Calculator
Swamp Cooler Temperature Calculator
Enter outdoor temperature and humidity to get the wet bulb, the supply air temperature your swamp cooler can reach, and a clear verdict on whether it will work.
A swamp cooler can only cool air toward the outdoor wet-bulb temperature. Enter the outdoor temperature and relative humidity for a typical hot afternoon and pick your pad type. The calculator works out the wet bulb and the temperature of the air coming out of your cooler.
How the calculation works
First, the calculator estimates the wet-bulb temperature from your dry-bulb temperature and relative humidity using the Stull (2011) equation. That is the lowest temperature water evaporation can bring the air to. Then it applies the pad's saturation efficiency:
The verdict uses the same thresholds as our Swamp Cooler Climate Index: supply air at or below 70°F is excellent, 70 to 74°F good, 74 to 78°F marginal and above 78°F poor.
Reference values
| Outdoor temp | 10% RH | 20% RH | 30% RH | 40% RH | 50% RH |
|---|---|---|---|---|---|
| 85°F | 58 | 63 | 67 | 70 | 73 |
| 95°F | 64 | 70 | 75 | 78 | 82 |
| 105°F | 71 | 77 | 82 | 87 | 90 |
| 115°F | 77 | 85 | 90 | 95 | 99 |
Read across any row and you can see why humidity matters so much: at 95°F, going from 10% to 40% humidity costs about 14°F of cooling. Note also that very hot air at a given relative humidity holds far more moisture, which is why 115°F at 30% humidity is a poor day for a swamp cooler.
Making the most of your result
- Marginal result? Check that your pads are fully and evenly wet, and consider upgrading to rigid media.
- Good result but a warm house? The problem is usually airflow or venting. Check relief air and your cooler size.
- Poor result? Read do swamp coolers work in humidity for alternatives and hybrid strategies.
Frequently asked questions
How cold can a swamp cooler make the air?
In theory, down to the outdoor wet-bulb temperature. In practice, rigid media gets about 85 to 90% of the way there and aspen pads about 60 to 80%, so the supply air lands a few degrees above the wet bulb.
At what humidity does a swamp cooler stop working?
There is no single cutoff because temperature matters too, but once the outdoor wet bulb passes about 72°F the supply air stays above roughly 75°F. At 95°F that happens at roughly 30% relative humidity.
Why is my house warmer than the supply air temperature?
Supply air picks up heat from sun on the roof and walls, people, lights and appliances as it moves through the house. A rise of 4 to 10°F between the register and the exhaust window is normal.
Where do I get my local humidity?
Use the current conditions from the National Weather Service or a nearby airport weather station. Afternoon readings matter most, since that is when you need the cooling.