Pipe Pressure Drop Calculator
Friction loss in pipes with Darcy-Weisbach and an iterated Colebrook friction factor, roughness presets, fitting K values and elevation, in kPa, bar or psi.
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About this toolWhat is the Pipe Pressure Drop Calculator?
Pressure drop decides pump size, pipe size and whether a system works at all. This calculator uses the Darcy-Weisbach equation, the method fluid mechanics texts treat as the general one, with the friction factor solved from the Colebrook-White equation by iteration rather than read off a chart. Laminar flow switches automatically to f = 64/Re, and the explicit Swamee-Jain value sits alongside so you can see how close the shortcut gets.
Pick a fluid (water at any temperature from 0 to 100 °C, or oils, seawater, air and others), enter the flow, bore and length, and choose a pipe material to fill in a typical roughness. Add elbows, tees, valves, entrances and exits from a list of textbook K values, or type your own total, and add an elevation change if the outlet is higher or lower than the inlet. The default case, 10 L/s of 20 °C water through 100 m of 100 mm commercial steel with four elbows and two gate valves, loses 17.0 kPa, of which 15.8 kPa is pipe friction.
Results come in Pa, kPa, bar, MPa or psi, as metres and feet of head, and per 100 m of pipe. A system curve plots friction loss against flow from zero to twice your design flow, ready to lay over a pump curve.
- Darcy-Weisbach with Colebrook-White solved by iteration, plus Swamee-Jain for comparison
- Automatic 64/Re in laminar flow and a warning in the transitional band
- Ten typical roughness presets and twelve fittings with textbook K values, all editable
- Fluid presets including water from 0 to 100 °C, seawater, oils and air
- Output in Pa, kPa, bar, MPa, psi and metres or feet of head, with a system curve chart
How to use the Pipe Pressure Drop Calculator
- Choose the fluid and its temperature, or enter density and viscosity yourself.
- Enter the flow rate, the pipe inside diameter and the pipe length.
- Pick the pipe material to set a typical roughness, or type the roughness you want, such as an aged-pipe value.
- Enter how many of each fitting and valve the line has, plus any other K values, and the elevation rise from inlet to outlet if it matters.
- Read the total drop, its parts (friction, fittings, elevation), Reynolds number, friction factor and head loss, then use the system curve to see how the loss grows with flow.
Worked examples
Water main section
Input
10 L/s water at 20 °C, 100 mm steel, 100 m, 4 flanged elbows, 2 gate valves
Output
v = 1.27 m/s, Re = 126,900, f = 0.01951, friction 15.79 kPa, fittings 1.21 kPa, total 17.0 kPa (1.74 m head)
Swamee-Jain gives f = 0.01960, 0.45% high. Swapping to uncoated cast iron raises the friction loss to 21.3 kPa.
US units, 2 inch Schedule 40
Input
100 gpm water, 2.067 in bore, 100 ft of steel
Output
v = 9.56 ft/s, f = 0.0209, Δp = 7.45 psi per 100 ft (17.2 ft of head)
The same run in PVC (ε = 0.0015 mm) loses 5.96 psi, 20% less, because the smoother wall lowers f to 0.0167.
Laminar oil line
Input
ISO VG 32 at 40 °C, 0.5 L/s, 25 mm bore, 10 m
Output
Re = 796, laminar, f = 64/Re = 0.0804, Δp = 14.3 kPa
Roughness does not matter in laminar flow, but viscosity does: the loss scales directly with μ.
Background and insight
Darcy-Weisbach says the friction loss is Δp = f (L/D)(ρv²/2): proportional to length, to velocity head and to a dimensionless friction factor f. In turbulent flow f depends on Reynolds number and relative roughness ε/D. Cyril Colebrook published his implicit formula in 1939 and Lewis Moody turned it into the familiar chart in 1944. Explicit fits such as Swamee-Jain (1976) and Haaland (1983) agree with Colebrook to within about 1 to 2% over the normal range, which this tool lets you check.
Roughness presets are typical values for new pipe from Moody and standard texts such as White's Fluid Mechanics: drawn tubing and plastics 0.0015 mm, commercial steel 0.045 mm, galvanized iron 0.15 mm, cast iron 0.26 mm and concrete from 0.3 to 3 mm. Pipes get rougher with corrosion and scale, sometimes by an order of magnitude, so ageing allowances matter for long-life systems.
Fitting losses use ΔP = K ρv²/2 with typical K values from textbook tables, for example 0.3 for a flanged regular 90° elbow, 1.5 for a threaded one, 0.15 for an open gate valve and 10 for an open globe valve. Published K values vary by maker and size, often by ±50%, so they are labelled typical and you can override them.
Common use cases
Pump selection
Add friction and fitting losses to the static lift to get the total head a pump must deliver.
Pipe sizing trade-offs
Compare the pressure drop of two bores to balance pipe cost against pumping energy.
Troubleshooting low flow
Check whether a long run, a globe valve or an aged rough pipe explains a pressure shortfall.
Cooling and process loops
Find the loss through each branch with the right fluid temperature and viscosity.
Tips and limitations
- Roughness of old pipe can be many times the new value. If a steel line is 20 years old, try 0.15 to 0.5 mm and see how sensitive the answer is.
- The Hazen-Williams formula is still common for water mains but only suits water near room temperature in turbulent flow. Darcy-Weisbach works for any Newtonian liquid.
- Do not mix Darcy and Fanning friction factors. Fanning f is a quarter of Darcy f; this tool shows both.
- Minor losses can dominate short, fitting-heavy runs such as pump skids. Long pipelines are usually dominated by pipe friction.
- For gases, if the drop exceeds roughly 10% of the inlet pressure the density changes too much for this incompressible method; use a compressible-flow method instead.
Frequently asked questions
How do you calculate pressure drop in a pipe?
Use Darcy-Weisbach: Δp = f × (L/D) × ρv²/2. Get the velocity from the flow and bore, the Reynolds number from the fluid properties, and the friction factor f from the Colebrook equation or a Moody chart. Add K × ρv²/2 for each fitting and ρgΔz for any rise.
What is the Colebrook equation?
An implicit formula for the turbulent Darcy friction factor: 1/√f = −2 log10(ε/(3.7D) + 2.51/(Re√f)). Because f appears on both sides it is solved by iteration; it usually converges in under ten steps.
What is the friction factor for laminar flow?
For fully developed laminar flow in a round pipe the Darcy friction factor is exactly 64/Re, independent of roughness. At Re = 1000, f = 0.064.
How accurate is the Swamee-Jain equation?
It matches Colebrook to within about 1 to 2% for 5,000 < Re < 10⁸ and 10⁻⁶ < ε/D < 0.01. That is far smaller than the uncertainty in pipe roughness, so it is fine for most design work.
What is the roughness of steel pipe?
New commercial steel is typically taken as 0.045 mm (0.00015 ft). Galvanized iron is about 0.15 mm and uncoated cast iron about 0.26 mm. Corroded or scaled pipe can be much rougher.
How do I convert pressure drop to head loss?
Divide by ρg: h = Δp/(ρg). For water at about 1000 kg/m³, 1 bar of loss is roughly 10.2 m of head, and 1 psi is about 2.31 ft.
What K value should I use for a 90 degree elbow?
Typical textbook values are about 0.3 for a flanged regular 90° elbow, 0.2 for a long-radius flanged one and 1.5 for a threaded regular elbow. Manufacturer data or the Crane TP-410 method, which ties K to pipe size, gives better numbers.
Is there an in-depth guide to the Pipe Pressure Drop Calculator?
Yes. “Darcy-Weisbach and the Colebrook Equation: Pipe Friction Loss in Practice” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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