Understanding evaporative cooling performance requires an understanding of psychrometrics. Evaporative cooling performance is dynamic due to changes in external temperature and humidity level. Under typical operating conditions, an evaporative cooler will nearly always deliver air cooler than 27 degrees Celsius (80 degrees Fahrenheit). A typical residential 'swamp cooler' in good working order should cool air to within 3–4 °C (5–7 °F) of the wet-bulb temperature.

In practice, it is simple to predict swamp cooler performance from standard weather report information. Because weather reports usually contain the dewpoint and relative humidity, but not the wet bulb temperature, a Psychrometric chart must be used to identify the wet bulb temperature. Once the wet bulb temperature and the dry bulb temperature are identified the cooling performance or leaving air temperature of the cooler may be determined:

LAT = DB – ((DB – WB) x E)

LAT = Leaving Air Temp

DB = Dry Bulb Temp

WB = Wet Bulb Temp

E = Efficiency of the evaporative media.

Evaporative media efficiency usually runs between 80% to 90% and the evaporation efficiency drops very little over time. Typical aspen pads used in residential evaporative coolers offer around 85% efficiency while celdec type of evaporative media offer efficiencies of 90% + depending on air velocity. The celdec media is more often used in large commercial and industrial installations.

As an example, in Las Vegas, Nevada with a typical summer design day of 108°F DB/66°F WB or about 8% relative humidity, the leaving air temperature of a residential cooler would be:

LAT = 108° – ((108° – 66°) x 85% efficiency)

LAT = 72.3°F

However, either of two methods can be used to estimate performance:

• Use a Psychrometric chart to calculate wet bulb temperature, and then add 6°F–8°F as described above.
• Use a rule of thumb which estimates that the wet bulb temperature is approximately equal to the ambient temperature, minus one third of the difference between the ambient temperature and the dew point. As before, add 6°F–8°F as described above.

Some rough examples clarify this relationship.

• At 32 °C (90 °F) and 15% relative humidity, air may be cooled to nearly 16 °C (61 °F). The dew point for these conditions is 2 °C (36 °F).
• At 32 °C (90 °F) and 50% relative humidity, air may be cooled to about 24 °C (75 °F). The dew point for these conditions is 20 °C (68 °F).
• At 40 °C (104 °F) and 15% relative humidity, air may be cooled to nearly 21 °C (70 °F). The dew point for these conditions is 8 °C (46 °F).

Because evaporative coolers perform best in dry conditions, they are widely used and most effective in arid, desert regions such as the southwestern USA , Middle East and northern Mexico.