Sensible Heat & Airflow Calculator
Work with the core air-side equation Q = 1.08 × CFM × ΔT: enter two values to get the third, with metric conversions and typical target airflows.
Open calculatorLast updated: 2026-09-15
How the calculation works
- The 1.08 factor bundles air density (0.075 lb/ft³) and specific heat (0.24 BTU/lb·°F) with the 60 min/h conversion.
- Leave one field blank to solve for it: airflow needed, heat delivered, or the resulting ΔT.
- The tonnage equivalent connects the answer to equipment sizing.
Worked example
Interpreting the result
This equation is the air-side heartbeat of HVAC: given equipment capacity and airflow it fixes the supply-to-return temperature split. Low airflow (clogged filter, weak blower) shows up as high ΔT, frozen coils in cooling, and heat exchanger stress in heating. Designers use 350–450 CFM per ton for cooling.
Assumptions
- Standard air at sea level — high altitude needs a corrected factor (1.08 × density ratio).
- Sensible heat only; latent (moisture) heat needs a separate wet-coil calculation.
Limitations
- Wet-coil (cooling with dehumidification) conditions need the total heat formula including latent load.
- High-altitude installations must correct for air density.
Sensible cooling (BTU/h) by airflow and temperature difference
| Airflow | 15 °F ΔT | 18 °F ΔT | 20 °F ΔT | 25 °F ΔT |
|---|---|---|---|---|
| 400 CFM | 6,480 | 7,776 | 8,640 | 10,800 |
| 800 CFM | 12,960 | 15,552 | 17,280 | 21,600 |
| 1,200 CFM | 19,440 | 23,328 | 25,920 | 32,400 |
| 1,600 CFM | 25,920 | 31,104 | 34,560 | 43,200 |
Q = 1.08 × CFM × ΔT for standard air. A ton of cooling is 12,000 BTU/h.
Worked scenario: how much air does a 2-ton system need?
A 24,000 BTU/h load with a 20 °F temperature drop across the coil needs 1,111 CFM of sensible capacity. Going the other way, 800 CFM at the same ΔT carries 17,280 BTU/h, or 1.44 tons.
Limits of the equation
- It covers sensible heat only. Removing moisture (latent heat) is separate, so total cooling is higher.
- The 1.08 factor assumes standard air at sea level. At high altitude the air is thinner and the factor drops.
- Real systems are specified at a design ΔT, which depends on the equipment and the humidity of the climate.
Frequently asked questions
What CFM per ton should I have?
350 CFM/ton in humid climates (better dehumidification), 400 in moderate, up to 450 in dry climates. Outside that range, check airflow and charge.
Why is my ΔT too high?
Usually low airflow — dirty filter, failing blower, or blocked returns. In heating, an over-fired furnace shows the same symptom.