Pressure Drop Pipe Calculator

The pressure drop along a straight pipe is computed using the Darcy–Weisbach relation:

ΔP_major = f · (L/D) · (ρV²/2)

Optional minor losses are added using a combined loss coefficient K:

ΔP_minor = K · (ρV²/2)

Total pressure drop and head loss:

ΔP = ΔP_major + ΔP_minor,   h_f = ΔP/(ρg)

Reynolds number and roughness ratio:

Re = ρVD/μ,   ε/D

Laminar flow uses f = 64/Re. Turbulent flow uses either Swamee–Jain (explicit) or Colebrook–White (iterative).

1. Choose an input mode: flow rate or average velocity.
2. Enter pipe length, inner diameter, and roughness with units.
3. Provide fluid density and dynamic viscosity in SI units.
4. Add a total K if fittings/valves are present.
5. Pick a friction method for turbulent flow, then calculate.
6. Use CSV/PDF export buttons to save the computed report.

1) What pressure drop represents

Pressure drop is the energy the fluid loses as it moves through a pipe. In design work, it sets the pump or fan duty, affects control valve authority, and can limit flow in long lines. Because losses scale strongly with velocity, small changes in diameter or flow can produce large changes in required driving pressure.

2) Major losses and the Darcy-Weisbach term

The dominant component in straight runs is the Darcy-Weisbach major loss, ΔP = f(L/D)(ρV²/2). Length increases losses linearly, while diameter enters both through L/D and velocity (V = 4Q/πD²), so undersized pipes can multiply losses rapidly. This calculator reports both ΔP and head loss h_f = ΔP/(ρg).

3) Flow regime data using Reynolds number

Reynolds number Re = ρVD/μ classifies the regime. Laminar flow is typically Re < 2300 and uses f = 64/Re. Transitional behavior often appears from about 2300 to 4000, where results can be sensitive. Fully turbulent conditions are common above 4000, and roughness begins to matter as Re grows.

4) Roughness values you can start with

Absolute roughness ε depends on material and condition. Typical starting points are PVC or smooth tubing around 0.0015 mm, commercial steel around 0.045 mm, and new cast iron around 0.26 mm. Because the correlation uses ε/D, the same surface can behave very differently in small versus large diameters.

5) Minor losses and typical K ranges

Fittings, entrances, and valves are modeled with a total K so that ΔP_minor = K(ρV²/2). As a rough guide, a smooth 90-degree elbow may be K ≈ 0.3 to 1.5, a fully open gate valve K ≈ 0.1 to 0.3, and a globe valve K ≈ 6 to 10. Add individual K values to build the total.

6) Fluid properties and temperature sensitivity

Density ρ affects dynamic pressure (ρV²/2), while viscosity μ drives Reynolds number and can shift the friction factor. Liquids with higher viscosity can move a system toward laminar behavior, reducing sensitivity to roughness but increasing f at a given velocity. Use property data at operating temperature for best results.

7) Interpreting outputs for sizing and checks

Use the reported velocity to sanity-check operability. For many water-like liquids, practical line velocities often fall roughly in the 0.5 to 3 m/s range, while gas lines can be higher depending on noise and pressure constraints. The calculator also estimates pump power from ΔP and flow, adjusted by efficiency if provided.

1) Which equation does this calculator use?

It uses the Darcy-Weisbach relation for straight-pipe (major) losses and optionally adds minor losses using a total K. The friction factor is computed from laminar flow, Swamee-Jain, or Colebrook-White methods.

2) What is the difference between Swamee-Jain and Colebrook-White?

Swamee-Jain is an explicit approximation that is fast and accurate for most turbulent cases. Colebrook-White is an implicit relation solved iteratively; it is useful when you want a classic reference method and close agreement across regimes.

3) How do I choose a roughness value?

Start with material-based values (for example, smooth plastic much lower than steel), then adjust for age, scaling, or corrosion. If you have test data or vendor specifications, prefer those and use the same units as the input field.

4) How do I estimate the total K for fittings and valves?

List each component in the line, look up its K in a handbook or manufacturer data, and sum the values. Partially open valves and sudden area changes can dominate, so use conservative K values when uncertain.

5) Why does pressure drop rise sharply with flow?

Loss terms scale with the dynamic pressure, which is proportional to V². Because velocity increases with flow rate, doubling flow can cause far more than double the pressure drop, especially when the diameter is small.

6) Can I enter velocity instead of flow rate?

Yes. Select the velocity mode and enter an average velocity. The tool converts velocity to flow using the pipe area, then computes Reynolds number, friction factor, and the resulting pressure drop.

7) Does the result include elevation (static head)?

No. The calculator reports friction and fitting losses only. For a full system requirement, add static elevation change and any required outlet pressure to the computed frictional pressure drop.

Tip: Run each case in the calculator to fill in ΔP for your conditions.