Pressure Drop & Friction Loss Calculator
Estimate the pressure loss in a pipe using the Darcy–Weisbach equation. Enter the flow in GPM, the pipe inside diameter in inches and the run length in feet, and the calculator returns the pressure drop in psi, the friction loss per 100 ft and the head loss in feet. Switch to SI for m³/s and metres.
SI: m³/s, Imperial: GPM
SI: m, Imperial: inches
SI: m, Imperial: ft
SI: m, Imperial: inches
Select your preferred units
Friction factor calculation
Typical Pipe Roughness Values
SI Units (meters)
Imperial Units (inches)
Free download
Get the printable pipe flow chart
Schedule 40 pipe sizes, water flow in GPM and LPM at four design velocities, and recommended velocities by application. One PDF, made to pin above the bench.
How It Works
The Pressure Drop Calculator uses the Darcy–Weisbach equation to estimate pressure losses in pipes due to friction. This equation is fundamental in fluid mechanics and is widely used in engineering applications.
Darcy–Weisbach Equation
The pressure drop is calculated using:
ΔP = f × (L/D) × (ρV²/2)
Where:
- • ΔP = Pressure drop
- • f = Friction factor
- • L = Pipe length
- • D = Pipe diameter
- • ρ = Fluid density
- • V = Flow velocity
Friction Factor Methods
Two methods are available for calculating the friction factor:
- • Haaland: Explicit equation, good for rough pipes
- • Colebrook: Implicit equation, more accurate but requires iteration
Worked Example
Problem: Calculate the pressure drop for water flowing at 0.01 m³/s through a 50mm new steel pipe, 10 meters long.
Given: Q = 0.01 m³/s, D = 0.05 m, L = 10 m, ε = 0.000045 m, ρ = 998 kg/m³, μ = 0.001 Pa·s
1. Velocity: V = Q/A = 0.01 / (π/4 × 0.05²) = 5.09 m/s
2. Reynolds: Re = ρVD/μ = 998 × 5.09 × 0.05 / 0.001 = 254,000 (turbulent)
3. Friction factor (Haaland): f ≈ 0.02024
4. ΔP = f × (L/D) × (ρV²/2) = 0.02024 × 200 × 12,940 ≈ 52,400 Pa ≈ 52.4 kPa (7.6 PSI)
Hose Pressure Drop
Flexible hoses typically have higher pressure drops than rigid pipe due to corrugations, smaller IDs, and rougher interior surfaces. Common hose pressure drops at typical flow rates:
| Hose Type | ID | Flow Rate | ΔP per 100ft |
|---|---|---|---|
| Garden hose | 5/8" | 10 GPM | 5–10 PSI |
| Hydraulic hose | 1/2" | 8 GPM | 15–25 PSI |
| Fire hose | 1-3/4" | 150 GPM | 10–15 PSI |
| Air hose | 3/8" | 20 CFM | 3–8 PSI |
Values are approximate and vary by manufacturer. Use our calculator with the appropriate roughness value for precise results.
Friction Loss per 100 ft of Schedule 40 Steel Pipe
US friction loss charts quote the pressure lost to pipe-wall friction per 100 feet of straight pipe, so runs of any length can be compared. The table gives psi per 100 ft for water at 68°F in new Schedule 40 steel (roughness 0.0018 in), worked out with the same Darcy–Weisbach and Haaland equations as the calculator above. A dash means the velocity is outside 1–10 ft/s, the usual design range.
| GPM | 1/2" | 3/4" | 1" | 1-1/2" | 2" | 3" | 4" |
|---|---|---|---|---|---|---|---|
| 2 | 2.0 | 0.51 | – | – | – | – | – |
| 5 | 10.9 | 2.7 | 0.81 | – | – | – | – |
| 10 | – | 9.7 | 2.9 | 0.35 | – | – | – |
| 20 | – | – | 10.7 | 1.2 | 0.36 | – | – |
| 50 | – | – | – | 7.0 | 2.0 | 0.28 | 0.07 |
| 100 | – | – | – | – | 7.4 | 1.0 | 0.26 |
| 200 | – | – | – | – | – | 3.8 | 0.96 |
Column headings are nominal pipe sizes; the calculation uses the Schedule 40 inside diameter (for example 2.067 in for 2"). Multiply by (run length ÷ 100) for a longer or shorter run, and add fittings as equivalent length. Smoother pipe such as PVC or copper loses less.
Tips for Reducing Pressure Drop
- • Increase pipe diameter — the most effective way to reduce pressure drop (scales with D&sup5;)
- • Reduce pipe length — use the most direct routing possible
- • Use smoother materials — PVC has ~30x less roughness than old steel
- • Minimize fittings — each elbow, tee, and valve adds equivalent pipe length
- • Keep pipes clean — scale buildup increases roughness over time
FAQ
Here you will find the answers to the frequently asked questions about pressure drop calculations.
Frequently Asked Questions
What is the Darcy–Weisbach equation?
The Darcy–Weisbach equation is a fundamental equation in fluid mechanics that relates the pressure drop in a pipe to the friction factor, pipe length, diameter, fluid density, and velocity. It's widely used in engineering to design and analyze piping systems.
What is friction loss, and how is it different from pressure drop?
Friction loss is the part of the pressure drop caused by the fluid rubbing against the pipe wall. In a straight, level pipe of constant bore the two are the same thing. US charts usually quote it per 100 feet of pipe (psi per 100 ft or feet of head per 100 ft), which is why the calculator reports that figure alongside the total. Fittings, valves and elevation changes add to the total pressure drop on top of pipe friction.
When should I use Haaland vs. Colebrook method?
Use the Haaland method for quick estimates and rough pipes, as it's an explicit equation that doesn't require iteration. The Colebrook method is more accurate and should be used for precise calculations, especially for smooth pipes or when high accuracy is required.
How does pipe roughness affect pressure drop?
Higher pipe roughness increases the friction factor, leading to greater pressure drops. Smooth pipes like PVC have low roughness values (around 0.0000015 m), while rough pipes like concrete can have roughness values up to 0.003 m or higher.
What units should I use for the calculations?
The calculator supports Imperial (US) and SI units. In Imperial, enter the flow in GPM, the pipe inside diameter and roughness in inches, and the length in feet. In SI, enter the flow in m³/s and the diameter, length and roughness in metres. Results are always shown in Pa, kPa, bar and psi, plus friction loss per 100 ft and per 100 m.
How accurate is this pressure drop calculation?
The accuracy depends on the input parameters and the chosen method. The Colebrook method is generally more accurate than Haaland, especially for smooth pipes. For most engineering applications, both methods provide sufficient accuracy within 5-10% of actual values.
Embed This Calculator
Add this free calculator to your website. Copy the code below and paste it into your page.
Related Calculators
Continue your analysis with these related engineering tools
Head Loss Calculator
K-value method for fittings and valve head loss
Pipe Sizing Calculator
Determine optimal pipe diameter for your flow requirements
Velocity Calculator
Calculate fluid velocity and Reynolds number
PSI to Feet of Head
Convert pressure in psi to head in feet of liquid
Feet of Head to PSI
Convert head in feet to pressure in psi
Total Dynamic Head Calculator
Pump head from lift, pressure, friction and fittings
Professional hydraulic engineering calculators for engineers, technicians, and students.
Main Tools
Advanced Tools
- Reynolds Number Calculator
- Head Loss Calculator
- Pump Power Calculator
- Total Dynamic Head Calculator
- Tank Fill/Drain Calculator
- Orifice Plate Calculator
- Weir Flow Calculator
- Manning's Equation Calculator
- Valve Cv Calculator
- Gas Flow Calculator
- Mass Flow Rate Calculator
- Pump Affinity Laws Calculator
- Heat Exchanger Calculator
- Tankless Water Heater Calculator
- Economic Pipe Diameter Calculator
Engineering Guides
Company & Resources
Related Tools
- Electrical Calculators Motor, cable and voltage drop calculators
- Heating Planner UK heat loss, radiator and heat pump calculators
- Solar Sizing Calculator Solar, battery and off-grid sizing calculators
© 2026 Flow Rate Calculator. All rights reserved. A Nomogram Works site.