Psychrometric Cooling Load Calculator

Every air-side load splits into two parts. Sensible load comes from the dry-bulb temperature change; latent load comes from the moisture change. Adding them gives an approximate total for the coil. The calculator also computes the total from the moist-air enthalpy difference, which is the more fundamental route, so you can see how close the shortcut constants are. If you only know a supply airflow target, the CFM / airflow calculator works the reverse direction.

These are the same air-side relationships used in a Manual J load calculation and described in the HVAC load calculation formula guide.

Sensible: 1.08 × 1,000 × (80 − 55) = 27,000 BTU/h. Latent: 0.68 × 1,000 × (90 − 60) = 20,400 BTU/h. Sum of the approximations: 47,400 BTU/h, with an approximate SHR of 27,000 ÷ 47,400 ≈ 0.57. By enthalpy: h in ≈ 33.30 and h out ≈ 22.50 Btu/lb, so Qt = 4.5 × 1,000 × 10.79 ≈ 48,575 BTU/h(about 4.05 tons), slightly above the shortcut sum because the enthalpy form includes the vapor's heat content as temperature changes. Defaults are example assumptions, not universal design conditions.

Values use the approximate constants and are illustrative. Notice how a large grains difference pulls SHR down even when the temperature drop is modest; that is a coil doing mostly dehumidification work. The last row is a dehumidification-focused case such as a unit that reheats or removes moisture without much cooling, not a typical comfort coil.

• Mixing up dry-bulb and wet-bulb temperatures — the sensible formula uses dry-bulb only; moisture is handled separately through grains.
• Entering relative humidity directly. RH must first be converted to a humidity ratio in grains/lb at the actual temperature, because the same RH holds far more moisture at higher temperatures.
• Forgetting the 1.08 and 0.68 constants are tied to standard-density air — at high elevation the air is less dense and both shrink.
• Adding the shortcut total and the enthalpy total together. They are two estimates of the same load, not two loads.
• Entering supersaturated leaving air — a humidity ratio above the saturation value at the leaving temperature is not a valid state.
• Sizing equipment on total load alone and ignoring SHR — a coil with the wrong SHR leaves the space cold and clammy or hot and dry.
• Using entering-air conditions as the leaving state. The temperature and grain differences must be taken across the coil, not at a single point.
• Assuming the coil dehumidifies. If the leaving grains equal the entering grains, latent load is zero and the coil is running dry.

The two air-side equations on this page are shortcuts for moving between points on a psychrometric chart. The chart, standardized in the ASHRAE Handbook - Fundamentals, plots every possible moist-air state on one diagram: dry-bulb temperature runs along the horizontal axis, the humidity ratio (grains of moisture per pound of dry air) runs up the vertical axis, and the curving lines crossing the field are constant wet-bulb temperature, constant relative humidity and constant enthalpy. Any two of those properties fix a single point, and once you have the point you can read off the others, including the dew point, where the air would begin condensing if cooled further.

A cooling coil traces a line from the entering air state to the leaving air state. When that line moves straight left at constant humidity ratio, the coil is doing pure sensible work and the latent load is zero. When the line also drops downward toward lower grains, the coil surface has fallen below the entering air's dew point, moisture is condensing and you pick up latent load. The slope of that process line is what the sensible heat ratio describes. The 1.08 constant captures the horizontal (temperature) move and 0.68 captures the vertical (moisture) move, both scaled by standard air density near sea level. At higher elevation the air thins out, so both constants shrink and the chart you would use changes with them.

Enthalpy ties the two halves together. It is the total heat content of the air in Btu per pound of dry air, sensible and latent combined. Reading the enthalpy difference across the coil and multiplying by the mass flow is a third route to the same total, and it is a useful check on the separate sensible and latent figures, since rounding the grains can quietly distort the split.

Most people size cooling equipment on total load alone: find the BTU/h, divide by 12,000 BTU/h per ton and pick a unit. That ignores one of the most important air-side properties for comfort, the sensible heat ratio. A space whose load has an SHR of 0.75 needs a coil that removes one unit of moisture heat for every three units of temperature heat. If the selected equipment delivers an SHR of 0.90, it can satisfy the thermostat on temperature before it has removed enough water, short-cycle and leave the room cool and clammy at a high relative humidity, well outside most comfort targets. ASHRAE guidance generally keeps indoor humidity under roughly 60 to 65% RH, with the exact comfort zone depending on temperature, clothing and activity.

AHRI publishes rated capacities for matched indoor and outdoor units, and those ratings list sensible and total capacity separately at standard conditions, which lets you back out the equipment SHR. The practical workflow is to establish the design loads first (a full ACCA Manual J gives sensible and latent totals), then use the leaving air state from this calculator to confirm a candidate coil can reach the supply moisture you need. Efficiency metrics such as SEER2, EER2 and HSPF2 tell you how cheaply the box runs; they say nothing about whether it dehumidifies, so SHR has to be checked independently, and Manual S selection works from sensible and latent capacity at design conditions.

• High-SHR loads: dry climates, tight envelopes and low occupancy — standard single-stage equipment sized close to the sensible load can work.
• Mid-SHR loads: mixed-humid climates and typical homes — variable-speed or two-stage equipment, with longer lower-airflow cycles, can strip more moisture.
• Low-SHR loads: humid climates, high occupancy, pools and kitchens — may need a dedicated dehumidifier or a dedicated outdoor-air system alongside the cooling coil.

Airflow is a lever that shifts SHR on a given coil. A common design figure is around 400 CFM per ton for good sensible capacity; slower airflow over the same coil lowers the coil surface temperature and increases moisture removal, within the manufacturer's limits (too little air risks coil freezing). You can test that trade-off directly by changing the airflow input here, or work the airflow target itself with the CFM calculator. For the whole-house load that feeds these numbers, start from the Manual J calculator, check capacity at design conditions with the Manual S calculator, and see the HVAC load calculation formula guide for how the pieces roll up into a single tonnage.

Use measured airflow, pressure-dependent properties and coil data for commissioning or selection. ASHRAE Handbook Fundamentals: Psychrometrics explains the properties and equations used here. The example inputs are stated scenarios and require project-specific verification. For residential design, ACCA Manual J, S and D address load calculation, equipment selection and duct design. This calculator does not certify a complete design or code compliance.