Home›Calculators›Construction›CFM Calculator

CFM Calculator

Construction

Calculate airflow in cubic feet per minute (CFM) from a duct's cross-sectional dimensions and air velocity. Free tool for HVAC technicians and builders.

Reviewed by the thecalcu.com team · Last updated July 9, 2026

260
260
1003,000

Airflow (CFM)

533.33
Duct Cross-Sectional Area
0.667

This calculator computes your Airflow (CFM), Duct Cross-Sectional Area from the values you enter.

Inputs
Duct WidthDuct HeightAir Velocity
Outputs
Airflow (CFM)Duct Cross-Sectional Area

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, using a measured velocity from an anemometer, and for early design work, where you're testing whether a candidate duct size can deliver a required CFM at a reasonable velocity.

Why Use a CFM Calculator?

Sizing a new duct run, verifying an existing system's delivered airflow, troubleshooting a room that isn't getting enough conditioned air, CFM is the number that ties duct geometry to actual performance. Calculating it directly from measurable inputs, duct size and velocity, avoids guesswork.

This calculator is especially useful for comparing duct size options during design: for the same target velocity, a wider or taller duct delivers proportionally more CFM, so you can quickly check whether upsizing a duct run gets you to a required airflow target without needing a larger, noisier, or more expensive fan.

Who Should Use This Calculator?

  • HVAC technicians verifying delivered CFM against a system's design specification using field-measured air velocity.
  • HVAC designers and estimators testing duct size options during early system design to hit required CFM targets.
  • Homeowners and DIYers troubleshooting a room with weak airflow, checking whether the duct feeding it is undersized for the intended CFM.
  • Building inspectors cross-checking installed ductwork against ventilation code requirements for bathrooms, kitchens, or mechanical ventilation systems.

What Insights Does the CFM Calculator Give You?

  • Airflow (CFM), the primary result, showing the volume of air moving through the duct per minute at the given velocity.
  • Duct Cross-Sectional Area, the calculated area in square feet, useful for cross-referencing against duct sizing charts or comparing multiple duct size options.

Together, these outputs let you verify whether a duct's size and air velocity combination meets a required airflow target, or work backward to test different duct sizes.

How to use this CFM calculator

  1. Enter the Duct Width in inches for the rectangular duct you're evaluating.
  2. Enter the Duct Height in inches.
  3. Enter the Air Velocity in feet per minute, either measured with an anemometer or selected as a design target for your application.
  4. Read the Airflow (CFM) result, this is the volume of air the duct delivers per minute at that velocity.
  5. Check the Duct Cross-Sectional Area figure if you need it for further sizing comparisons.
Show formula & methodology ↓Show less ↑

Formula & Methodology

Duct cross-sectional area:
A = (W × H) ÷ 144

Airflow:
CFM = A × 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 × 8 in duct with an air velocity of 800 fpm:

- Duct area: (12 × 8) ÷ 144 = 96 ÷ 144 = 0.667 sq ft
- Airflow: 0.667 × 800 = 533.33 CFM

Frequently Asked Questions

How do I calculate CFM from duct size and air velocity?
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, enter your duct width, height, and measured or design air velocity.
What is CFM and why does it matter in HVAC?
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, undersized ductwork or fans starve a room of airflow, while oversized systems waste energy and can create noise or comfort issues.
How do I measure air velocity in a duct?
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, supply ducts commonly run 700-900 fpm in residential work, with return ducts often slightly lower.
What's a typical air velocity for residential ductwork?
Residential supply ductwork commonly targets 700-900 fpm for main trunk lines and slightly lower velocities for branch runs, balancing airflow capacity against noise, 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.
How does duct size affect CFM for the same air velocity?
CFM scales directly with duct cross-sectional area for a fixed air velocity, 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.
Can I use this calculator for round ducts?
This calculator is set up for rectangular duct dimensions (width × height). For round ducts, first calculate the cross-sectional area using π × radius², convert that area to square feet, and then multiply by your air velocity in fpm to get CFM, the underlying airflow formula (area × velocity) is the same regardless of duct shape.
What CFM do I need for a bathroom exhaust fan?
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.
How does duct velocity relate to noise and pressure drop?
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, balancing capacity, noise, and energy efficiency.
Why is duct area divided by 144 in this calculation?
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.
What's the difference between CFM and static pressure?
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, 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.
Can this calculator help size an HVAC duct run?
Yes, by rearranging the relationship, 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.
What other calculators help with HVAC and ventilation sizing?
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.
Also known as
duct CFM calculatorairflow calculatorcubic feet per minute calculatorHVAC CFM calculatorduct airflow velocity calculator