Calculation sequence
Step-by-step method
- Calculate local velocity pressure using entered density and velocity.
- Convert hydraulic diameter to the same length basis as duct length.
- Apply Darcy friction factor and length-to-diameter ratio.
MechanicalAdvancedDuct Systems
Duct Pressure-Loss Calculator
Calculate straight-duct pressure loss with Darcy–Weisbach from hydraulic diameter, length, velocity, air density, and friction factor.
Why it matters: A transparent pressure-loss calculation distinguishes geometry, velocity pressure, and friction factor.
Calculated result
Engineering evaluation
Straight-duct pressure loss (ΔP)0.14 in. w.g.
- Local velocity pressure (Pv)
- 0.14 in. w.g.
- Calculated friction rate (FR)
- 0.14 in. w.g./100 ft
- Length-to-diameter ratio (L/Dh)
- 50 ratio
Engineering interpretation
Calculated straight-duct loss is 0.14 in. w.g., equivalent to 0.14 in. w.g./100 ft.
Important limitations
- The entered friction factor must be the Darcy factor. Add fitting, equipment, leakage, and system-effect losses separately.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Pv = ρV²/2
L/Dh = length-to-diameter ratio
ΔP = f(L/Dh)Pv
FR = ΔP/L
Current substitution
Pv = 0.14 in. w.g.; ΔP = 0.02×50×0.14 = 0.14 in. w.g..
Equation legend
Inputs and calculated quantities
- L
- Straight duct length — ft or m
- Dh
- Hydraulic diameter — in. or mm
- V
- Average air velocity — fpm or m/s
- ρ
- Air density — lb/ft³ or kg/m³
- f
- Darcy friction factor — ratio
- ΔP
- Straight-duct pressure loss — in. w.g.
- Pv
- Local velocity pressure — in. w.g.
- FR
- Calculated friction rate — in. w.g./100 ft
- L/Dh
- Length-to-diameter ratio — ratio
Applications
How this calculation is used
Building design
Evaluate a straight-duct segment before assembling a system pressure budget.
Construction
Check the pressure consequence of an installed size or length change.
Commissioning
Compare measured pressure loss with a calculated straight-duct expectation.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe one stable operating condition and use consistent measurement boundaries.
- Fluid or air properties and geometric dimensions are representative of the evaluated system.
Limitations
- This is a preliminary evaluation and field-verification tool, not a final sizing or code-compliance determination.
- Manufacturer data, adopted codes, project criteria, and professional engineering review remain controlling.
Field use
Verification procedure
- Confirm the approved reference, measurement plane, and instrument calibration.
- Stabilize the system and record dimensions and operating data.
- Compare the calculated result with project documents and manufacturer data before disposition.
Common engineering mistakes
- Using outside dimensions rather than clear inside dimensions.
- Combining values from different loads, device positions, or measurement conditions.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsCurrent handbook chapters covering duct design, fluid flow, heat transfer, insulation, and measurement; verify the licensed edition during professional review.
- SMACNA Technical StandardsIndustry duct-design, construction, leakage, and air-system reference context; protected tables are not reproduced.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement