CFM per room, from load or from a system split.

Short answer

Room airflow in CFM = room load in Btu/h divided by (1.08 x the temperature difference between supply air and room air in F). A 6,000 Btu/h room with a 20 F difference needs 278 CFM. To split a whole system, each room gets the total CFM times its share of the total load.

Worked example

A bedroom has a 6,000 Btu/h cooling load and the supply air arrives 20 F colder than the room: 6,000 / (1.08 x 20) = 278 CFM. For a split, a 1,200 CFM system serving rooms with loads of 9,000, 4,000, 3,000, 2,500, 1,000 and 2,000 Btu/h (total 21500) sends 502 CFM to the living room, 223 to the kitchen, 167 to the first bedroom and the rest in proportion.

Formula, assumptions and sources

The sensible-heat equation is Btu/h = 1.08 x CFM x temperature difference. ACCA's HVAC blog derives the 1.08 as air density 0.075 lb/ft3 x specific heat 0.24 Btu/lb-F x 60 minutes per hour, and cautions that it applies to air at sea level and near room temperature. It covers sensible heat only; moisture removal is not included. The split divides the total airflow by each room's share of the summed loads, which gives a starting allocation, not a duct design. The DOE air distribution guideline treats the required CFM for each room as the key input to choosing a supply outlet. This tool does not calculate the room load itself (that takes a Manual J-style load calculation) and it does not account for ventilation or exhaust airflow; for that, use the CFM and Air Changes per Hour Calculator.

Source: ACCA HVAC Blog, The Physics of Heat and Air Flow (A. Bailes, 2 May 2018) (retrieved 2026-10-15)

Source: U.S. DOE Building America, Advanced Strategy Guideline: Air Distribution Basics and Duct Design (IBACOS, December 2011) (retrieved 2026-10-15)

Safety and limits

This is a planning aid, not professional advice. Duct, furnace and air-conditioner design affects comfort, combustion safety and code compliance; a licensed HVAC contractor should perform a load calculation, size the system and verify airflow on site.

FAQ

How do I calculate CFM per room?

Divide the room's sensible load in Btu/h by 1.08 times the temperature difference in F between the supply air and the room. Or split the whole system's CFM among rooms in proportion to their loads.

Is CFM per square foot a reliable way to size a room?

It is only a rough starting point. Room load depends on windows, insulation, orientation and use, so airflow follows the load rather than the floor area. A load calculation gives the real figure.

Where does 1.08 come from?

It is air density 0.075 lb/ft3 times specific heat 0.24 Btu/lb-F times 60 minutes per hour. It assumes standard air near sea level and covers sensible heat only.

Is this the same as ventilation CFM from air changes per hour?

No. This tool sizes heating and cooling airflow from load. Ventilation airflow from room volume and air changes per hour is on the CFM and Air Changes per Hour Calculator.