Sensible Heat Formula & Airflow Calculator (Q = 1.08 × CFM × ΔT)
Use the sensible heat equation to find delivered BTU/h from airflow and temperature split, the CFM from heat and ΔT, or the ΔT to expect, plus CFM per ton, the 350–450 CFM per ton check, kW output, altitude correction and a table of splits by airflow.
How it works
Q_sensible (BTU/h) = 1.08 × CFM × ΔT(°F) [1.08 = 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/h]; metric Q (W) = 1.2 × L/s × ΔT(°C); CFM/ton = CFM ÷ tons
The sensible heat equation is the most used formula in HVAC: the heat carried by an airstream equals its mass flow times the specific heat of air times the temperature change, and at standard density the constants collapse to 1.08 when airflow is in cubic feet per minute and temperature in Fahrenheit. Technicians use it backwards to measure airflow from a furnace's heat input and its supply-return temperature rise, forwards to check whether a coil is delivering its rated sensible capacity, and as a sanity check via CFM per ton — around 400 for a typical cooling system, lower for dehumidification, higher in dry climates. At altitude the air is thinner, so the constant falls with the density ratio; a fan moving 1,200 CFM in Denver carries about 17% less heat than at sea level.
Worked example
1,200 CFM with an 18 °F split delivers 1.08 × 1,200 × 18 = 23,328 BTU/h, about 1.9 tons of sensible cooling; on a 3-ton unit that is 400 CFM/ton — right on target. At 1,000 CFM the same heat would need a 21.6 °F split; a 90,000 BTU/h furnace with a 50 °F rise needs about 1,670 CFM.
Frequently asked questions
Why does the total capacity of my air conditioner not match this?
The sensible formula ignores latent (moisture) heat. In humid conditions 25–35% of a coil's capacity goes to condensing water, so the total is Q_total = 4.5 × CFM × Δh (enthalpy, BTU/lb) — measure wet-bulb temperatures for that.
Where do I measure the temperatures?
Return air just before the coil or furnace, and supply air after the plenum has mixed but before any leaks or duct losses — a few feet down the trunk. Use the same probe for both to avoid calibration error.
Related calculators
Standard HVAC field formulas (sensible heat equation, fan laws, ASHRAE 62.2 and blower-door conventions) at sea-level air density unless noted. Use manufacturer data, a Manual J/D/S load calculation and a licensed HVAC technician for design and commissioning decisions.