Free HVAC Tool
HVAC Duct Size Calculator
Enter room square footage (or CFM) to get the recommended round duct diameter, air velocity in FPM, and a noise rating. Supports supply branches, return ducts, and main trunks — with flex duct adjustment.
See HVAC Contractor SoftwareCalculate the Right HVAC Duct Size
Enter room size or a known CFM requirement to get an instant round duct diameter recommendation with velocity and noise check.
Required CFM
200
Min. duct size
8" round
Air velocity
573 FPM
Velocity rating
Good
Round duct options
| Diameter | Max CFM | Velocity at 200 CFM | Noise rating |
|---|---|---|---|
| 8" round Recommended | 240 | 573 FPM | Good |
| 9" round | 320 | 453 FPM | Good |
| 10" round | 410 | 367 FPM | Low |
This is a simplified estimate. Proper duct sizing requires an ACCA Manual J load calculation and Manual D duct design by a licensed HVAC contractor or engineer. Duct length, fittings, static pressure, and system balancing all affect actual sizing requirements.
How HVAC Duct Sizing Works
Start with the airflow requirement (CFM)
Every room or zone needs a specific amount of conditioned air. A rough residential rule of thumb is 1 CFM per square foot of floor area for cooling in a moderate climate, adjusted for ceiling height and climate severity. The more rigorous approach is a Manual J load calculation that accounts for insulation, window area, orientation, and local climate.
Select a duct size that handles the CFM at good velocity
Each duct size has a range of CFM it can carry while keeping air velocity in the comfortable zone (400–700 FPM for supply branches). Too small → high velocity, noisy airflow, and higher static pressure drop. Too large → low velocity, poor throw at registers, and potential stratification.
Adjust for flex duct friction
Flexible duct has approximately 25% more friction than smooth rigid metal duct at the same airflow. If you're using flex duct, size up one diameter compared to the rigid duct equivalent. Also keep flex duct stretched tight — kinked or compressed flex duct dramatically increases friction loss.
Verify with a full Manual D calculation
This calculator gives a starting point, not a final design. A complete ACCA Manual D calculation accounts for total duct length, number of elbows and fittings, system total external static pressure, and balancing across all branches. Always have a licensed HVAC contractor perform a Manual D design for new installations.
Common HVAC Duct Sizing Mistakes
Undersized main trunk
The main supply trunk must carry the total system airflow before it branches out. An undersized trunk causes high static pressure, forcing the blower to work harder, reducing efficiency, and shortening equipment life.
Using flex duct with same size as rigid
Flex duct requires a one-size-larger diameter compared to rigid metal for the same airflow. Using identical sizes for both materials results in an undersized flex run with noisy, high-velocity airflow.
Too many registers on one branch
Splitting one branch run to serve too many registers divides airflow too thin, leaving far rooms cold in summer. Each branch should be sized for its own load, not overloaded with additional tees downstream.
Ignoring return air capacity
An undersized return duct creates negative pressure in the living space, pulling unconditioned air through gaps and reducing system efficiency. Returns should be sized generously — at least equal in total area to the supply system.
Sharp bends in flex duct
A 90-degree bend in flexible duct can reduce effective diameter by 40% and increase friction by 50% or more. Route flex duct in gentle curves whenever possible. Sharp bends are one of the most common causes of poor airflow in residential retrofits.
Not accounting for equipment static pressure
Every air handler has a maximum external static pressure (ESP) it can operate against. Undersized ductwork raises static pressure, forcing the system into an operating range where airflow drops and efficiency falls. Match duct system design to the equipment's rated ESP.
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HVAC Duct Sizing FAQ
How do I calculate duct size?
Calculate required CFM first (roughly 1 CFM per sq ft for moderate climates). Then select a round duct size where the air velocity falls in the comfortable range: 400–700 FPM for supply branches, 400–800 FPM for returns, 700–900 FPM for main trunks.
What size duct do I need for a 12×12 room?
A 144 sq ft room needs roughly 100–150 CFM. A 7-inch round duct (rated for 90–175 CFM) or a 6×8-inch rectangular equivalent is typical. In hot climates or rooms with large west-facing windows, size up to 8 inches.
What is CFM in HVAC?
CFM (Cubic Feet per Minute) is the volume of air a duct or system moves per minute. Residential systems typically move 350–450 CFM per ton of cooling. A 3-ton system moves about 1,200 CFM total, distributed across all supply registers.
What is the correct air velocity for HVAC ducts?
Supply branch ducts: 400–700 FPM. Return ducts: 400–800 FPM. Main supply trunks: 700–900 FPM. Above 900–1,000 FPM, ductwork generates noticeable noise. Below 400 FPM, air may not reach registers with adequate pressure.
What is Manual D duct sizing?
ACCA Manual D is the industry standard for residential duct system design. It calculates duct sizes based on friction rate, equivalent duct lengths, and airflow from a Manual J load calculation. Most codes now require Manual D for new construction.
How many CFM per ton of AC?
Residential systems are designed for 350–450 CFM per ton, with 400 CFM/ton being most common. A 3-ton system moves about 1,200 CFM. Under-ducted systems can't move enough air, causing the coil to freeze and reducing efficiency.
What is the difference between supply and return ducts?
Supply ducts carry conditioned air from the air handler to rooms. Return ducts carry room air back to be reconditioned. Returns are typically fewer, larger ducts; supplies are more numerous, smaller branches. A balanced system has approximately equal supply and return capacity.
Can I use flex duct instead of rigid?
Yes, but flex duct has 25% more friction than rigid metal. Size flex duct one diameter larger than the equivalent rigid duct recommendation. Keep flex duct stretched tight and avoid sharp bends — kinked flex duct dramatically reduces airflow.
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Book a Free DemoThis calculator provides simplified estimates. Proper duct sizing requires an ACCA Manual J and Manual D calculation by a licensed HVAC contractor.