CFM = Cubic Feet per Minute. Measures airflow through ducts. 1 CFM ≈ 1.7 m³/h. Residential AC designed at 400 CFM/ton cooling capacity.

Fresh air requirements?

ASHRAE 62.2: 0.35 ACH + 7.5 CFM per 100 sq ft. Bedrooms typically need 30-50 CFM fresh air.

Duct too big or small?

Too small → noisy, fan overload. Too big → low velocity, distant vents weak, poor distribution. Size for 0.08-0.15 in.wg/100ft friction loss.

Airflow Calculator (CFM) — Free HVAC Duct Sizing

🌬️ Airflow Parameters

Mode

Duct Diameter (in)

Velocity (FPM)

Presets:

Airflow (CFM)

📊 Details

Duct Area

Velocity

ACH (1000sqft x 8ft)

Cooling Estimate

FAQ

What is CFM?

CFM (Cubic Feet per Minute) is the standard unit for measuring airflow. CFM = duct area (ft²) × velocity (FPM). Residential HVAC systems typically deliver ~400 CFM per ton of cooling capacity.

How do I choose the right duct diameter?

Duct diameter depends on airflow and velocity. Residential systems target 500-700 FPM; commercial 800-1200 FPM. Too high = noisy; too low = insufficient airflow. Formula: diameter = 2 × √(CFM / (π × velocity/144)).

What is ACH (Air Changes per Hour)?

ACH measures how many times the room's air volume is replaced per hour. Standard homes need 4-6 ACH, kitchens 10-15 ACH, factories 6-20 ACH depending on process. ACH = CFM × 60 / room volume (ft³).

How does CFM relate to cooling capacity?

HVAC industry standard: 1 ton of cooling = 400 CFM. So 500 CFM ≈ 1.25 tons. Actual design must account for climate zone, window area, and insulation levels.

Airflow Calculator (CFM)