CFM Calculator
Use this tool to calculate airflow in cubic feet per minute (CFM) using the method that matches your project.
- Duct Method: CFM = Duct Area (ft²) × Air Velocity (ft/min)
- Room + ACH Method: CFM = (Room Volume × Air Changes per Hour) ÷ 60
- Volume/Time Method: CFM = Volume (ft³) ÷ Time (min)
Tip: This method is commonly used for HVAC duct design and fan sizing.
Good for whole-room ventilation requirements in homes, offices, and workshops.
Use this when you know the exact volume and how quickly it must be exchanged.
What is cubic feet per minute (CFM)?
Cubic feet per minute (CFM) is a standard unit used to describe airflow volume. It tells you how many cubic feet of air move through a fan, duct, vent, or space each minute. If you are sizing an exhaust fan, checking HVAC duct performance, or designing a ventilation system, CFM is one of the core numbers you need.
In practical terms, higher CFM means more air movement. But “more” is not always better. You want airflow that is matched to the room, equipment, noise goals, static pressure, and energy use.
When to use each CFM formula
1) Duct Size + Velocity
Use this method when you know the duct dimensions and average air velocity. This is very common during HVAC design and balancing.
- Convert duct dimensions to feet (if entered in inches).
- Calculate area: width × height.
- Multiply by velocity in feet per minute.
Formula: CFM = Area (ft²) × Velocity (ft/min)
2) Room Volume + ACH
Use this when you are targeting a specific air change rate. ACH means how many times the entire room air volume is replaced each hour.
- Find room volume: length × width × height.
- Multiply by desired ACH.
- Divide by 60 to convert from hourly flow to per-minute flow.
Formula: CFM = (Volume × ACH) ÷ 60
3) Known Volume + Time
Use this approach when you know the amount of air you must move and the allowed time window. This is useful for purge ventilation or process control.
Formula: CFM = Volume ÷ Time
Worked examples
Example A: Duct airflow
A rectangular duct is 24 in × 12 in with velocity of 900 ft/min. Convert dimensions to feet: 24 in = 2 ft, 12 in = 1 ft. Area = 2 × 1 = 2 ft². CFM = 2 × 900 = 1,800 CFM.
Example B: Room ventilation by ACH
A room is 20 ft × 15 ft × 9 ft, and target ACH is 6. Volume = 20 × 15 × 9 = 2,700 ft³. Required CFM = (2,700 × 6) ÷ 60 = 270 CFM.
Example C: Volume over time
You need to clear 1,500 ft³ of air in 5 minutes. CFM = 1,500 ÷ 5 = 300 CFM.
Common mistakes to avoid
- Mixing inches and feet without converting units.
- Using air velocity from one point as if it represents full-duct average velocity.
- Ignoring static pressure and assuming fan free-air CFM is real-world CFM.
- Selecting extremely high CFM without checking noise, drafts, and power consumption.
Quick selection tips
- For bathrooms and kitchens, local code and fixture requirements often define minimum airflow.
- For workshops, include contaminant source strength in addition to ACH targets.
- For grow rooms and server spaces, include heat load—not just room volume.
- Always verify the fan curve at your expected static pressure.
FAQ
Is CFM the same as ACH?
No. CFM is airflow rate; ACH is how many room air replacements happen each hour. They are related, but not identical.
Does a higher CFM fan always improve air quality?
Not always. Better filtration, source control, and distribution can matter just as much as raw airflow.
Can this calculator be used for both supply and exhaust?
Yes. The math is the same. Just make sure your input data (velocity, volume, time, or ACH target) reflects your actual design condition.
Use the calculator above to estimate your airflow requirement quickly, then validate with equipment specs and installation conditions for final selection.