0.26 mm, galvanized iron 0.15 mm, plastic 0.0015 mm. Old or scaled pipes are rougher; KSB uses 0.30 mm for slightly incrusted steel pipe.

Why does the diameter matter so much?

At the same flow rate, head loss is roughly inversely proportional to the 5th power of the bore, so a narrower pipe loses far more. KSB states that 5 % less diameter means up to 30 % more loss – in the example 0.207 instead of 0.161 bar.

How do I include valves and bends?

Add their loss coefficients and enter the sum as Σζ. Each adds ζ · v²/2g; with Σζ = 3.5 the example grows from 1.64 m to 1.86 m.

Does it work for liquids other than water?

Yes, Darcy-Weisbach applies to all liquids. Enter the kinematic viscosity and the density of the liquid at its temperature. The result is a calculated value for a full pipe and steady flow.

Sources and legal basis

As of: 9 October 2026 · Editorial policy