Choose what to solve for
Enter flow rate and inside diameter to calculate total friction pressure drop.
Pipe
Fluid & Flow
Fittings
Calculate
How to use the pipe pressure drop calculator
Choose ΔP, Q, or D at the top. Flow and diameter are solved by repeatedly evaluating the same Darcy–Weisbach and fitting-loss model. Each solution is back-checked against the entered pressure drop. Diameter is the actual inside diameter; fitting geometry ratios stay fixed as diameter changes. A positive available friction pressure drop is required for Q and D. Negative path-specific fitting K values are supported in ΔP mode only because they can make the inverse problem non-unique.
This calculator estimates frictional pressure loss through one constant-flow, constant-inside-diameter pipe section. Straight-pipe loss is calculated with the Darcy-Weisbach equation, while fittings and valves are added through loss coefficients. Use the actual inside diameter of the pipe, the fluid properties at the expected operating temperature, the design flow rate, and the fittings that are physically in that section.
The total pressure drop shown above is the sum of straight-pipe friction and the selected fitting losses. It is a frictional loss, not the complete pressure requirement for an entire process. If the line changes elevation, discharges into a vessel at a different pressure, passes through equipment with its own pressure drop, or includes a pump or turbine, those effects must be handled separately in the system energy balance.
When to use it
- Single-phase liquid flow through a full circular pipe.
- A pipe section with one flow rate and one inside diameter.
- Preliminary sizing, line-loss checks, pump-system calculations, and comparison of piping alternatives.
- Systems where density and viscosity can reasonably be treated as constant through the section.
For a branched network, calculate each constant-flow section separately. A tee or wye can be included in the section containing the junction, but the downstream branches should use their own branch flow rates and diameters.
Core assumptions
- The fluid is single phase and behaves approximately as a Newtonian fluid.
- The pipe is full, circular, and represented by one actual inside diameter.
- Density, viscosity, and roughness are constant over the modeled section.
- Fitting coefficients represent the selected geometry and operating condition closely enough for engineering estimation.
- Elevation head, vessel pressure differences, heat transfer, pumps, control-system behavior, and equipment pressure drops are outside this calculation unless entered separately as a loss coefficient.
Common mistakes
- Using nominal pipe size instead of actual inside diameter. Pressure drop is very sensitive to diameter because diameter changes both velocity and the L/D term.
- Mixing Darcy and Fanning friction factors. This calculator uses the Darcy friction factor. The Fanning factor is one quarter of the Darcy value.
- Using the wrong viscosity. Enter dynamic viscosity here, not kinematic viscosity. Fluid viscosity can change substantially with temperature.
- Using roughness with the wrong units. Absolute roughness and diameter are converted internally, but the roughness value must correspond to the pipe material and condition you actually intend to model.
- Double-counting fittings. Do not add an equivalent length for a fitting and also add its K value unless you intentionally want both losses.
- Using the wrong velocity basis for a junction K. Tee and wye coefficients are path-specific. This calculator uses the combined-leg reference basis described in the fitting input.
- Treating a partially open valve like a fully open valve. Use vendor data or a custom K when the valve position or trim differs materially from the built-in case.
How to interpret the result
Pressure loss increases rapidly with velocity. In turbulent flow, doubling flow through the same pipe often produces roughly four times the frictional loss, although the exact change also depends on the friction factor. If fittings dominate the result, inspect the fitting breakdown. If straight pipe dominates, diameter, length, flow rate, and roughness are usually the main levers.
A negative path-specific tee or wye K can occur with some junction correlations because K is defined from pressure differences and a selected reference velocity. It does not mean the fitting creates energy.
Related tools and reference
Use the Reynolds Number Calculator to check flow regime, the Darcy Friction Factor Calculator to inspect f directly, and the K Factor ↔ Equivalent Length Calculator when converting loss methods.
Reference background: the U.S. Department of Energy fluid-flow handbook discusses head loss, Darcy friction, Reynolds number, and equivalent-length methods. DOE fluid-flow handbook.