Pipe Pressure Drop Calculator
Find pressure loss in a pipe (Darcy-Weisbach).
Results update as you type.
How it works
As fluid flows through a pipe, friction causes a pressure drop, and this calculator computes it with the Darcy-Weisbach equation, the standard method in fluid mechanics. You enter the Darcy friction factor, the pipe length and diameter, the fluid density, and the flow velocity, and it returns the pressure drop in pascals, bar, and PSI. The equation multiplies the friction factor by the ratio of length to diameter and by the dynamic pressure, which is half the density times velocity squared. Each factor has an intuitive effect: longer pipes lose more pressure, narrower pipes lose much more (diameter is in the denominator and also affects the friction factor and velocity), denser fluids and faster flow raise the loss with velocity squared, so doubling the speed quadruples the friction loss. This calculation is essential for designing piping systems, sizing pumps, and ensuring adequate flow and pressure in water supply, HVAC, oil and gas, and process plants. The friction factor depends on the flow regime and pipe roughness; it is found from the Reynolds number and relative roughness using the Moody chart or Colebrook equation for turbulent flow, or 64 divided by the Reynolds number for laminar flow. This calculator takes the friction factor as an input so you can use the value appropriate to your conditions. Keep units in SI for a result in pascals. Enter the friction factor, dimensions, density, and velocity to find the pressure drop.
ΔP = f × (L/D) × (ρv²/2)
The Darcy-Weisbach equation gives the pressure drop from friction in a pipe as the friction factor times the length-to-diameter ratio times the dynamic pressure (half the fluid density times velocity squared). Longer, narrower pipes and faster flow all increase the pressure loss.
Source:Fluid mechanics (Darcy-Weisbach)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter darcy friction factor (f), pipe length, pipe diameter, fluid density and flow velocity.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Pressure drop updates instantly as you type — no button to press.
Frequently asked questions
What is the Darcy-Weisbach equation?
The Darcy-Weisbach equation calculates the pressure drop due to friction as fluid flows through a pipe. It equals the friction factor times the length-over-diameter ratio times the dynamic pressure (half the density times velocity squared). It is the most widely used and accurate equation for pipe friction losses in engineering.
How does pipe diameter affect pressure drop?
Smaller diameters dramatically increase pressure drop. Diameter appears in the denominator of the length-to-diameter ratio, and a smaller pipe also forces higher velocity for the same flow rate, which raises the loss with the square of velocity. So even a modest reduction in diameter can greatly increase the pressure needed to maintain flow.
What is the friction factor?
The Darcy friction factor represents how much friction the pipe imposes on the flow. It depends on the Reynolds number (flow regime) and the pipe's relative roughness. For laminar flow it is 64 divided by the Reynolds number; for turbulent flow it comes from the Colebrook equation or the Moody chart. This calculator uses the friction factor you provide.
Why does velocity have such a strong effect?
Pressure drop depends on the dynamic pressure, which is proportional to velocity squared. This means doubling the flow velocity quadruples the friction loss. High velocities cause disproportionately large pressure drops, which is why oversizing pipes to keep velocity moderate is a common way to reduce pumping energy and pressure requirements.
How is pressure drop used in design?
Engineers use pressure drop to size pumps, ensure adequate flow and pressure at the point of use, and choose pipe diameters that balance material cost against pumping energy. The total pressure the pump must provide includes friction losses along all pipes plus fittings and elevation changes, so calculating pressure drop is central to piping system design.