liminfoDuct Sizing Calculator
Free web tool: Duct Sizing Calculator
Typical: 0.08-0.10 for low velocity, 0.15-0.20 for medium
Results
Velocity Guidelines
About Duct Sizing Calculator
The Duct Sizing Calculator is a free, browser-based HVAC engineering tool that computes the required round duct diameter using the equal friction method. You enter the airflow in CFM, the duct run length in feet, the duct material (galvanized steel or flexible duct), and a target friction rate in inches of water gauge per 100 feet. The calculator iteratively solves the Darcy-Weisbach equation combined with the Colebrook-White friction factor formula to find the exact diameter that matches your target pressure drop, then rounds up to the nearest whole inch and displays the equivalent in millimeters.
This tool is used by HVAC engineers, mechanical contractors, building designers, and students preparing for engineering exams. The equal friction method keeps the pressure drop per unit length constant throughout the duct system, simplifying balancing and ensuring consistent airflow to each zone. The calculator uses air density of 0.075 lb/ft³ and dynamic viscosity of 1.21×10⁻⁵ lb/(ft·s) — standard conditions at sea level and 70°F — and applies material roughness values of 0.0003 ft for galvanized steel and 0.003 ft for flexible duct.
In addition to the recommended duct diameter, the results show air velocity in both FPM and ft/s, the computed friction rate, and the total pressure drop across the specified duct length. A velocity guideline panel highlights whether your calculated velocity falls within acceptable ranges: 700–1,200 FPM for main ducts, 600–900 FPM for branch ducts, and 500–750 FPM for supply registers. Equivalent rectangular duct dimensions at three common aspect ratios are also displayed for situations where a round duct cannot be installed.
Key Features
- Iterative Darcy-Weisbach solver with Colebrook-White friction factor for precise diameter results
- Equal friction method — enter target friction rate (in.wg/100ft) and get the matching duct diameter
- Supports galvanized steel (roughness 0.0003 ft) and flexible duct (roughness 0.003 ft)
- Displays round duct diameter in both inches (rounded up) and millimeters
- Shows air velocity in FPM and ft/s with color-coded guideline comparison (green/yellow/red)
- Calculates total pressure drop across the full duct length at the specified friction rate
- Provides equivalent rectangular duct dimensions at multiple aspect ratios
- 100% client-side processing — no data ever leaves your browser
Frequently Asked Questions
What is the equal friction method for duct sizing?
The equal friction method sizes each duct section to have the same pressure drop per unit length (typically 0.08–0.10 in.wg per 100 ft for low-velocity systems). This approach simplifies system balancing because all branches experience similar resistance, reducing the need for damper adjustments. The calculator finds the duct diameter that achieves your specified friction rate using the Darcy-Weisbach equation.
What friction rate should I use for HVAC duct design?
For low-velocity residential and light commercial systems, a target friction rate of 0.08–0.10 in.wg per 100 ft is standard. Medium-velocity commercial systems typically use 0.15–0.20 in.wg per 100 ft. Higher friction rates allow smaller ducts but increase fan energy consumption and noise. The calculator accepts any target value so you can optimize for your specific project requirements.
What is the difference between galvanized steel and flexible duct roughness?
Galvanized steel has a surface roughness of about 0.0003 ft, while flexible duct has a much higher roughness of 0.003 ft due to its corrugated interior. This ten-fold difference significantly affects the friction factor calculated by the Colebrook-White equation, resulting in larger required diameters for flexible duct at the same airflow and friction rate target.
Why does the calculator round the diameter up?
Standard round duct is manufactured in whole-inch increments. Rounding the calculated exact diameter up to the next whole inch ensures the actual duct can physically carry the required airflow without exceeding the target friction rate. A slightly larger duct than required is always preferable to undersizing, which would increase pressure drop and reduce airflow to the served zone.
What air velocity is acceptable in supply ducts?
ASHRAE guidelines recommend 700–1,200 FPM for main supply ducts, 600–900 FPM for branch ducts, and 500–750 FPM for final supply registers. Velocities above 1,200 FPM in main ducts typically generate objectionable noise. The calculator flags your computed velocity in green (acceptable), yellow (borderline), or red (too high) based on these thresholds.
How do I convert round duct diameter to rectangular duct dimensions?
The calculator provides three equivalent rectangular duct options at aspect ratios of approximately 1:1, 1.2:0.85, and 1.5:0.7. These are approximations based on equal hydraulic diameter. For precise equivalence, use the ASHRAE circular equivalent formula: D_e = 1.30 × (ab)^0.625 / (a+b)^0.25, where a and b are the rectangular duct dimensions.
What is the Colebrook-White equation and why is it used here?
The Colebrook-White equation is an implicit formula that calculates the Darcy friction factor f for turbulent flow in pipes and ducts: 1/√f = −2 log₁₀(ε/(3.7D) + 2.51/(Re√f)), where ε is surface roughness, D is diameter, and Re is the Reynolds number. It is the most accurate friction factor correlation for fully turbulent flow and is the basis for ASHRAE duct design charts. The calculator solves it iteratively using 20 Newton-Raphson steps.
Can this calculator be used for exhaust or return air ducts?
Yes. The same equal friction method and Darcy-Weisbach equations apply to exhaust and return air duct sizing. Simply enter the appropriate airflow (CFM) and select the same target friction rate as your supply system. For return air grilles and low-velocity return paths, a lower friction rate (0.05–0.08 in.wg/100 ft) is often more appropriate to minimize pressure drop and noise.