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Evaporative Cooler Supply Temperature Calculator

A direct evaporative (swamp) cooler can only cool air towards the outdoor wet-bulb temperature. Enter the weather and the pad effectiveness to see the supply temperature, humidity and water use.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system
Outdoor humidity as

From the cooler’s or media maker’s data. A DOE Building America report gives up to 93 % for thick (10–12 in) rigid media and 50 % to at most 80 % for older 2 in excelsior pads.

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Supply-air temperature
71°F
Supply-air relative humidity
73%

Supply air at about 71 °F and 73 % RH, evaporating 5.6 gal of water an hour.

Temperature drop
24°F
Outdoor wet-bulb (the lowest possible supply temperature)
65°F
Water evaporated
5.61gal/h
Sensible cooling of the airstream
51,840BTU/h
Show the working
  1. Supply temperature T = T_db − ε × (T_db − T_wb)
    95 − 0.8 × (95 − 65) = 71 °F
  2. Wet-bulb stays constant through the pad, which sets the supply humidity
    44.2 → 82.7 gr/lb, 73 % RH
  3. Water evaporated = dry-air flow × ΔW
    46.78 lb/h = 5.61 gal/h (plus any bleed-off)
  4. Sensible cooling = 1.08 × density ratio × CFM × ΔT
    51,840 BTU/h

The formula

Saturation effectiveness compares the temperature drop the cooler achieves with the most possible, the wet-bulb depression:

  • Supply temperature = outdoor dry-bulb − effectiveness × (outdoor dry-bulb − outdoor wet-bulb)

The wet-bulb temperature stays essentially constant through the pad, so the supply humidity follows from the supply temperature and the wet-bulb. Water evaporated is the dry-air flow times the gain in humidity ratio.

Effectiveness of different pads

A Department of Energy Building America report notes that well-designed coolers with thick (10 to 12 inch or more) rigid media can reach about 93 percent effectiveness, while older coolers with 2 inch excelsior pads reach 50 percent to at most 80 percent. Use the manufacturer’s figure when you have it. Dirty or dry pads lower it.

Where evaporative cooling works

Evaporative cooling works best where the air is dry and the wet-bulb is low. In humid weather the wet-bulb is high and the cooler cannot bring the air down much. The air also gets more humid, so the house needs open windows or relief vents for it to escape.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Outdoor dry-bulb
95 °F
Outdoor humidity as
Wet-bulb temperature
Outdoor wet-bulb
65 °F
Saturation effectiveness
80 %
Cooler airflow
2,000 CFM
Site elevation
0 ft

Result

Supply-air temperature
71 °F
Supply-air relative humidity
73 %
Temperature drop
24 °F
Outdoor wet-bulb (the lowest possible supply temperature)
65 °F
Water evaporated
5.61 gal/h
Sensible cooling of the airstream
51,840 BTU/h

Supply air at about 71 °F and 73 % RH, evaporating 5.6 gal of water an hour.

Assumptions

  • Adiabatic process at constant wet-bulb (EnergyPlus engineering reference); saturation effectiveness is constant.
  • Water use counts evaporation only; coolers also bleed off water to limit mineral build-up.

Limitations

  • Raises indoor humidity; works best in dry climates and needs open windows or relief vents for the air to leave.
  • Effectiveness falls if pads are dirty, dry or bypassed.

Frequently asked questions

Can a swamp cooler get below the wet-bulb temperature?

No. A direct evaporative cooler can approach the outdoor wet-bulb but not go below it; 100 % effectiveness would reach it exactly.

How much water does it use?

The tool shows evaporation only. Most coolers also bleed off some water to limit mineral build-up, which adds to the total.

Does altitude matter?

Slightly. Thinner air carries less heat per CFM, so the cooling in BTU/h and the water evaporated are lower for the same airflow.

Sources

  1. EnergyPlus Engineering Reference (v9.6), Evaporative Coolers — Direct Evaporative Cooler — U.S. Department of Energy EnergyPlus documentation, hosted by Big Ladder Software. Saturation efficiency ε = (T_db,in − T_db,out) ÷ (T_odb − T_owb); at 100 % the leaving dry-bulb would equal the entering wet-bulb; the process is treated as adiabatic at constant wet-bulb.
  2. New Evaporative Cooling Systems: An Emerging Solution for Homes in Hot Dry Climates with Modest Cooling Loads (L. Kinney) — Southwest Energy Efficiency Project for the U.S. DOE Building America Program (hosted by BuildItSolar). Direct saturation effectiveness is analogous to cooling efficiency and is measured relative to the wet-bulb depression; well-designed systems with 10–12 in or thicker media can reach 93 %, older 2 in excelsior pads 50 % to at most 80 %.
  3. PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
  4. Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.

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