Duct Pressure Loss Calculator
Example Data Table
| Case | Duct | Airflow | Length | Total K | Typical Use |
|---|---|---|---|---|---|
| 1 | 18 in round | 1200 cfm | 80 ft | 1.40 | Main branch estimate |
| 2 | 24 in × 12 in | 1500 cfm | 100 ft | 2.10 | Rectangular supply run |
| 3 | 400 mm round | 0.80 m³/s | 30 m | 1.80 | Metric design check |
Formula Used
Area for round duct: A = πD² / 4
Area for rectangular duct: A = width × height
Hydraulic diameter: Dh = 2ab / (a + b)
Velocity: V = Q / A
Velocity pressure: Pv = ρV² / 2
Reynolds number: Re = ρVDh / μ
Laminar friction factor: f = 64 / Re
Turbulent friction factor: f = 0.25 / [log10(e / 3.7Dh + 5.74 / Re^0.9)]²
Straight duct loss: ΔP = f × (L / Dh) × Pv
Fitting loss: ΔP = K × Pv
Total pressure loss: ΔPtotal = straight duct loss + fitting loss
How to Use This Calculator
Enter the airflow and select its unit. Choose round or rectangular duct. Add diameter, or enter width and height. Select the correct dimension unit. Enter straight duct length, roughness, air temperature, and elevation. Add elbow, damper, and extra fitting coefficients. Press Calculate to view results above the form. Use the CSV or PDF buttons to save the same calculation.
Duct Pressure Loss Planning
Duct Pressure Loss Planning
Duct pressure loss affects comfort, noise, fan power, and balancing work. A small error can move the selected fan away from its useful operating point. This calculator gives a structured way to review straight duct friction and fitting losses before final design.
Why Pressure Loss Matters
Air moving through a duct rubs against the wall. It also loses energy at elbows, dampers, entries, exits, transitions, and other fittings. The fan must overcome these losses while still delivering the required airflow. Low pressure loss usually reduces noise and power demand. High pressure loss can cause weak airflow and poor room control.
Engineering Approach
The tool uses hydraulic diameter, air velocity, density, Reynolds number, relative roughness, and a Darcy friction factor. Round and rectangular ducts are handled with different geometry. Rectangular ducts use hydraulic diameter. This makes the friction equation work with non-round sections. Fittings are added with total loss coefficients. This gives a practical ASHRAE-style estimate for early sizing and checking.
Input Quality
Good inputs matter. Use measured duct length when possible. Include straight duct, main runs, branch runs, and critical path sections. Choose realistic roughness for the duct material. Galvanized steel is smoother than flexible duct. Flexible duct often needs extra allowance. Add fitting coefficients from your design standard or project notes. When values are unknown, use conservative estimates and review the result later.
Reading the Result
The total pressure loss is shown in pascals and inches of water. The calculator also reports velocity, velocity pressure, Reynolds number, friction factor, and loss per length. High velocity may mean noise risk. Low velocity may mean larger ducts and more material. Compare several duct sizes until pressure, velocity, and available space all make sense.
Using Results Safely
This calculator supports planning, comparison, and education. It does not replace signed engineering documents. Real systems include leakage, branch interaction, dampers, coils, filters, terminals, and installation defects. Always check local codes, manufacturer data, and professional design standards before construction. Use the export buttons to keep a calculation record with each design revision.
For best results, test one change at a time. Save each case. Then compare fan pressure, duct size, and fitting choices without confusion. This supports clearer design decisions later.
FAQs
What does duct pressure loss mean?
It is the pressure drop caused by airflow through duct walls and fittings. The fan must overcome this resistance to move the required air volume.
Does this calculator support round ducts?
Yes. Select round duct and enter the diameter. The calculator uses the circular area and the diameter as hydraulic diameter.
Does this calculator support rectangular ducts?
Yes. Select rectangular duct and enter width and height. The calculator uses hydraulic diameter for the friction calculation.
What is fitting K value?
K value is a loss coefficient for fittings. Elbows, dampers, transitions, entries, and exits can each add pressure loss.
Why is air density included?
Air density changes with temperature and elevation. Density affects velocity pressure, Reynolds number, and total pressure loss.
What roughness should I enter?
Use the roughness for the duct material. Smooth metal ducts use lower values. Flexible or rough ducts need higher values.
Can I use this for final HVAC design?
Use it for planning and checking. Final design should follow project standards, codes, equipment data, and professional review.
Why are results shown in Pa and in. w.g.?
Both units are common in duct design. Pascals help metric work. Inches of water gauge help many HVAC fan selections.