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CFM Calculator.

Calculate airflow from room volume and air changes per hour.

On-device calculationNo signup
01

Set your values

Results update as you type.

Volume: 2,400

Volume

2,400.000
airChangesPerHour: 6

airChangesPerHour

6.000
CFM: 240

CFM

240.000

Use Cases

HVAC system sizing

Determine the minimum airflow needed to achieve a target ventilation rate for a room, helping select appropriate fans or HVAC equipment.

Example: A 20 ft × 15 ft × 10 ft room with 6 ACH needs 300 CFM.

Ventilation planning for indoor spaces

Ensure adequate fresh air supply for offices, classrooms, or residential rooms by calculating required CFM based on occupancy and usage.

Example: A 30 ft × 20 ft × 12 ft warehouse with 4 ACH requires 480 CFM.

Frequently Asked Questions

How do I calculate CFM for a room?
Enter the room length, width, and height in feet, and the desired air changes per hour (ACH). The calculator multiplies the room volume (L×W×H) by ACH and divides by 60 to give airflow in cubic feet per minute (CFM).
What is ACH and how do I choose it?
ACH stands for air changes per hour, the number of times the entire room volume is replaced with fresh air in one hour. Typical values: 4-6 for bedrooms, 6-8 for kitchens, 8-12 for bathrooms, and higher for industrial spaces. Consult HVAC guidelines for your specific use.
Can I use this calculator for any room shape?
This calculator assumes a rectangular room. For irregular shapes, divide the space into rectangular sections, calculate CFM for each, and sum the results. The formula remains the same: volume × ACH / 60.

Tips & Common Mistakes

Tips

  • Measure the room dimensions accurately in feet. Use the longest and widest points for irregular rooms, but remember this gives an overestimate.
  • Choose ACH based on the room's purpose: higher for kitchens and bathrooms, lower for living areas. Check local building codes for minimums.
  • For rooms with high ceilings, consider the occupied zone height (e.g., 8-10 ft) instead of full ceiling height for ventilation calculations.
  • This calculator gives the theoretical airflow. In practice, add 10-20% to account for duct losses and filter resistance.

Common Mistakes to Avoid

  • Using inches or meters instead of feet for dimensions, leading to incorrect volume and CFM results.
  • Confusing ACH with air changes per minute. Remember to divide by 60 to convert from per hour to per minute.
  • Forgetting to account for multiple rooms or open spaces – calculate each room separately and sum the CFM for total system requirement.

Last updated: August 13, 2026