When to use: Use to determine the required supply airflow (CFM) for a zone based on its cooling or heating load. The formula CFM = BTU/hr ÷ (1.08 × ΔT) converts load to airflow, where ΔT is the temperature difference between supply air and room setpoint (typically 15–25°F). Also calculates the minimum outdoor air required per ASHRAE 62.1 based on occupancy and floor area.
This calculator converts cooling or heating loads (BTU/hr) to supply airflow in CFM using the psychrometric sensible heat formula, and separately calculates the minimum outdoor air required per ASHRAE 62.1. Mechanical engineers use it to size air terminals, VAV boxes, and air handling units for individual zones.
The fundamental sensible heat formula for air is Q = 1.08 × CFM × ΔT, where Q is the sensible load in BTU/hr, 1.08 is the volumetric heat capacity of air at standard conditions, and ΔT is the temperature differential between the room and supply air. Rearranged for airflow: CFM = Q / (1.08 × ΔT). A typical cooling supply air temperature is 55°F delivered to a 75°F room, giving ΔT = 20°F.
For heating, the same formula applies with a higher supply air temperature — hot water coils deliver 105–120°F supply air to a 70°F space. If heating and cooling use the same terminal unit, the system is sized for whichever mode requires more airflow, which is almost always cooling in commercial buildings.
The resulting CFM must then be checked against the ASHRAE 62.1 minimum outdoor air requirement. If the ventilation OA flow exceeds the load-based CFM, the system must supply the higher of the two values to maintain acceptable indoor air quality.
ASHRAE Standard 62.1 (Ventilation and Acceptable Indoor Air Quality) governs minimum outdoor air rates for commercial and institutional spaces. The Ventilation Rate Procedure requires outdoor air based on both occupancy (CFM/person, the Rp component) and floor area (CFM/ft², the Ra component). Table 6-1 lists rates by space type.
ASHRAE Standard 55 sets thermal comfort requirements that influence supply air temperature selection. ACCA Manual D governs duct sizing downstream of the airflow calculation. Local energy codes based on ASHRAE 90.1 limit supply air temperatures and require minimum airflows for VAV boxes.
Supply air temperature differential is a key design choice. A larger ΔT (25°F) requires less CFM for the same load, reducing duct sizes and fan energy — but delivers colder air that can cause drafts near diffusers. A smaller ΔT (15°F) requires more CFM but provides more uniform mixing. Standard practice for comfort cooling is ΔT = 18–20°F.
CFM per ton is a useful sanity check: 350–450 CFM/ton is typical for commercial comfort cooling. Values below 300 CFM/ton suggest the system is undersized for sensible cooling; values above 500 CFM/ton may indicate low sensible heat ratios requiring more latent capacity. Always verify that the OA fraction does not exceed the system's ability to condition the outdoor air without freezing or excessive humidity.
Enter the zone sensible cooling or heating load in BTU/hr from your load calculation software or a manual calculation. Enter the design temperature differential — 20°F is the default for cooling. Enter floor area and design occupancy to compute the ASHRAE 62.1 minimum outdoor air requirement.
Compare the load-based CFM to the OA-based CFM. If OA exceeds supply CFM, increase either the supply airflow or consider a dedicated outdoor air system (DOAS). The CFM per ton result confirms the system is within the industry-standard range of 350–450 CFM/ton.
1.08 is the volumetric specific heat of air at standard conditions (70°F, sea level): 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/hr = 1.08 BTU·min/(ft³·°F·hr). At high altitudes, air density decreases and the constant drops proportionally, requiring more CFM for the same load.
Commercial AHUs deliver supply air at 55°F from the cooling coil. After duct heat gain, the air may be 57–60°F at the diffuser. For sensitive applications, 52°F LAT is used. Underfloor air distribution systems deliver air at 62–65°F due to lower air velocity and longer mixing path.
In lightly loaded but densely occupied spaces — conference rooms, classrooms, reception areas — the ASHRAE 62.1 OA requirement can exceed the load-based CFM. This means the system must supply more airflow than the thermal load requires. A dedicated outdoor air system (DOAS) decouples ventilation from thermal conditioning in these cases.
At higher elevations, air is less dense. The 1.08 constant assumes sea-level density of 0.075 lb/ft³. At 5,000 ft, air density is about 0.062 lb/ft³, reducing the constant to approximately 0.89. This means you need about 20% more CFM to deliver the same BTU/hr, which also affects fan sizing.
350–450 CFM/ton is the standard range for commercial comfort cooling systems. Residential systems are typically designed at 400 CFM/ton. High sensible heat ratio applications (dry climates) may use 450–500 CFM/ton, while high-latency applications (pools, kitchens) may use 250–300 CFM/ton to maximize dehumidification.
Try our HVAC Studio
More calculators, simulators, and guides for this discipline.