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May 2, 2026

CFM Calculator

CFM / Airflow Calculator

Calculate airflow in CFM using room size and ACH, sensible heat load, or duct size and velocity. This version is rebuilt to be more WordPress-friendly and uses a safer PDF export through the browser print dialog.

Enter your values

Pick the method that matches your use case. This tool is made for normal users as well as HVAC learners and professionals.

Use the same unit for all room dimensions.
Common values are often 4 to 12 depending on the room.
Example: 15 ft or 4.5 m
Enter the full inside width.
Do not skip ceiling height. That mistake gives the wrong airflow result.
Use sensible load, not total load, for the standard airflow formula.
Choose the same unit as your load value.
Usually room air temperature minus supply air temperature.
The tool converts it before calculating CFM.
This method gives a practical sensible airflow estimate. It does not include latent load, humidity, altitude correction, or full psychrometric design.
Use the same unit as your duct size.
Use average duct air speed, not fan advertised airflow.
FPM is common in duct design.
Enter full diameter, not radius.
Inside duct width.
Inside duct height.
This method calculates airflow from duct cross-sectional area and average air velocity. Real delivered airflow can change because of fittings, fan curve, duct losses, and leakage.

Result

Your result appears here with clear conversions and useful details.

You changed some inputs after the last calculation. Please calculate again before exporting the PDF.

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Choose a method, enter valid values, and click Calculate CFM.

Required airflow

0 CFM

CFM 0
m³/h 0
L/s 0

Calculation details

    Formula used

    Proper ventilation is often an afterthought until windows start fogging up or kitchen smells linger for days. If your airflow is too low, pollutants and moisture stay trapped inside. If it is too high, you are wasting energy and creating unnecessary noise.

    The secret to a healthy indoor environment is finding the right CFM. This calculator helps you determine the exact amount of air movement your space requires to stay fresh, dry, and safe.

    What is CFM?

    CFM stands for Cubic Feet per Minute. It is a measurement of velocity that tells you how many cubic feet of air pass through a specific point in sixty seconds.

    In the HVAC and ventilation world, CFM measures how much air a fan or blower can move. A higher CFM means more air is being exchanged, which is vital for removing steam from bathrooms or smoke from kitchens.

    What This Calculator Helps You Calculate

    Our tool provides a precise airflow estimate based on your specific needs. It calculates:

    • Bathroom Ventilation: The power needed to prevent mold and mildew growth.
    • Kitchen Exhaust: The strength required to clear smoke, grease, and cooking odors.
    • Whole Room Air Exchange: How much air is needed to refresh the entire volume of a room periodically.
    • Duct Sizing Hints: Helps you understand if your existing vents can handle the required airflow.

    Who Should Use This Tool?

    Correct airflow is essential for both comfort and structural integrity:

    • Homeowners: When replacing an old, noisy bathroom fan or upgrading a range hood.
    • DIY Enthusiasts: Planning a workshop or a home gym that requires fresh air.
    • Contractors: To verify that a ventilation system meets local building codes.
    • Growers: To ensure indoor gardens have enough CO2 exchange and temperature control.

    Understanding the Input Fields

    To get an accurate airflow estimate, you need a few basic details:

    Room Volume

    Enter the length, width, and height of your room. Airflow is calculated based on the total volume of air in the space, not just the floor area.

    Air Changes Per Hour (ACH)

    Different rooms have different needs. A bathroom needs its air replaced about 8 times per hour to deal with steam, while a living room might only need it 4 times per hour. Our calculator provides presets for these standard rates.

    Application Type

    Are you venting a stove? If so, you need to consider the "Heat Output" (BTUs) of your burners. The calculator allows you to toggle between general ventilation and specific kitchen exhaust needs.

    How the CFM Formula Works

    The basic math for general ventilation is straightforward:$$CFM = \frac{Volume \times ACH}{60}$$

    This means you take the total cubic feet of the room, multiply it by the number of times you want the air replaced in an hour, and divide by 60 minutes. For example, a 1,000 cubic foot room needing 6 air changes per hour requires a 100 CFM fan.

    Step-by-Step: How to Use the Calculator

    1. Measure the Room: Input your room’s length, width, and ceiling height.
    2. Choose the Room Type: Select the environment (e.g., Bathroom, Kitchen, Basement).
    3. Adjust for Duct Length: If your vent pipe is very long or has many turns, increase your required CFM by 20% to account for air resistance.
    4. Review the Result: Compare the calculated CFM against the rating on the fan you plan to purchase.

    Common Mistakes to Avoid

    • Undersizing Bathroom Fans: Most building codes require at least 50 CFM for a small bathroom, but for a full bath with a shower, you should aim for at least 1 CFM per square foot.
    • Ignoring Sones: While CFM measures power, "Sones" measure noise. Don't buy a high-CFM fan that is so loud you never want to turn it on.
    • Using Small Vents: If you buy a 150 CFM fan but attach it to a 3-inch duct, the air will be "choked," and the fan will be loud and inefficient.

    Tips for Better Air Quality

    • Run Fans Longer: Leave your bathroom fan running for 15 minutes after your shower to ensure all moisture is removed.
    • Cross-Ventilation: Whenever possible, open a window or door slightly when running a high-power exhaust fan to prevent "backdrafting."
    • Clean the Filters: Grease buildup in kitchen hoods can reduce your actual CFM by over 50%.

    Why This Calculator is Useful

    Ventilation is about more than just comfort; it is about protecting your health and your home. Over-ventilating wastes expensive heated or cooled air, while under-ventilating leads to structural rot and poor air quality. This tool takes the guesswork out of the equation.

    FAQ Section

    How much CFM do I need for a bathroom?

    The standard rule is 1 CFM for every square foot of floor space. For a 70-square-foot bathroom, you would need a 70 CFM fan. If you have a jetted tub, you should add an extra 50 CFM.

    What size range hood do I need for a gas stove?

    For gas ranges, you should have 100 CFM for every 10,000 BTUs of total burner output. If your stove is 30,000 BTUs, you need at least a 300 CFM hood.

    Does a higher CFM mean a fan is better?

    Not necessarily. A fan should be matched to the room size. A fan that is too powerful for a small room will pull out your climate-controlled air too quickly, increasing your utility bills.

    How do I calculate CFM for a grow room?

    Calculate the room volume (L x W x H) and aim to exchange the entire air volume every 1 to 3 minutes.