Pressure Drop Calculator
Calculate pressure drop step by step: ΔP = (f × L ÷ D + ΣK) × ρv² ÷ 2 (Darcy–Weisbach).
In short
Formula: ΔP = (f × L ÷ D + ΣK) × ρv² ÷ 2 (Darcy–Weisbach). f = 64/Re if laminar; otherwise Swamee–Jain: f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]².
Pressure drop
43.0009 kPa
- Friction factor f
- 0.01915
- Reynolds number
- 74,850
- Pipe friction loss
- 43.0009 kPa
- Fitting losses
- 0 kPa
- Head loss
- 4.3937 m
- psi
- 6.2368
What this calculator does
Calculate pressure drop step by step: ΔP = (f × L ÷ D + ΣK) × ρv² ÷ 2 (Darcy–Weisbach). Worked example, questions and limitations included.
Use it to turn Pipe length, Inside diameter, Mean velocity, Fluid density, and the other shown inputs into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this fluid calculation.
Inputs and what they mean
- Pipe length
- — number value.
- Inside diameter
- — number value.
- Mean velocity
- — number value.
- Fluid density
- — number value.
- Dynamic viscosity
- — number value.
- Absolute roughness ε
- — From pipe material data.
- Sum of fitting loss coefficients ΣK (optional)
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
Swamee–Jain is an explicit approximation of Colebrook, valid roughly for 5,000 < Re < 10⁸.
Transitional flow (2,300–4,000) is uncertain; results there are approximate.
Roughness depends on pipe material and age; you enter it.
ΔP = (f × L ÷ D + ΣK) × ρv² ÷ 2 (Darcy–Weisbach). f = 64/Re if laminar; otherwise Swamee–Jain: f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]².
Inputs used: Pipe length, Inside diameter, Mean velocity, Fluid density, Dynamic viscosity, Absolute roughness ε, Sum of fitting loss coefficients ΣK (optional).
Worked example
100 m of 50 mm smooth pipe, water at 1.5 m/s
- Re = 74,850 → turbulent.
- Swamee–Jain gives f ≈ 0.0192.
- ΔP = 0.0192 × 100/0.05 × 998 × 1.5²/2 ≈ 43.1 kPa.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Incompressible, single-phase, fully developed flow.
- Transitional regime approximate.
- Elevation change not included (add ρgΔz separately).
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this pressure drop calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Common questions
What are fitting losses?
Elbows, valves and tees add losses expressed as K values from manufacturer or handbook tables. Enter their sum to include them.
Why is roughness required?
In turbulent flow, wall roughness affects friction strongly. New plastic is very smooth; old steel is much rougher.
Can I use it for gas?
Only if the pressure drop is small compared with absolute pressure (roughly under 10%), so density stays nearly constant. Check the formula, example, and limitations on this page before using the result for a real fluid decision.
How do I use the Pressure Drop Calculator?
Enter the required values for Pipe length, Inside diameter, Mean velocity, Fluid density, Dynamic viscosity, and the other fields shown. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Pressure Drop Calculator use?
ΔP = (f × L ÷ D + ΣK) × ρv² ÷ 2 (Darcy–Weisbach). f = 64/Re if laminar; otherwise Swamee–Jain: f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]². The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Pressure Drop Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.