CFM Calculator

How CFM and ACH Work Together

CFM measures airflow rate, while ACH measures how often that airflow replaces the room's entire air volume. The formula connecting them is CFM = (Volume × ACH) / 60. The division by 60 converts hours to minutes. A 20×15-foot room with 8-foot ceilings has 2,400 cubic feet. At 6 ACH, it needs (2,400 × 6) / 60 = 240 CFM.

Why does ACH vary by room type? Bathrooms generate moisture and odors, requiring faster air replacement (6-8 ACH). Living rooms and bedrooms have lower pollutant loads and use 4-6 ACH for comfort. Kitchens need 8-10 ACH to clear cooking byproducts. Commercial spaces like restaurants or gyms demand 10-20 ACH due to high occupancy and activity.

ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) publishes ACH standards for various building types. Code minimum for habitable rooms is often 0.35 ACH for infiltration, but mechanical ventilation adds 2-4 ACH. Tighter modern homes with low infiltration rely entirely on mechanical ventilation to hit target ACH, making accurate CFM calculations critical for indoor air quality.

CFM for HVAC System Sizing

Central HVAC systems use a rule of thumb: 400 CFM per ton of cooling capacity. A 3-ton air conditioner moves 1,200 CFM. This matches the airflow needed to absorb heat from the evaporator coil without freezing it. Too little airflow causes ice buildup; too much reduces dehumidification and comfort.

Heating systems use the same airflow in most cases. Furnaces and heat pumps work best with consistent CFM year-round. However, high-efficiency furnaces with modulating burners sometimes adjust airflow in heating mode to 300-350 CFM per ton for better comfort in mild weather. Installer manuals specify the exact CFM range for each model.

Room-by-room CFM distribution matters as much as total system CFM. A 1,200 CFM system serving six rooms doesn't deliver 200 CFM to each unless the ductwork is sized and balanced properly. Bedrooms might get 100-150 CFM each, while the open living area receives 400-500 CFM. Use this calculator to determine each room's needs based on volume and desired ACH, then design duct sizes accordingly. Balancing dampers fine-tune airflow after installation to match the calculated CFM targets.

Ventilation CFM vs. Conditioning CFM

HVAC CFM splits into two categories: conditioning CFM and ventilation CFM. Conditioning CFM is the recirculated indoor air that passes through the air conditioner or furnace for temperature control. Ventilation CFM is fresh outdoor air brought in to dilute indoor pollutants. Both are essential but serve different roles.

Conditioning CFM typically ranges from 400-450 CFM per ton, totaling 1,200-1,500 CFM for a typical home with a 3-ton system. This air loops continuously through the ductwork, cycling 8-12 times per hour. Ventilation CFM is much smaller: ASHRAE 62.2 requires 30-60 CFM for most homes, depending on size and occupancy. This fresh air dilutes CO₂, VOCs, and other indoor pollutants.

Energy recovery ventilators (ERVs) and heat recovery ventilators (HRVs) provide ventilation CFM while minimizing energy loss. They exchange heat between incoming fresh air and outgoing stale air, preconditioning the outdoor air before it enters the living space. Without heat recovery, 60 CFM of unconditioned outdoor air can add 5-10% to heating and cooling costs. This calculator determines total room CFM; subtract existing conditioning CFM to find how much supplemental ventilation CFM is needed.