Sensible + Latent CFM - HVAC Calculator Toolbox

About this calculator This calculator determines the supply airflow required to offset a known sensible load and, separately, a known latent load, then highlights whichever airflow is higher — that is the one the unit has to deliver. The sensible side works in either heating or cooling mode by using the magnitude of ΔT between the unit leaving and space temperatures. The latent side is cooling-only: it uses ΔW, the difference between the space humidity ratio and the humidity ratio of the supply air at its leaving dew point. Both apply a density correction for altitude. Two air-density bases are supported: Cooling Design evaluates moist-air density and specific heat at the selected weather station's 0.4% cooling design condition, while Standard uses the textbook 1.08 and 0.69 factors corrected for altitude. This is the same airflow math as the Traditional RTU vs Paragon HVAC calculator, without the RTU or energy comparison. The latent inputs are optional — leave them blank to use it as a sensible-only calculator.

Use when You need to size supply airflow for known sensible and latent loads at a given supply and space condition, and want to know which load sets the airflow.

Formula Sensible CFM = Sensible Load ÷ (k_s × |ΔT|) | Latent CFM = Latent Load ÷ (k_l × ΔW)

Variables

Sensible Load Sensible heating or cooling load in BTU/hrLatent Load Latent cooling load in BTU/hr — optional; leave blank to size for the sensible load onlyUnit Leaving Temperature / Dew Point Dry bulb and dew point of the supply air leaving the unit, in °FDesired Space Temperature / Humidity Design indoor dry bulb in °F, plus one humidity property — relative humidity, wet bulb, dew point, or humidity ratioAltitude Installation altitude in feet — adjusts for air densityAir Density Basis Sets the k factors — Cooling Design uses moist air at the weather station's 0.4% cooling design condition; Standard uses 1.08 and 0.69 corrected for altitude. We recommend Cooling Design for high % OA designs and Standard for low % OA designs

Sensible Load (BTU/hr)

Sensible heating or cooling load in BTU per hour

Latent Load (BTU/hr)

Latent cooling load in BTU per hour

Unit Leaving Temperature (°F)

Dry bulb temperature of air leaving the unit

Unit Leaving Dew Point (°F)

Dew point of air leaving the unit

Desired Space Temperature (°F)

Target indoor space dry bulb temperature

Space Humidity Input

Which space humidity property you know

Desired Space Relative Humidity (%)

Target indoor relative humidity

Altitude (ft)

Installation altitude — autofilled from weather station

Cooling Design

Standard (1.08)

Air Density Basis

We recommend using Cooling Design for high % OA designs and Standard for low OA % designs

ΔT Dry Bulb (°F) —

Sensible CFM —

ΔW Humidity Ratio (gr/lb) —

Latent CFM —

How is supply airflow determined from sensible and latent loads?

Supply air has to do two jobs: carry away heat and carry away moisture. The sensible heat equation is Sensible Load (BTU/hr) = kS × CFM × |ΔT|, where ΔT is the difference between supply (unit leaving) and space dry bulb temperature. The latent heat equation is Latent Load (BTU/hr) = kL × CFM × ΔW, where ΔW is the space humidity ratio minus the supply air humidity ratio, in grains per pound of dry air. Solving each for CFM gives two airflows, and the larger one governs: delivering it satisfies both loads, while delivering the smaller leaves one of them unmet. With Standard, kS = 1.08 × Density Factor and kL = 0.69 × Density Factor — constants built from standard air density (0.075 lb/ft³), the specific heat of air (0.24 BTU/lb·°F) or the latent heat of vaporization (1076 BTU/lb), and 60 min/hr; the Density Factor adjusts for altitude. With Cooling Design, both are evaluated from moist-air density at the weather station's 0.4% cooling design dry bulb and mean coincident wet bulb at site pressure — hot, humid design air is less dense than standard air, so this basis yields somewhat higher required CFM. A larger ΔT or ΔW requires less airflow for the same load. Typical design ΔT for comfort cooling is 15–25 °F. When the latent airflow governs, lowering the leaving dew point is usually more effective than adding airflow.

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