Sensible Heat & Airflow Calculator
BTU, CFM and temperature rise — where the famous 1.08 is density times specific heat times sixty, and falls to 0.93 in Denver.
This is sensible heat only — the part that changes the thermometer. Moisture being condensed or evaporated carries latent heat that this does not count, which on a humid cooling day is a large share of the real load. Nothing is uploaded.
The 1.08 is three unit conversions, not a fudge factor
Air weighs 0.075 lb per cubic foot and holds 0.24 BTU per pound per degree, and there are 60 minutes in an hour. Multiply the three and you get 1.08 — a volume flow turned into a mass flow and then into an hourly heat rate. There is nothing fitted or approximate about the arithmetic.
| Airflow | Rise | Sensible heat |
|---|---|---|
| 200 CFM | 20 °F | 4,320 BTU/h |
| 400 CFM | 20 °F | 8,640 BTU/h |
| 800 CFM | 20 °F | 17,280 BTU/h |
| 1200 CFM | 20 °F | 25,920 BTU/h |
| 1600 CFM | 20 °F | 34,560 BTU/h |
The relation is linear in both terms, so it reads three ways with no rearranging: heat from airflow and rise, airflow from heat and rise, rise from heat and airflow. A technician needs all three and they are the same equation.
But the density in it is sea level, so the constant is not constant
0.075 lb per cubic foot is air at sea level. Thinner air carries less heat per cubic foot, so the factor falls with altitude — 0.931 at 5,000 feet, 0.798 at ten thousand. Using 1.08 in Denver overstates the capacity of an airstream by 16%.
| Altitude | Air density | Factor | 400 CFM at 20 °F | 1.08 overstates by |
|---|---|---|---|---|
| Sea level (0 ft) | 100% | 1.080 | 8,640 BTU/h | 0% |
| A low valley (1000 ft) | 97% | 1.049 | 8,390 BTU/h | 3% |
| Foothills (2500 ft) | 93% | 1.003 | 8,025 BTU/h | 8% |
| Denver (5000 ft) | 86% | 0.931 | 7,445 BTU/h | 16% |
| A mountain town (7000 ft) | 81% | 0.875 | 7,004 BTU/h | 23% |
| A ski resort (10000 ft) | 74% | 0.798 | 6,380 BTU/h | 35% |
Note the error is always in the same direction. The sea-level rule never understates capacity — it flatters the equipment, everywhere above sea level, by more the higher you go. That is the sort of bias worth knowing about before sizing anything in the mountains.
How to use
- Enter the airflow in CFM.
- Enter the temperature rise across the coil or heater.
- Enter your altitude, which changes the factor.
- Read the heat, and how far the sea-level rule would have been out.
Frequently asked questions
What is the sensible heat formula?
BTU per hour equals 1.08 times CFM times the temperature rise in Fahrenheit. It is the standard relation for the heat an airstream carries, and every term in it is a unit conversion.
Where does the 1.08 come from?
Air weighs 0.075 lb per cubic foot and holds 0.24 BTU per pound per degree, and there are 60 minutes in an hour. Multiply the three and you get 1.08 — a volume flow turned into a mass flow and then into an hourly rate.
Is it a fudge factor?
No, and that is worth knowing. Nothing about it is fitted or empirical. It is three constants multiplied together, which is why it appears in every textbook with the same value rather than as a range.
Does 1.08 work everywhere?
No — the 0.075 is sea-level air. Thinner air carries less heat per cubic foot, so the factor falls with altitude: about 0.93 at 5,000 feet and 0.80 at ten thousand.
How wrong is it at altitude?
Using the sea-level figure in Denver overstates the capacity of an airstream by roughly 15%, and by about a third at 10,000 feet. The error is always in the same direction, so the rule flatters the equipment everywhere above sea level.
Can I work backwards from a load?
Yes — the relation is linear in both airflow and temperature rise, so it reads three ways with no rearranging. Heat from airflow and rise, airflow from heat and rise, or rise from heat and airflow.
Does this include humidity?
No. This is sensible heat only — the part that changes the thermometer. Moisture being condensed or evaporated carries latent heat that this does not count, which on a humid cooling day is a large share of the real load.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.