CFM Calculator
Airflow, Duct Velocity & Air Changes
CFM from Room Size
Calculate required airflow based on room dimensions and air changes per hour.
CFM from Cooling Capacity
Calculate required airflow based on system capacity using 400 CFM per ton rule.
Duct Velocity Calculator
Calculate air velocity in ducts or find required duct size for a target velocity.
Air Changes Per Hour (ACH)
Calculate ACH from known CFM and room volume.
Formulas
CFM = (Volume × ACH) / 60
CFM from room size
CFM = Tons × 400
CFM from capacity (rule of thumb)
Velocity = CFM / Area
FPM from airflow and duct size
ACH = (CFM × 60) / Volume
Air changes per hour
Recommended ACH by Space
Bedroom
5-6 ACH
Living Room
6-8 ACH
Office
6-10 ACH
Kitchen
15-20 ACH
Bathroom
15-25 ACH
Restaurant
12-15 ACH
Gym
15-20 ACH
Hospital Room
6-12 ACH
Operating Room
20-25 ACH
Duct Velocity Guidelines
Supply Trunk: 700-900 FPM
Supply Branch: 500-700 FPM
Return Duct: 500-700 FPM
>1000 FPM = Noise issues