CFM Calculator
ConstructionCalculate airflow in cubic feet per minute (CFM) from a duct's cross-sectional dimensions and air velocity. Free tool for HVAC technicians and builders.
Airflow (CFM)
533.33
Duct Cross-Sectional Area
0.667
What is a CFM?
A CFM calculator computes airflow in cubic feet per minute based on a duct's cross-sectional dimensions and the air velocity moving through it. CFM is the standard unit HVAC professionals use to specify and verify airflow for heating, cooling, and ventilation systems, and this calculator gives you a fast way to convert duct size and velocity into a usable CFM figure.
The underlying relationship is simple: airflow rate equals cross-sectional area times velocity. This calculator handles the unit conversion from duct dimensions in inches to area in square feet automatically, so you can plug in a duct's width and height directly along with a measured or design air velocity in feet per minute.
This tool is useful both for verifying existing ductwork โ using a measured velocity from an anemometer โ and for early design work, where you're testing whether a candidate duct size can deliver a required CFM at a reasonable velocity.
How to use this CFM calculator
- Enter the Duct Width in inches for the rectangular duct you're evaluating.
- Enter the Duct Height in inches.
- Enter the Air Velocity in feet per minute โ either measured with an anemometer or selected as a design target for your application.
- Read the Airflow (CFM) result โ this is the volume of air the duct delivers per minute at that velocity.
- Check the Duct Cross-Sectional Area figure if you need it for further sizing comparisons.
Formula & Methodology
Duct cross-sectional area: A = (W ร H) รท 144 Airflow: CFM = A ร V Where A is duct area (sq ft), W is duct width (in), H is duct height (in), and V is air velocity (fpm). Worked example: For a 12 in ร 8 in duct with an air velocity of 800 fpm: - Duct area: (12 ร 8) รท 144 = 96 รท 144 = 0.667 sq ft - Airflow: 0.667 ร 800 = 533.33 CFM
Frequently Asked Questions
Convert the duct's cross-sectional area from square inches to square feet by multiplying width times height and dividing by 144, then multiply that area by the air velocity in feet per minute to get CFM. This calculator does both steps automatically โ enter your duct width, height, and measured or design air velocity.
CFM stands for cubic feet per minute, the standard unit for measuring airflow rate in HVAC systems, ventilation design, and exhaust fan sizing. Correct CFM sizing ensures a space gets adequate heating, cooling, or ventilation air โ undersized ductwork or fans starve a room of airflow, while oversized systems waste energy and can create noise or comfort issues.
Air velocity is typically measured with an anemometer or velometer held at the duct opening or through a designated test port, expressed in feet per minute (fpm). For design purposes rather than field measurement, target velocity is often selected based on duct type and application โ supply ducts commonly run 700-900 fpm in residential work, with return ducts often slightly lower.
Residential supply ductwork commonly targets 700-900 fpm for main trunk lines and slightly lower velocities for branch runs, balancing airflow capacity against noise โ higher velocities move more air through a smaller duct but generate more noise and pressure drop. Commercial and industrial systems can run considerably higher depending on application and noise tolerance.
CFM scales directly with duct cross-sectional area for a fixed air velocity โ doubling the duct's cross-sectional area doubles the CFM it can move at the same velocity. This is why upsizing a duct is a common way to increase airflow capacity without needing a more powerful fan to push air faster through a smaller opening.
This calculator is set up for rectangular duct dimensions (width ร height). For round ducts, first calculate the cross-sectional area using ฯ ร radiusยฒ, convert that area to square feet, and then multiply by your air velocity in fpm to get CFM โ the underlying airflow formula (area ร velocity) is the same regardless of duct shape.
A common rule of thumb for bathroom exhaust fans is roughly 1 CFM per square foot of bathroom floor area, with a minimum of about 50 CFM for smaller bathrooms, though larger bathrooms or those with separate shower/tub areas may need more per local code. This calculator helps verify the CFM your duct run can actually deliver at a given velocity, which should meet or exceed your fan's rated output.
Higher air velocity through a duct increases both noise (from turbulence at duct walls, fittings, and grilles) and static pressure drop, which forces the fan or blower to work harder. This is why HVAC designers typically target a velocity range rather than maximizing airflow through the smallest possible duct โ balancing capacity, noise, and energy efficiency.
Duct width and height are usually measured in inches, so multiplying them gives area in square inches. Since airflow velocity is conventionally expressed in feet per minute, the area needs to be converted to square feet (dividing by 144, since there are 144 square inches in a square foot) before multiplying by velocity to get a correct CFM result.
CFM measures the volume of air moved per minute, while static pressure measures the resistance the air encounters moving through ducts, filters, and fittings, expressed in inches of water column. Both matter for proper HVAC system design โ a fan's rated CFM typically drops as static pressure increases, so real-world airflow depends on the full duct system, not just duct size and fan rating alone.
Yes, by rearranging the relationship โ if you know your required CFM (from a load calculation or fan spec) and a target velocity, you can test different duct width and height combinations in this calculator until the resulting CFM matches your requirement, which helps confirm a workable duct size before installation.
The [Air Conditioner BTU Calculator](/air-conditioner-btu-calculator/) and [AC Tonnage Calculator](/ac-tonnage-calculator/) help size cooling capacity for a space, while the [Gallons Per Minute Calculator](/gallons-per-minute-calculator/) covers analogous flow calculations for liquid systems like hydronic heating or plumbing.
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