VENTILATION CFM
About this calculator
This ventilation CFM calculator gives the required mechanical ventilation rate for a residence per ASHRAE 62.2 (and the IECC residential ventilation standard that references it). The whole-house base rate is 0.03 CFM per ft² of conditioned floor area + 7.5 CFM per occupant, with occupancy assumed at bedrooms + 1. The calculator also lists the local exhaust CFM required for kitchens (intermittent vs continuous), bathrooms, and laundry per the same standard. ESTIMATE ONLY — actual installed flow must be measured at the grille with a flow hood and verified by a licensed HVAC contractor.
How to use this calculator
Enter the conditioned floor area in ft² (heated/cooled space only) and the number of bedrooms (ASHRAE 62.2 assumes occupancy = bedrooms + 1, so a 3-bedroom home assumes 4 occupants). Pick the kitchen ventilation strategy: intermittent means a 100 CFM range hood used during cooking; continuous means an always-on low-flow system at 5 ACH of the kitchen volume.
Enter the number of bathroom exhaust fans (ASHRAE 62.2 specifies 50 CFM intermittent or 20 CFM continuous each). The calculator returns the whole-house ventilation requirement plus a breakdown of local exhaust totals.
Worked example
For a 2,000 ft² home with 3 bedrooms, intermittent kitchen, 2 bathrooms:
Whole-house: 0.03 × 2,000 + 7.5 × 4 = 60 + 30 = 90 CFM. This needs to run 24/7 from a dedicated supply or balanced ventilation system (HRV/ERV preferred).
Kitchen exhaust: 100 CFM intermittent range hood. Bathroom exhaust: 2 × 50 = 100 CFM intermittent. Laundry: 50 CFM. Total local exhaust: 250 CFM during peak use.
For a 4,000 ft² 5-bedroom home: whole-house = 0.03 × 4,000 + 7.5 × 6 = 120 + 45 = 165 CFM continuous. Larger HRV needed (Panasonic Intelli-Balance 100 or similar at $400-$700).
For a 1,200 ft² 2-bedroom apartment: whole-house = 0.03 × 1,200 + 7.5 × 3 = 36 + 22.5 = 58.5 CFM. A bath fan running continuously can usually handle this in apartment-scale homes.
Common mistakes & waste factors
Skipping mechanical ventilation in tight homes. Modern post-2015 builds with blower-door ratings under 3 ACH50 have nearly zero natural infiltration — they MUST have mechanical ventilation or moisture/CO₂/VOCs build up rapidly.
Running the bath fan as the only whole-house ventilation. A 50 CFM bath fan running 24/7 burns ~50W of motor energy and brings in unconditioned outdoor air with no heat recovery. An HRV/ERV at $400-$1,000 saves significantly on heating/cooling load.
Mixing local exhaust and whole-house numbers. Local exhaust (kitchen, bath, laundry) is intermittent during use. Whole-house ventilation is continuous baseline. Both are required separately.
Forgetting kitchen makeup air. Range hoods over 400 CFM trigger the IRC requirement for makeup air — a balanced supply that compensates for the exhaust pull. Without it, large hoods backdraft combustion appliances.
Rules of thumb
ASHRAE 62.2: 0.03 × ft² + 7.5 × (bedrooms + 1) = whole-house CFM (continuous).
Kitchen: 100 CFM intermittent range hood OR continuous at 5 ACH of kitchen volume.
Bathroom: 50 CFM intermittent OR 20 CFM continuous each.
Laundry: 50 CFM exhaust (or vented dryer counts).
Tight homes (<3 ACH50 blower door): mechanical ventilation REQUIRED. Older leaky homes (>10 ACH50) often meet the rate from natural infiltration alone.
HRV/ERV recovers 60-85% of heat from exhaust air — pays back over 5-10 years vs straight bath-fan exhaust.
Kitchen range hoods over 400 CFM require dedicated makeup air per IRC.
Common questions
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