Enter Duct Data
Formula Used
The calculator uses the Darcy-Weisbach relation for duct friction. It also adds fitting losses through the selected minor loss coefficient.
ΔPmajor = f × (L / Dh) × (ρV² / 2)
ΔPminor = K × (ρV² / 2)
ΔPtotal = ΔPmajor + ΔPminor
For a round duct, Dh = D. For a rectangular duct, Dh = 2WH / (W + H). Velocity is V = Q / A. Reynolds number is Re = ρVDh / μ. Laminar friction factor is f = 64 / Re. Turbulent friction factor uses the Swamee-Jain equation.
How to Use This Calculator
- Select a round or rectangular duct.
- Enter airflow and choose the proper airflow unit.
- Enter inside duct dimensions. Use the same dimension unit.
- Add straight length, equivalent fitting length, and total K value.
- Choose roughness, temperature, pressure, and fan efficiency.
- Press the calculate button. Read the result above the form.
- Use the CSV button to save the calculated report.
Example Data Table
| Case | Shape | Airflow | Size | Length | Typical use |
|---|---|---|---|---|---|
| Branch run | Round | 250 CFM | 8 in diameter | 35 ft | Small room supply |
| Main supply | Rectangular | 1,200 CFM | 18 in × 10 in | 80 ft | Medium air handler |
| Quiet return | Rectangular | 900 CFM | 24 in × 12 in | 60 ft | Low velocity return |
| Long duct | Round | 700 CFM | 12 in diameter | 140 ft | Remote zone supply |
Why Duct Pressure Drop Matters
Duct pressure drop is the resistance that air meets while moving through a duct. It affects fan selection, energy use, room comfort, and system noise. A small error can create weak airflow at diffusers. A large error can force an oversized fan. Good estimates help designers balance cost, comfort, and efficiency.
Key Factors Behind the Result
Airflow is the first driver. More airflow raises velocity. Higher velocity raises dynamic pressure quickly. Duct size also matters. A small duct creates faster air and larger friction. Length adds more wall contact. Fittings add extra turbulence. Elbows, transitions, dampers, and grilles can increase total loss. Rough material also increases friction. A smooth metal duct usually loses less pressure than a rough flexible run.
Round and Rectangular Ducts
Round ducts use their inside diameter directly. Rectangular ducts need a hydraulic diameter. This value converts width and height into a comparable flow size. The calculator uses area for velocity. It uses hydraulic diameter for friction. This keeps the estimate useful for both shapes. Very flat rectangular ducts should be checked with care. Their flow can be uneven near corners.
Reading the Output
Total pressure drop is shown in pascals, inches of water, millimeters of water, and psi. These units help compare HVAC reports, fan curves, and field instruments. Friction rate is also shown. It explains the loss per length. Velocity helps check noise risk. Reynolds number describes the flow pattern. Fan air power estimates the minimum power transferred to air. Actual motor input will be higher because fans and drives are not perfect.
Using Results in Design
Use the result as a planning estimate. Compare it with the available static pressure of the fan. Add losses for filters, coils, diffusers, and return paths. Check every critical run, not only the shortest route. The worst run often controls fan pressure. After installation, measure static pressure and airflow. Field results may differ because joints leak, dampers move, and fittings are not always ideal.
Practical Tips
Keep velocity moderate when quiet operation matters. Avoid sudden area changes. Use long radius elbows when space allows. Seal joints to reduce leakage. Use realistic roughness and fitting data. Do not hide large loss values by rounding too early. A careful pressure drop estimate supports better comfort and lower operating cost.
Where the Method Fits
The method is best for steady air in common ventilation ducts. It assumes the duct is filled and the dimensions are known. It does not replace certified design software for complex networks. Still, it gives a transparent check. You can change one input and see the effect at once. This is useful during early sizing. It is also useful when checking shop drawings. For final work, confirm local codes, manufacturer data, balancing notes, and project specifications.
Always document assumptions, because each assumption affects the final selected fan pressure during later review stages.
FAQs
What is duct pressure drop?
It is the pressure loss created as air moves through duct walls, turns, transitions, dampers, and other restrictions. Higher loss means the fan must work harder to deliver the same airflow.
Which pressure unit should I use?
Pascals are common in engineering calculations. Inches of water gauge are common in HVAC field work. This calculator shows several units, so you can compare designs and fan data easily.
Does duct shape change pressure drop?
Yes. Shape affects area, hydraulic diameter, velocity, and wall contact. A rectangular duct with the same area as a round duct may still have a different friction result.
What is hydraulic diameter?
Hydraulic diameter is a flow size used for non-round ducts. For rectangular ducts, it equals two times width times height, divided by width plus height.
What does the K value mean?
K is a minor loss coefficient for fittings. It converts fitting turbulence into pressure loss using dynamic pressure. Add K values for elbows, entries, dampers, and transitions.
Should I use equivalent length or K values?
Use whichever data you have. Equivalent length adds fittings to the friction length. K values add direct minor loss. Avoid double counting the same fitting in both fields.
Why does velocity matter?
Velocity controls dynamic pressure and noise risk. Higher velocity can reduce duct size, but it often increases pressure loss, fan energy, vibration, and sound problems.
What roughness should galvanized duct use?
A typical starting value is about 0.15 mm. Actual roughness can change with lining, age, seams, flexible sections, dust, and installation quality.
Can this size a fan?
It helps estimate duct loss for fan selection. Final fan pressure should also include filters, coils, louvers, diffusers, return ducts, safety margins, and balancing needs.
Why is field pressure different?
Installed systems often include leakage, crushed flexible duct, closed dampers, extra fittings, dirty filters, and measurement errors. These factors can change real pressure drop.
Is the result valid for liquids?
No. This tool is built for air in ventilation ducts. Liquid pipe calculations need different fluid properties, fittings, roughness assumptions, and sometimes pump curve checks.