CFM per ton calculator

What CFM per ton means

CFM per ton is blower airflow divided by cooling capacity, with a ton being 12,000 BTU/h. The site works to 350 to 400 CFM per ton: 400 in a dry climate, 350 where latent load matters and you want the coil colder so it pulls more water out of the air. A three-ton system at 400 moves 1,200 CFM. The duct sizing guide starts from the same number.

The rule of thumb is a starting point, not the design. On a certified house the design fan airflow is taken from the manufacturer's expanded performance data and written down separately for cooling and heating, and a third-party rater can grade the airflow actually delivered.

Sources: EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14), Item 5.2 and Footnote 34 (read 2026-10-18) · EPA ENERGY STAR, National Rater Field Checklist v3.1/3.2/3.3 (Rev. 14), Item 5a.1 (read 2026-10-18)

How to use the calculator

1. Cooling target: enter the tonnage and pick the climate. If you have a measured airflow from a flow hood or the blower table at the measured static pressure, enter it for a grade.
2. Measured from temperature rise: enter the furnace's output, the return and supply temperatures, and the rise range printed on the rating plate.
3. Read the dial. Cool means inside the band or the plate range; warm means the airflow is off and the panel says which way.
4. Carry the CFM to the duct size calculator, which takes airflow as its input and returns the round or rectangular size.

Airflow from temperature rise

A furnace puts a known output into the air, so the temperature rise across it tells you how much air there was. The sensible heat equation does it: CFM = BTU/h ÷ (1.08 × ΔT). The 1.08 is standard air folded into one constant: 0.075 lb per cubic foot, times 60 minutes, times a specific heat of 0.24 BTU per pound per °F.

Use output, not input: an 80,000 BTU/h input furnace at 96% AFUE puts 76,800 BTU/h into the air. The rise has to land inside the range on the furnace's rating plate. A rise above the range means too little air across the heat exchanger; a rise below it means too much air, or a burner firing short of its rating. The furnace sizing guide walks the same check from the sizing side.

When the airflow is low

Low CFM per ton is almost never the blower's fault. It is a loaded filter, a dirty coil, an undersized return or flex duct that was never pulled tight, and each of those shows up as high static pressure. Measure it with the static pressure calculator and read the static pressure guide for what each component should cost. The total has to sit below the limit of the equipment, so turning the blower up to hide a restriction only trades one fault for another. On the cooling side, low airflow drops the coil temperature until it freezes, which is a branch of the AC not cooling decision list.

Sources: DOE Building America Solution Center, Air Conditioning (read 2026-09-04) · EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 5.4 and Footnote 36 (read 2026-09-04)

From total airflow to the rooms

The CFM-per-ton total is the trunk. Split it room by room in proportion to each room's share of the cooling load, not its floor area; the Manual J load worksheet gives those shares. Then size each branch for its share. The federal design checklist names ACCA Manual D for that step, with the duct system designed for the equipment actually selected.

Sources: EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 5.1 (read 2026-09-04)