Pressure Loss Calculator
This pressure loss calculator finds the pressure drop along a pipe from friction, using the Darcy equation. Enter the friction factor, length, diameter, velocity, and fluid density and the pressure loss calculator returns the pressure loss in kPa and bar plus the head loss. It works as a pressure drop calculator for water, air, and other fluids in pipes and ducts.
Results
The Pressure Loss Calculator is part of our Fluids collection. Browse more fluids to size, convert, and check your work alongside this tool.
What the pressure loss calculator shows
As fluid flows through a pipe, friction with the walls steals pressure along the way, and the pressure loss calculator measures how much. It uses the Darcy equation, which grows with the pipe length and the square of the velocity and falls with the diameter. Whether you run it as a pressure drop calculator or a friction losses calculator, it gives the pressure a pump or fan must make up to keep the flow moving.
The pressure loss formula
The pressure loss formula is the Darcy equation, friction factor times length over diameter times the velocity head:
f is the friction factor, L the length, D the diameter, ρ the density, and v the velocity. Because velocity is squared, doubling the flow speed quadruples the loss. Dividing the result by the fluid weight per unit volume gives the head loss in meters. This pressure drop equation is the backbone of the pressure loss calculator.
Pressure drop calculator example
Water in a 100 m pipe, 50 mm across, at 2 m/s, with a friction factor of 0.02. The pressure drop calculator gives:
So the pipe loses 80 kPa, or 0.8 bar, about 8.16 m of head. Halve the velocity and the loss drops to a quarter, since the velocity is squared.
How to use the pressure loss calculator
Enter the friction factor, the pipe length in meters, the diameter in millimeters, the flow velocity in meters per second, and the fluid density, then the pressure loss calculator returns the pressure loss and the head loss. Use about 0.02 for the friction factor in typical turbulent water flow, and 1000 kg per cubic meter for water. This online pressure loss calculator works for ducts too, with air density near 1.2.
Reading the pressure loss calculator result
The pressure loss is the pressure the friction removes; a pump or fan has to supply at least this much on top of any height and equipment losses to keep the flow going. The head loss in meters is the same loss written as a height of fluid, which is handy for pump curves. A large loss usually means the velocity is too high or the pipe too small, so widening the pipe is the strongest fix. The pressure loss calculator gives the friction part; add fittings and elevation for the total.
When to use the friction losses calculator
- Sizing a pump with the friction losses calculator
- Checking a pipe run with the pressure drop calculator
- Finding duct loss with a ducting pressure drop calculator
- Comparing pipe sizes by their head loss calculation
Common pressure loss calculator mistakes
- Wrong density. Use 1000 for water and about 1.2 for air, or the pressure loss will be far off.
- Diameter in the wrong unit. Enter the diameter in millimeters; the equation converts it to meters.
- Ignoring fittings. This gives the straight pipe friction only; add elbows, valves, and elevation for the total.
Tips for reducing pressure loss
- Widen the pipe to cut loss fast, since diameter has a strong effect.
- Keep the velocity moderate, because loss rises with its square.
- Add fittings and elevation separately for the full pump head.
The pressure drop equation and head loss calculation
The pressure drop equation splits the loss into pieces you can act on. The length over diameter term says a longer, narrower pipe loses more, and the velocity head term, half rho v squared, says the loss climbs with the square of the speed. That squared velocity is why the same pipe at 3 m/s loses nine times what it loses at 1 m/s. A head loss calculation restates the pressure loss as a height of fluid, which lines up with pump curves that are drawn in meters of head, so a pump rated for 20 m of head can overcome about 20 m of friction plus lift. Dividing the pressure loss by the fluid weight per unit volume, rho times g, makes that conversion, and the online pressure loss calculator shows both forms.
Pressure loss equation for duct and pipe
The same pressure loss equation covers a water pipe and an air duct, with only the density changing. For water, rho is about 1000, and to calculate pressure drop in duct you switch rho to roughly 1.2 for air, which is why duct losses are small in absolute pressure but still important against the low pressure a fan makes. A pressure drop calculator air duct result is often quoted in pascals rather than kPa for that reason. The friction factor is the other variable: it depends on whether the flow is smooth or turbulent and on the wall roughness, so a ductwork pressure drop calculator for flexible duct uses a higher factor than smooth steel pipe. Pick the density and friction factor for your fluid and pipe, and the equation handles both worlds.
Pressure drop formula reference
Pressure loss for water in a 100 m pipe, 50 mm across, at a friction factor of 0.02, from the pressure drop formula:
| Velocity | Pressure loss | Head loss |
|---|---|---|
| 1 m/s | 20 kPa | 2.04 m |
| 1.5 m/s | 45 kPa | 4.59 m |
| 2 m/s | 80 kPa | 8.16 m |
| 3 m/s | 180 kPa | 18.35 m |
Pressure Loss Calculator FAQ
What is the pressure loss formula?
The pressure loss formula is the Darcy equation, ΔP = f (L / D)(ρ v² / 2). The pressure loss calculator applies it to give the drop in kPa and the head loss in meters.
Why does velocity matter so much?
Velocity is squared in the pressure drop equation, so doubling the flow speed quadruples the loss. Slowing the flow or widening the pipe cuts the loss sharply.
What friction factor should I use?
About 0.02 for typical turbulent water flow. The exact value depends on the Reynolds number and pipe roughness, so a rough pipe or laminar flow needs a different figure.
Does it work for air ducts?
Yes. As a ducting pressure drop calculator, use the air density near 1.2 kg per cubic meter instead of 1000 for water, and the same equation applies.