Where does the constant 1.08 come from?

It is the product of standard air density (0.075 lb/ft3), the specific heat of air (0.24 BTU/lb-F), and 60 minutes per hour: 0.075 x 0.24 x 60 = 1.08. It converts a BTU/h sensible load and a temperature difference directly into CFM at sea-level standard conditions.

What supply-air temperature difference should I use?

For cooling, a delta-T of 18 to 22 F between room air and the supply diffuser is typical. A larger delta-T means less airflow but risks cold drafts and dumping; a smaller delta-T raises airflow and fan energy. 20 F is a common design default.

Why is the latent constant 0.68?

Latent airflow uses Q = CFM x 0.68 x d-grains, where 0.68 comes from 0.075 lb/ft3 x 60 min/h x 1076 BTU/lb / 7000 grains per pound. It converts a latent load and a humidity-ratio difference in grains per pound into the airflow needed to remove that moisture.

Should I size to the sensible or latent airflow?

Size the duct to the larger of the two. In most comfort cooling applications the sensible airflow governs, but in humid climates or high-occupancy spaces the latent requirement can dominate, so check both and use whichever is greater.

Does altitude change the result?

Yes. Air density drops with altitude, so the 1.08 and 0.68 constants decrease and the required CFM rises. At about 5,000 ft the constants are roughly 17 percent lower. This tool uses sea-level standard air; apply a density correction for high-altitude jobs.

HVAC CFM Airflow Calculator

The HVAC CFM airflow calculator turns a room’s heat load into the supply airflow you actually need to deliver. It uses the standard sensible-heat equation that every mechanical designer relies on to size diffusers, VAV boxes, and air-handler branches.

How it works

Required airflow comes from rearranging the two classic air-side heat equations:

sensible:  Q_s = CFM x 1.08 x dT
latent:    Q_l = CFM x 0.68 x d-grains

so:
CFM_sensible = Q_s / (1.08 x dT)
CFM_latent   = Q_l / (0.68 x d-grains)

The constant 1.08 bundles standard air density, the specific heat of air, and the 60-minutes-per-hour conversion. The latent constant 0.68 does the same but for moisture removal, using the latent heat of vaporization and 7000 grains per pound. You size the duct to whichever airflow is larger.

Example and tips

A room with a 12,000 BTU/h sensible load and a 20 F supply temperature difference needs 12000 / (1.08 x 20) = 556 CFM. If that same room also carries a 3,000 BTU/h latent load with a 25 gr/lb difference, the latent airflow is 3000 / (0.68 x 25) = 176 CFM, so the sensible requirement governs and you design for about 556 CFM. Keep your delta-T realistic: pushing it above 22 F to shrink ducts often causes cold drafting at the diffusers.