Room Airflow Calculator
About This Tool
This calculator determines the airflow (CFM) each room needs from your HVAC system. It compares load-based airflow against ASHRAE 62.2 ventilation minimums and recommends appropriately sized supply registers. Results feed into the Duct Size Calculator and Duct System Planner.
How to Use
- Add rooms with type, size, and BTU load
- Set system type and supply air temperature
- Review CFM, register sizes, and system total
How to Use
- Add rooms with type, size, and BTU load
- Set system type and supply air temperature
- Review CFM, register sizes, and system total
Methodology
Load-based airflow uses the sensible heat formula: CFM = BTU/h ÷ (1.08 × ΔT), where ΔT is the supply air temperature differential (20°F for cooling per ACCA, 55°F for heating). Ventilation airflow follows ASHRAE Standard 62.2: CFM = (floor area ÷ 100) + (occupants × 7.5). The design CFM for each room is the larger of load-based and ventilation-based values, ensuring both comfort and air quality. Register sizes are selected to keep face velocity below 500 FPM to minimize noise.
Understanding Your Results
The "Design CFM" column shows the airflow each room actually needs — it is the higher of load-based CFM and ventilation CFM. This ensures both thermal comfort and adequate fresh air. The recommended register size is the smallest standard register that can handle the room's CFM without excessive noise. If multiple registers are needed, the count is shown. Use these values when ordering supply grilles and when sizing branch ducts in the Duct System Planner.
Practical Examples
Example: A 288 sq ft living room with 3 occupants and a cooling load of 6,336 BTU/h (Zone 4A, average insulation). Load CFM = 6,336 ÷ (1.08 × 20) = 293 CFM Ventilation CFM = (288 ÷ 100) + (3 × 7.5) = 25.4 CFM Design CFM = 293 CFM (load-driven) Recommended register: 10×12 (290 CFM capacity) — 1 register needed. At 293 CFM in a 2,304 cu ft room (288 × 8), the ACH = (293 × 60) ÷ 2,304 = 7.6 air changes per hour.
Airflow Tips
Kitchens need more CFM than bedrooms of the same size because they generate significant internal heat from cooking appliances. If a room shows multiple registers needed, consider using two smaller registers on opposite walls instead of one large one — this improves air distribution and comfort. The total system CFM determines your blower size. Make sure the selected equipment can deliver the required CFM at the system's static pressure. In heating mode, the higher delta-T (55°F vs 20°F) means less CFM is needed for the same BTU capacity. Design your duct system for the cooling CFM since it is almost always higher.
Sources
All calculations are performed locally in your browser. No data is sent to any server.