HVAC CFM Calculator
Calculate the required airflow rate from air changes per hour and room size.
Unit System
Air Changes Per Hour
How many times per hour the full room air volume is replaced.
Room Size
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Fill in ACH and room size to see the result.
Disclaimer: This calculator provides estimates for informational purposes only. Results are based on simplified ACH formulas and may not reflect actual airflow requirements for your specific installation. Duct losses, system static pressure, equipment selection, and local building codes are not accounted for. Always consult a licensed HVAC engineer or contractor before making design or equipment decisions.
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FAQs
Frequently Asked Questions
How do you calculate required CFM for a room?
Most calculators use: CFM = (Room Volume × ACH) ÷ 60, where volume is L × W × H and ACH is air changes per hour.
What does CFM mean in HVAC?
CFM (cubic feet per minute) is the amount of air moved each minute through a system, vent, or fan.
What is ACH and why does it matter?
ACH (air changes per hour) estimates how many times per hour the air in a space is replaced. Higher ACH usually means more required airflow (CFM).
What ACH should I use?
It depends on the space and goal (comfort vs ventilation vs exhaust). Many people start with lower ACH for living/bedrooms and higher ACH for kitchens/baths/utility areas, then confirm with local code / design guidance.
Why does ceiling height affect my CFM result?
Because airflow need is based on volume. Higher ceilings increase cubic feet, so the calculator returns a higher CFM for the same ACH.
Is this calculator for ventilation CFM or HVAC supply CFM?
Typically it's best for ventilation/exhaust sizing (ACH-based). HVAC supply airflow for heating/cooling can be different and depends on load calculations, duct design, and equipment.
What's the difference between fresh-air ventilation and recirculated air?
Fresh air brings outdoor air in (or exhausts indoor air out). Recirculated air is air moved around inside the building (supply/return). Both are airflow, but the sizing targets differ.
Why does the calculator show a range instead of one number?
Real airflow needs vary by space use, leakage, filtration, duct losses, fan curves, and installation. A range is more realistic than a single "perfect" number.
How do I calculate ACH if I know my fan's CFM?
Use: ACH = (CFM × 60) ÷ Room Volume.
Can I use this for bathroom exhaust fan sizing?
Yes—bath fans are often chosen by required CFM (and sometimes by code minimums). Use the calculator as a starting estimate, then verify local requirements.
What is "400 CFM per ton" and when is it used?
It's a common rule-of-thumb for cooling airflow (supply CFM per ton of AC capacity). It's not a ventilation formula and can vary by climate, humidity goals, and equipment.
How do I convert between CFM and m³/h?
Approximate: m³/h = CFM × 1.699 and CFM = m³/h ÷ 1.699.
Why is my measured airflow lower than the calculated CFM?
Common causes include dirty filters/coils, undersized ducts, crushed flex duct, closed dampers/registers, high static pressure, or inadequate return air.
Does duct size matter for CFM?
Yes. Duct size affects air velocity and pressure drop. Even if your target CFM is correct, ducts that are too small (or too long/restricted) can prevent delivering it.
Is this calculator suitable for whole-house airflow?
It's best for room/zone estimates. Whole-building ventilation usually follows a different method based on floor area + bedrooms/occupants (and local standards).