CFM to BTU Calculator — Heating & Cooling Airflow

Divide the BTU per hour by (1.08 × ΔT in °F) for heating: CFM = BTU/h ÷ (1.08 × ΔT). Example: 12,000 BTU/h across a 30°F temperature rise is 12,000 ÷ (1.08 × 30) = 370 CFM. For sensible cooling use 1.10 instead of 1.08: 12,000 BTU/h across a 20°F drop is 545 CFM. The ΔT is the air temperature change across the furnace, coil or heater — not the room's setpoint difference.

Use CFM = BTU/hr ÷ (1.08 × ΔT). A 60,000 BTU/hr furnace with a 40°F rise moves 60,000 ÷ 43.2 = 1,389 CFM; at a 50°F rise only 1,111 CFM. Notice the lever: hotter air carries more heat per cubic foot, so the same furnace can push fewer CFM — that is why high-rise furnaces pair with smaller blowers.

Multiply: BTU/h = 1.08 × CFM × ΔT for heating. Example: 221 CFM warmed 25°F carries 221 × 1.08 × 25 = 5,967 BTU/h — about a 6,000 BTU/h (0.6 MBH) output. For cooling, use 1.10 × CFM × ΔT: 400 CFM dropping 20°F across a coil absorbs 8,800 BTU/h of sensible heat.

It depends on the air temperature change. Heating at a 30°F rise: 12,000 ÷ (1.08 × 30) = 370 CFM. At a 25°F rise: 444 CFM. As a sensible-cooling duty with a 20°F drop: 12,000 ÷ (1.10 × 20) = 545 CFM. This is why a '12,000 BTU' unit ships with different blower taps — the airflow matches the ΔT the equipment was designed around.

Rearrange the formula: ΔT = BTU/h ÷ (1.08 × CFM). A furnace delivering 80,000 BTU/h through 1,600 CFM is running an 80,000 ÷ (1.08 × 1,600) = 46°F rise. Compare that measured rise against the nameplate's rated range (typically 30–70°F): a rise well above rating means starving the heat exchanger with too little airflow — the classic cracked-exchanger risk.

Residential gas furnaces move roughly 30–45 CFM per 10,000 BTU/h of output, set by the rated temperature rise. An 80,000 BTU/h furnace at a 40°F rise needs 1,852 CFM — 23 CFM per 10,000 BTU/h. The exact answer always comes from CFM = BTU/h ÷ (1.08 × rated rise), with the rise printed on the furnace nameplate.

MBH means thousands of BTU per hour — 1 MBH = 1,000 BTU/h. Convert MBH to CFM with CFM = MBH × 1,000 ÷ (1.08 × ΔT). Example: 25 MBH across a 35°F rise is 25,000 ÷ 37.8 = 661 CFM. Commercial coil schedules and boiler ratings use MBH heavily, so this is the fastest unit to internalize for plan reviews.

CFM = kW × 3,412 ÷ (1.08 × ΔT) ≈ kW × 3,160 ÷ ΔT. A 10 kW duct heater across a 25°F rise needs 10 × 3,160 ÷ 25 = 1,264 CFM. Skim below about 45 CFM per kW and element surface temperatures climb toward their limit switches — electric duct heaters list a minimum CFM per kW for exactly this reason.

A boiler itself does not move air — the coil or radiator it feeds does. For a hydronic air handler, apply the same formula with the coil's air-side output: CFM = coil BTU/h ÷ (1.08 × ΔT). A 100,000 BTU/h boiler driving a coil that delivers 80,000 BTU/h to air across a 35°F rise needs 80,000 ÷ 37.8 = 2,116 CFM. Size the blower to the coil's air-side duty, not the boiler's firing rate.

BTU measures energy (heat), CFM measures volume (air). They connect only through ΔT: the same 1,000 CFM carries 10,800 BTU/h at a 10°F difference and 43,200 BTU/h at 40°F. That is why no single 'BTU to CFM' factor exists — any converter that skips ΔT is guessing. Every result on this page pairs a heat rate with the temperature change that delivers it.

The factor bundles air density (lb per ft³) times specific heat (BTU per lb-°F) times 60 min. Cooling air leaves the coil colder and denser — about 0.073 lb/ft³ versus 0.075 for room air — and industry convention rounds the product to 1.10 for sensible cooling. For total capacity including latent (moisture) load, engineers use the enthalpy formula BTU/h = 4.5 × CFM × Δh instead, which this calculator deliberately does not touch.

Yes — the 1.08 factor assumes sea-level air density. At 5,000 ft the density drops about 17%, so the effective factor falls to roughly 0.90 and the same furnace moving the same CFM delivers 17% less heat. High-altitude installations use derated appliances and adjusted factors; this calculator's results are sea-level planning values.