Evaporative Cooler Performance Calculator
Enter outdoor conditions, elevation, and cooler performance to estimate the supply air your direct evaporative cooler can deliver. The result also shows its evaporation rate, sensible capacity, and the airflow needed for an entered sensible load.
Estimated supply-air temperature (°F)73.2
- Outdoor wet-bulb temperature (°F)
- 68.47
- Supply-air relative humidity
- 79.41%
- Temperature drop (°F)
- 26.8
- Evaporated water (gal/h)
- 14.5
- Sensible capacity at target temperature (BTU/h)
- 33,939
- Required airflow for entered load (CFM)
- 4,420
- Entered load met
- 100%
Moist-air calculation details
| Quantity | Value |
|---|---|
| Site atmospheric pressure (kPa) | 97.72 |
| Entering humidity ratio (kg/kg dry air) | 0.01 |
| Outlet humidity ratio (kg/kg dry air) | 0.01 |
| Dry-air mass flow (kg/s) | 2.55 |
Assumes steady direct evaporation, delivered airflow, and adequate relief or exhaust air from the space.
Evaporated water excludes purge, bleed-off, drift, leaks, reservoir dumping, and pump losses.
How to use this calculator
- Enter outdoor dry-bulb temperature and relative humidity from the same weather observation or sensor.
- Enter site elevation so the calculation can account for the lower air density at altitude.
- Use the media saturation effectiveness from the manufacturer, or a conservative value for its condition and face velocity.
- Enter delivered airflow, target indoor temperature, and estimated sensible heat load to compare capacity with demand.
How evaporative cooler performance is calculated
A direct evaporative cooler adds water to outdoor air. As liquid water evaporates, it takes heat from the air, lowering dry-bulb temperature while raising humidity. The limiting temperature is the outdoor wet-bulb temperature, not the outdoor dew point. A real pad does not usually reach that limit, so the calculator applies the entered saturation effectiveness.
First, the calculator converts the entering relative humidity to a humidity ratio. It uses Buck’s saturation-pressure equation and local pressure from elevation: P = 101.325 × (1 − 2.25577 × 10^-5 × z)^5.2559. The entering vapor pressure is pv = RH × pws(T), and humidity ratio is W = 0.621945 × pv ÷ (P − pv).
Wet-bulb temperature is found by matching the entering moist-air enthalpy to saturated-air enthalpy. The calculation then estimates supply temperature with To = Ti − ε × (Ti − Twb), where Ti is outdoor dry bulb, Twb is wet bulb, and ε is media effectiveness. It preserves approximate moist-air enthalpy through the pad to recover outlet humidity.
Airflow matters twice. Entering specific volume converts CFM to dry-air mass flow at the actual site pressure, and water use is the mass flow multiplied by the increase in humidity ratio. Sensible capacity is the dry-air mass flow times moist-air specific heat and the positive difference between target room temperature and supply temperature. The required CFM result scales the entered airflow by the ratio of the entered load to that calculated capacity.
What moves the answer most
Outdoor wet-bulb depression is the main driver. Hot, dry air has a large gap between dry bulb and wet bulb, while humid air has little evaporative cooling potential. Media effectiveness determines how much of that gap the cooler captures. Elevation reduces air mass per CFM, so a high-elevation cooler needs more delivered airflow for the same sensible capacity. Supply air must also be cooler than the target room temperature; otherwise it cannot offset a sensible load.
What this calculator leaves out
This is a steady-state direct-evaporation estimate, not a whole-building indoor-temperature model. It does not model fan heat, duct heat gain, leakage, pad condition, water distribution, relief-air paths, changing weather, or indoor moisture accumulation. Water shown is evaporation only, so purge, bleed-off, drift, leaks, reservoir dumping, and pump losses can make actual water supplied higher.
Worked example
At 100°F and 20% relative humidity, 1,000 ft elevation, and 85% saturation effectiveness, the calculator finds an outdoor wet-bulb temperature of about 68.5°F. A 5,000 CFM cooler therefore supplies air at about 73.2°F, a 26.8°F temperature drop, with outlet relative humidity near 79%.
With an 80°F target room temperature, that airflow provides about 33,900 BTU/h of sensible capacity and evaporates about 14.5 gallons per hour. For a 30,000 BTU/h sensible load, the same conditions require about 4,400 CFM, assuming adequate exhaust or relief air and no additional duct or fan heat.
Common questions
What saturation effectiveness should I use for aspen or rigid media?
Use the manufacturer’s tested value when it is available at the intended airflow. Clean, well-watered direct-evaporative media commonly performs around 70% to 90%, but effectiveness falls with scale, poor water distribution, excessive face velocity, air bypass, or worn pads. A lower value is a prudent planning estimate when the cooler condition is unknown.
How much can a swamp cooler lower the temperature?
The maximum theoretical drop is the difference between outdoor dry-bulb and wet-bulb temperature. Actual supply temperature reaches only the entered fraction of that difference. Hot, dry weather can support a large drop, while humid weather may offer very little cooling even when the outdoor temperature is high.
Why does evaporative cooling work poorly in humid weather?
Humid air is already closer to saturation, so less water can evaporate into it. That makes wet-bulb temperature close to dry-bulb temperature and leaves little temperature reduction for the media to capture. The cooler also delivers more moisture indoors, which can further limit comfort if the building cannot exhaust air.
How much water does an evaporative cooler actually evaporate?
The evaporation result is the water transferred into the airstream from its change in humidity ratio and delivered airflow. It is not the same as total water supplied to the unit. Purge or bleed-off water, overflows, leaks, drift, reservoir draining, and maintenance can make metered use materially higher.
How many CFM does my evaporative cooler need?
The required-airflow result divides your entered sensible heat load by the cooling produced per CFM at the selected weather and target temperature. It is only meaningful when supply air is cooler than the target room temperature. The building must also have adequate relief openings or exhaust so the supplied air can leave without pressurizing the space.
Why does elevation increase the required airflow?
At higher elevation, atmospheric pressure and air density are lower. One CFM therefore carries less dry-air mass, and less mass is available to carry sensible heat away. The temperature result changes little from elevation alone, but the sensible capacity and water evaporation per CFM decline.