Duct Sizing Calculator — Correct Duct Diameter for HVAC Airflow
Calculate the correct duct size for your HVAC system based on required CFM airflow and velocity limits.
How to Use This Calculator
- Enter the required airflow in CFM for the duct run (see room CFM requirements above)
- Select the design velocity — use Low for bedrooms, Standard for most supply ducts
- Choose round or rectangular duct based on your installation constraints
- Click "Calculate Duct Size" to get the recommended diameter or dimensions
- Use the actual velocity result to confirm the selected size is within acceptable range
How the Calculation Works
area_sqft = cfm / velocity_fpm
area_sqin = area_sqft × 144
// For round duct:
diameter_in = sqrt(4 × area_sqin / π)
// Round up to next standard size: 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20
recommended_diameter = next_standard_size(diameter_in)
actual_area_sqin = π × (recommended_diameter / 2)²
actual_velocity = cfm / (actual_area_sqin / 144)
// For rectangular duct:
rect_side = round(sqrt(area_sqin))
rect_w = round_up_to_even(rect_side)
rect_h = ceil(area_sqin / rect_w) Duct sizing starts by dividing the required airflow (CFM) by the target air velocity (FPM) to get the required cross-sectional area in square feet. Multiplying by 144 converts to square inches. For round duct, the diameter is derived from the area using the circle area formula (A = π r²), then rounded up to the nearest standard duct size. Actual velocity is recalculated at the standard size. For rectangular duct, the required area is converted to a near-square aspect ratio, rounded to even-inch dimensions, which minimizes friction losses.
Variables:
- cfm: Required airflow in cubic feet per minute
- velocity_fpm: Target air velocity in feet per minute (600–1000 FPM typical)
- area_sqin: Minimum required duct cross-section in square inches
- diameter_in: Calculated round duct diameter before rounding to standard size
- recommended_diameter: Next standard duct size that meets or exceeds the requirement
- actual_velocity: Actual air velocity at the recommended standard duct size
Pro Tips
- Supply ducts should target 600–900 FPM velocity — too fast causes noise, too slow loses pressure
- Return ducts run at lower velocity (400–700 FPM) to reduce noise near living areas
- Flexible duct has approximately 20% more friction loss than rigid sheet metal — upsize by one diameter when using flex
- Always insulate ducts running through unconditioned spaces (attics, crawlspaces) to prevent condensation and heat loss
Recommended Products
Frequently Asked Questions
What CFM velocity should I use for supply ducts?
For residential supply ducts, 600–900 FPM is the standard range. Use 600 FPM for quiet areas like bedrooms, 800 FPM for standard main trunk lines, and up to 1,000 FPM only for commercial or high-velocity systems. Velocities above 900 FPM in residential ductwork create noticeable noise at the registers.
What is the difference between round and rectangular ductwork?
Round ductwork is more efficient — it has a lower surface area per unit of cross-section, which means less friction loss. Rectangular duct is used when clearance is limited (tight joist bays, low ceilings). For equal airflow, a rectangular duct will be slightly larger than a round duct due to higher friction. Aspect ratios above 4:1 should be avoided as efficiency drops significantly.
How do I calculate the CFM needed for each room?
A simplified method: multiply the room square footage by 1 CFM per sq ft for standard rooms, or use ACCA Manual D for precise calculations. Bedrooms typically need 50–100 CFM, living rooms 100–200 CFM. Your HVAC system total CFM is typically 400 CFM per ton of cooling capacity. Distribute CFM proportionally to room size and heat load.
Should I use flex duct or rigid sheet metal?
Rigid sheet metal is more efficient — flex duct has 20–30% higher friction loss, especially when kinked or compressed. Use rigid metal for main trunk lines and longer runs. Flex duct is acceptable for final 6-foot connections from a rigid branch to the register box. Never use flex duct for runs longer than 8 feet or with more than two bends.