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). This calculator’s duct function uses that 1.08. ΔT is the temperature split between supply and return air — the amount the equipment heats or cools the air in one pass.
The 24,000 BTU/hr, the 20°F split, and 1,111 CFM are the worked example already on this page.
Worked example at a 28°F split, not a claim that 28°F is typical: a 2-ton (24,000 BTU/hr) input is 24,000 ÷ (28 × 1.08) = 24,000 ÷ 30.24 ≈ 794 CFM, or about 397 CFM per ton. At a 20°F split the same 24,000 BTU/hr is 1,111 CFM. The split you type is the input. The formula does not pick it.
CFM by tonnage at three splits
| 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 |
Per-run airflow
Divide the formula’s CFM by the number of supply runs if you want a per-run figure. The worked example below does that for six runs. Diameter is not in this formula.
Diagnosing an existing system
Measure supply and return temperatures and use those as ΔT. The formula runs both directions — measured CFM × ΔT × 1.08 gives delivered BTU/hr. This page does not print a normal split.
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Size the equipment first in the HVAC load calculator, then work its BTU through the CFM calculator above.
A clogged filter restricts the airflow this chart assumes: how to clean an air conditioner filter. The loop that airflow serves is how an air conditioner works.
Worked example on this site’s duct tab
The duct tab calls ductCFM: BTU/hr ÷ (ΔT × 1.08). One ton is 12,000 BTU/hr, so a 3-ton cooling rate is 36,000 BTU/hr and a 2-ton rate is 24,000 BTU/hr. At a 20°F supply-to-return split, ΔT × 1.08 is 21.6. ductCFM(36000, 20) is 36,000 ÷ 21.6 = 1,666.666… CFM, and the ΔT 20 column above rounds that to 1,667. ductCFM(24000, 20) is 24,000 ÷ 21.6 = 1,111.111… CFM, which that column rounds to 1,111. Split the 2-ton result across 6 supply runs and each run is 1,111.111 ÷ 6 = 185.185 CFM.
Open the calculator, choose Duct CFM, and enter 36000 with a ΔT of 20, or 24000 with a ΔT of 20. The result card shows the unrounded quotient. Square footage does not belong in that tab. The room-BTU tab is where square footage, ceiling height, windows, sun, and insulation are entered. CFM starts from a BTU/hr figure you already have.
At a 28°F split, 24,000 ÷ (28 × 1.08) = 24,000 ÷ 30.24 ≈ 794 CFM, and 794 ÷ 2 ≈ 397 CFM per ton. At a 20°F split the same 24,000 BTU/hr is 1,111 CFM, about 556 CFM per ton. The split is an input. This page does not use a CFM-per-ton rule in place of the formula.
Common mistake
Reading a duct diameter off the CFM. The formula returns an airflow. Diameter, friction rate, and available static pressure are ACCA Manual D. This page does not include a diameter table. The ASHRAE handbook is the engineering reference for the air-side quantities. It does not replace a measurement of the equipment’s external static pressure, and the blower still has to be able to move the CFM against that pressure.
The other mistake is typing the room’s square footage into the duct tab and treating the result as a system size. Size the equipment on the room-BTU tab, or from a Manual J, then bring that BTU/hr here.
Frequently asked questions
How many CFM is a 3-ton system at a 20°F split?
ductCFM(36000, 20) is 36,000 ÷ (20 × 1.08) = 1,666.666… CFM. This page’s chart rounds that to 1,667. One ton is 12,000 BTU/hr, so 3 tons is 36,000 BTU/hr.
Does that CFM pick the duct diameter?
No. ACCA Manual D sizes the duct from friction rate, material, and available static pressure. The CFM is the airflow target. A diameter guessed from CFM alone is not a Manual D.
Why does a 28°F split not match the 20°F column?
At a 28°F split, 24,000 ÷ (28 × 1.08) ≈ 794 CFM, about 397 CFM per ton. At a 20°F split the same 24,000 BTU/hr is 1,111 CFM, about 556 CFM per ton. Those are two inputs to the same formula. The chart’s three columns are three different splits.
Sources & Standards
Related standards. These pages describe the publications and do not print the rule-of-thumb factors used in the calculators: