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.

What is the HVAC CFM Airflow Calculator?

Calculate required supply airflow in CFM from a room's sensible and latent heat load using Q = CFM x 1.08 x dT and Q = CFM x 0.68 x d-grains. Sizes zone-level airflow for AHUs and VAV boxes. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

HVAC CFM Airflow Calculator

Name: HVAC CFM Airflow Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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 7,000 grains per pound. You size the duct to whichever airflow is larger.

Worked example

A private office with a 12,000 BTU/h sensible cooling load and a 20 °F supply temperature difference:

CFM = 12,000 / (1.08 × 20) = 556 CFM

The same office has a 3,000 BTU/h latent load from occupants and a humidity-ratio difference of 25 grains/lb:

CFM = 3,000 / (0.68 × 25) = 176 CFM

The sensible requirement governs — design the branch for 556 CFM. If the latent load were the larger figure (common in high-occupancy spaces or humid climates), you would design to the latent airflow and check whether the coil can handle the sensible load at that higher airflow.

Choosing the right supply-air delta-T

The delta-T is a design choice, not a fixed constant. It affects diffuser selection, duct size, and occupant comfort:

Delta-T (°F)	Effect
Less than 15	High airflow, larger ducts, warmer supply, lower drafting risk
18–22	Typical comfort cooling range; 20 °F is the standard design default
More than 25	Low airflow, smaller ducts, risk of cold drafts and condensation at diffusers

A larger delta-T reduces the required CFM and lets you use smaller ducts — attractive for retrofits with limited ceiling depth. But pushing beyond 22–25 °F in occupied spaces risks “dumping” cold air that falls straight down from ceiling diffusers, creating uncomfortable drafts at head height. When in doubt, stay at 20 °F and size the diffuser for the resulting velocity.

Altitude correction

The constants 1.08 and 0.68 assume standard sea-level air density (0.075 lb/ft³). At altitude, air is less dense, so the same CFM carries less energy and the required airflow rises. At roughly 5,000 ft elevation, multiply the calculated CFM by approximately 1.20. Always apply a density correction for high-altitude projects — ignoring it undersizes the system.

For the most accurate altitude correction, calculate the actual air density at your project elevation and temperature, then substitute it into the heat equation in place of the standard 0.075 lb/ft³ factor. Published altitude correction factors assume a standard atmosphere, which may differ from actual site conditions if the climate is particularly hot or humid.