Duct CFM: the BTU ÷ (ΔT × 1.08) guide
One formula turns equipment size into the airflow your ducts must carry — with the worked numbers for every common system size.
The formula
CFM = BTU/hr ÷ (ΔT × 1.08). The 1.08 constant is air's heat-carrying capacity at sea level (0.075 lb/cu ft density × 0.24 BTU/lb-°F × 60 min/hr). ΔT is the temperature split between supply and return air — the amount the equipment heats or cools the air in one pass.
Worked example: a 2-ton (24,000 BTU/hr) AC at a typical cooling ΔT of 27–28°F: CFM = 24,000 ÷ (28 × 1.08) ≈ 794 CFM — which is exactly where the industry shorthand of ~400 CFM per ton comes from. Setups with a smaller split (20°F) run more CFM per ton; drier-coil designs with a bigger split run less. Your equipment's spec sheet and measured split settle it.
CFM by tonnage, at the usual ΔTs
| System | ΔT 20°F | ΔT 25°F | ΔT 30°F |
|---|---|---|---|
| 1.5 ton (18,000 BTU/hr) | 833 | 667 | 556 |
| 2 ton (24,000 BTU/hr) | 1,111 | 889 | 741 |
| 2.5 ton (30,000 BTU/hr) | 1,389 | 1,111 | 926 |
| 3 ton (36,000 BTU/hr) | 1,667 | 1,333 | 1,111 |
| 4 ton (48,000 BTU/hr) | 2,222 | 1,778 | 1,481 |
| 5 ton (60,000 BTU/hr) | 2,778 | 2,222 | 1,852 |
Heating with a gas furnace? ΔT is much larger — 40–60°F is normal, so a 60,000 BTU/hr furnace at ΔT 50 needs only 1,111 CFM. Heat pumps sit near 20–25°F in heating.
Per-run airflow
Divide total CFM by the number of supply runs to sanity-check the branch layout. At 1,200 CFM across 8 runs, each branch carries 150 CFM — typical residential branches at 6–7 inch round carry 100–150 CFM. If your per-run number forces 8-inch branches everywhere, the trunk is starved or the run count is too low.
| Duct (round, typical residential) | Carries, approx CFM |
|---|---|
| 5 inch branch | 60–80 |
| 6 inch branch | 100 |
| 7 inch branch | 150 |
| 8 inch branch | 200 |
| 12 inch trunk | 800–1,000 |
| 14 inch trunk | 1,200–1,400 |
Diagnosing an existing system
Measure supply and return temps a few feet from the equipment with a probe thermometer. Cooling split much under 16°F usually means low airflow (dirty coil/filter, undersized returns); much over 22°F on AC suggests low charge or, in humid climates, deliberately low airflow for dehumidification. The formula runs both directions — measured CFM × ΔT × 1.08 gives you delivered BTU/hr.
Measuring your own system: a dual-probe thermometer, a vane anemometer for register velocity, and a static-pressure manometer are the three tools techs actually use.
Size the equipment first in the HVAC load calculator, then work its BTU through the CFM calculator above.