Convert CFM to BTU/hr from airflow and the temperature difference (1.08 × CFM × ΔT, typical 20 °F included), or find the CFM needed for a BTU load.
Quick overview: Convert airflow in CFM to sensible heating or cooling capacity in BTU/hr using the standard HVAC relationship BTU/hr = 1.08 × CFM × ΔT.
- 500 CFM at 20°F ≈ 10,800 BTU/hr
- 400 CFM at 20°F ≈ 8,640 BTU/hr
- 1,000 CFM at 20°F ≈ 21,600 BTU/hr
- 500 CFM at 30°F ≈ 16,200 BTU/hr
The 1.08 factor is based on standard indoor air conditions and combines air density, specific heat, and minutes-to-hours conversion.
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This page converts airflow in cubic feet per minute (CFM) to a sensible heat rate in British thermal units per hour (BTU/hr). HVAC technicians and engineers use this relationship as one check in a broader system assessment; it does not independently size equipment or ductwork. The relationship between air volume and thermal energy depends on air density, specific heat, and the temperature differential across the air stream.
CFM to BTU Formula
The standard sensible heat equation for air at sea-level, near-room-temperature conditions is:
Where:
- CFM is the volumetric airflow rate
- ΔT is the temperature difference in °F
- BTU/hr is the sensible heating or cooling capacity
The inverse form is CFM = BTU/hr / (1.08 × ΔT).
Where the 1.08 Factor Comes From
The number 1.08 is not a universal constant. It is the product of three properties of dry air at standard conditions, typically sea level and near 70°F:
ρ is the air density, approximately 0.075 lb/ft³ at standard conditions. Cp is the specific heat of air, approximately 0.24 BTU per pound per degree Fahrenheit. The factor of 60 converts minutes to hours. When these conditions change, the 1.08 factor changes as well, which is why altitude and temperature corrections matter for accurate HVAC calculations.
CFM to BTU/hr Reference Table
The table below shows BTU/hr output for common CFM values across several temperature differentials, all using the standard 1.08 factor.
| CFM | ΔT 10°F | ΔT 15°F | ΔT 20°F | ΔT 25°F | ΔT 30°F | ΔT 40°F |
|---|---|---|---|---|---|---|
| 100 | 1,080 | 1,620 | 2,160 | 2,700 | 3,240 | 4,320 |
| 200 | 2,160 | 3,240 | 4,320 | 5,400 | 6,480 | 8,640 |
| 400 | 4,320 | 6,480 | 8,640 | 10,800 | 12,960 | 17,280 |
| 500 | 5,400 | 8,100 | 10,800 | 13,500 | 16,200 | 21,600 |
| 600 | 6,480 | 9,720 | 12,960 | 16,200 | 19,440 | 25,920 |
| 800 | 8,640 | 12,960 | 17,280 | 21,600 | 25,920 | 34,560 |
| 1,000 | 10,800 | 16,200 | 21,600 | 27,000 | 32,400 | 43,200 |
| 1,500 | 16,200 | 24,300 | 32,400 | 40,500 | 48,600 | 64,800 |
| 2,000 | 21,600 | 32,400 | 43,200 | 54,000 | 64,800 | 86,400 |
| 3,000 | 32,400 | 48,600 | 64,800 | 81,000 | 97,200 | 129,600 |
At 400 CFM and a 30°F temperature difference, the standard sensible formula gives 12,960 Btu/h. This is sensible heat only; it does not establish total cooling tonnage or validate a CFM-per-ton design ratio.
Altitude Correction for the 1.08 Factor
At the same temperature and composition, lower absolute air pressure reduces density. For actual volumetric airflow, use the density appropriate to the operating conditions; no single elevation threshold determines when correction is needed. The sensible relationship is Btu/h = (60 × density × specific heat) × CFM × ΔT.
For sensible heat, the correction multiplier relative to 1.08 is (density ÷ 0.075) × (specific heat ÷ 0.24), with density in lb/ft³ and specific heat in Btu/(lb·°F). Enter a documented adjusted factor in Optional settings.
For example, if a documented operating condition gives a sensible factor of 0.90, 800 CFM with a 20°F rise corresponds to 800 × 0.90 × 20 = 14,400 Btu/h. This is a conditional heat-balance result, not a universal altitude derating of equipment capacity.
Temperature Effects on Air Density
Air density also varies with temperature. The standard 0.075 lb/ft³ applies near 70°F. At higher or lower temperatures, the density shifts, changing how much heat energy each cubic foot of air can carry. This matters in industrial applications such as process heating, furnace flue gas analysis, and cold-storage ventilation.
Use air properties at representative operating conditions. Temperature and pressure alone do not define a correction for moist air or a different gas mixture.
For dry air at the same pressure, density varies approximately inversely with absolute temperature. Specific heat also varies, particularly across wide temperature ranges. Use measured or documented air properties rather than assuming a universal percentage error for 1.08.
Sensible Heat vs. Total Heat vs. Latent Heat
The equation BTU/hr = 1.08 × CFM × ΔT calculates sensible heat only, meaning the energy associated with a temperature change in air. In cooling applications, removing moisture from air also requires energy, called latent heat. The three common HVAC heat formulas are shown below.
| Heat Type | Formula | What It Measures |
|---|---|---|
| Sensible | 1.08 × CFM × ΔT | Temperature change only |
| Latent | 0.68 × CFM × ΔW (grains) | Moisture removal only |
| Total | 4.5 × CFM × Δh (BTU/lb) | Temperature plus moisture change |
The sensible and latent fractions depend on entering-air conditions, moisture load, equipment and operating point. Choose the matching calculator mode. The latent shortcut uses humidity-ratio difference in grains of water per pound of dry air; the total shortcut uses enthalpy difference in Btu per pound of dry air. These factors approximate a standard-air mass-flow basis.
CFM per Ton of Cooling
The table below assumes 400 CFM per US refrigeration ton for arithmetic comparison, where 1 ton = 12,000 Btu/h. The assumed ratio is not a universal commissioning target.
| System Size | Tons | Total BTU/hr | Typical CFM | Sensible BTU/hr at ΔT 20°F |
|---|---|---|---|---|
| Small room unit | 1.5 | 18,000 | 600 | 12,960 |
| Small residential | 2.0 | 24,000 | 800 | 17,280 |
| Average home | 3.0 | 36,000 | 1,200 | 25,920 |
| Large home | 4.0 | 48,000 | 1,600 | 34,560 |
| Large residential | 5.0 | 60,000 | 2,000 | 43,200 |
Use manufacturer performance and commissioning data to select airflow. Changing airflow can affect sensible and latent capacity, and the result depends on the equipment and conditions. The assumed table ratios do not replace those data or establish the sensible fraction.
When the Standard Formula Is Not Enough
Use Btu/h = ρ × Cp × 60 × CFM × ΔT with properties and actual volumetric airflow on a compatible basis. Large temperature ranges, unusual pressure, moisture changes or other gases require appropriate property or psychrometric analysis. For total heat, the standard-air shortcut 4.5 × CFM × Δh also requires a compatible dry-air mass-flow basis; 4.5 is not universal.
