Pipe Pressure Drop Calculator (Darcy-Weisbach)
Pressure drop through a straight pipe from the Darcy-Weisbach equation, with the friction factor from the iterated Colebrook equation for turbulent flow and the exact 64/Re law for laminar flow. Results include the flow velocity, Reynolds number, friction gradient per metre, head loss, and a warning when the flow sits in the transitional band where no single law applies.
How to use this calculator
- Enter the volume flow rate, the pipe inside diameter, and the straight length of pipe. Add the equivalent lengths of fittings separately and enter the total.
- Enter the absolute roughness of the pipe material, and the density and viscosity of the fluid at the working temperature.
- Enter any static height rise between the inlet and the outlet, or leave it at zero for a horizontal line.
- Read the total pressure drop, the flow regime, the Darcy friction factor, and the pressure drop per metre used to check the gradient along the run.
Frequently asked questions
What is the difference between the Colebrook and Swamee-Jain friction factors?
Colebrook is the implicit equation that matches the experimental data and has to be solved by iteration. Swamee-Jain is an explicit approximation that gets within about one percent of it over the normal range of Reynolds numbers and roughness ratios, which is why it is used as the starting guess for the iteration and then quoted alongside as a cross check. If the two disagree by more than a couple of percent, the iteration has not converged or the inputs are outside the range where either applies.
Why is the pressure drop so uncertain between Reynolds numbers of 2300 and 4000?
That band is genuinely transitional. Intermittent turbulent patches appear in otherwise laminar flow, and the friction factor depends on how the flow was started, on vibration and on the pipe entry, so it is not repeatable from run to run. It can jump by a factor of two. Design equipment to avoid sitting in this band rather than to predict it accurately.
Does the pressure drop depend on how rough the pipe is or on the ratio of roughness to diameter?
On the ratio. Roughness enters Colebrook as e/D, so the same 0.045 mm commercial steel roughness is negligible in a 500 mm main and significant in a 12 mm tube. This is also why the friction factor stops depending on Reynolds number at high speeds for a rough pipe: the roughness elements stick out through the laminar sublayer and dominate, giving the fully rough regime.
What flow velocity should I design a liquid line for?
Most process and hydraulic lines are sized for 1 to 3 m/s. Below about 0.3 m/s suspended solids settle and air does not separate out. Above about 3 m/s erosion, noise, water hammer and pressure drop all climb steeply, since the drop goes with the square of the velocity. Pump suction lines are deliberately run slower than discharge lines to protect the net positive suction head.