TradeCalc

Duct Size Calculator

Calculate the required duct size for your HVAC system based on airflow (CFM), duct shape, and maximum velocity per ASHRAE guidelines.

How to Size HVAC Ductwork

Duct Sizing Fundamentals

Proper duct sizing ensures that air moves through the HVAC system at the right velocity — fast enough to reach all rooms, but slow enough to avoid noise and excessive pressure drop. The three key variables are CFM (airflow volume), velocity (speed in FPM), and friction rate (pressure loss per 100 ft). The most common residential duct sizing method is the equal friction method, which targets about 0.08–0.10 inches WC per 100 ft.

The Equal Friction Method

Duct Area (sq ft) = CFM / Max Velocity (FPM)

Round Diameter = 2 × √(Area / π)

For rectangular ducts: Width = Area (sq in) / Height (in). Use the equivalent diameter formula to compare to round duct performance.

Maximum Velocity Limits (ASHRAE)

Main supply duct	→ 1,500 FPM max
Branch supply duct	→ 1,200 FPM max
Return air duct	→ 800 FPM max
Flexible duct	→ 800 FPM max

Worked Example

Scenario: A main supply duct needs to deliver 400 CFM. Target velocity: 1,500 FPM.

Required area = 400 / 1,500 = 0.267 sq ft = 38.4 sq in
Round duct diameter = 2 × √(38.4 / π) = 6.99 in → round up to 7" (or standard 8")
With an 8" round duct: actual area = π × 4² = 50.3 sq in → velocity = 400 / (50.3/144) = 1,145 FPM ✓
For a rectangular duct at 8" height: width = 38.4 / 8 = 4.8" → use 5" × 8"
Equivalent diameter of 5×8 = 1.3 × (40)^0.625 / (13)^0.25 = 6.1"

Practical Tips

When in doubt, go bigger. Oversized ducts are quieter; undersized ducts whistle and waste energy.
Flexible duct has higher friction than hard pipe — add 20–30% to the calculated size.
Use turning vanes in rectangular elbows and keep bends gradual to minimize pressure loss.
Never use a duct smaller than the outlet of the air handler or furnace.

Code References

ASHRAE Fundamentals Chapter 21, ACCA Manual D, SMACNA Duct Standards

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Frequently Asked Questions

How do I determine the CFM needed for a room?

The basic rule is: CFM = Room Area (sq ft) × Ceiling Height × ACH / 60. For cooling, typical ACH ranges from 4-8 depending on room type. Alternatively, divide the total system CFM proportionally by room load. A bedroom might need 80-150 CFM, while a living room might need 200-400 CFM.

What is the equal friction method for duct sizing?

The equal friction method sizes ducts so that the pressure drop per 100 feet of duct is approximately constant throughout the system (typically 0.08–0.10 in. WC/100ft). This ensures balanced airflow with minimal noise. It is the most commonly used method for residential and light commercial duct design.

What velocity is too high for HVAC ducts?

Per ASHRAE guidelines: main supply ducts should not exceed 1,500 FPM, branch ducts should not exceed 1,200 FPM, return air ducts should stay below 800 FPM, and flex duct should be below 800 FPM. Higher velocities cause excessive noise, increased static pressure, and energy waste.