Calculator
Swamp Cooler Water Usage Calculator
Estimate how many gallons per hour, day and month your evaporative cooler uses, including bleed-off, and what that water costs at your local rate.
A swamp cooler uses water in two ways: the water that evaporates to cool the air, and the water deliberately dumped (bled off) to keep minerals from building up. This calculator estimates both from your cooler's airflow and the weather.
How the calculation works
The temperature drop across the pads comes from the outdoor wet bulb and pad efficiency, exactly as in our cooling temperature calculator. The heat removed from the air stream is 1.08 × CFM × temperature drop BTU per hour. Dividing by the latent heat of water (about 1,060 BTU per pound) and the weight of a gallon (8.34 pounds) gives:
Bleed-off is added as a percentage of evaporation. Read bleed-off and water quality to set the right rate for your water.
Worked example: 5,000 CFM cooler on a 98°F, 18% humidity afternoon
The wet bulb is about 67.5°F, a depression of roughly 30.5°F. Rigid media at 88% drops the air about 27°F. Evaporation is 5,000 × 27 ÷ 8,186 = about 16.4 gallons per hour on the hottest afternoon hours. Mornings and evenings are cooler with smaller drops, so a full day averages well below that peak. With 50% bleed-off the peak is about 25 gallons per hour.
Why real-world use is often lower
- Many coolers deliver 20 to 40% less airflow than their rating once ducts are attached.
- The temperature drop is smaller in the morning and evening, when the cooler often runs on low speed.
- Pads that are partly dry evaporate less (and cool less).
For a full discussion, including how cooler water use compares with the water used to generate electricity for air conditioning, read how much water a swamp cooler uses.
Frequently asked questions
How much water does a whole-house swamp cooler use per day?
A typical 4,000 to 6,500 CFM unit evaporating 5 to 12 gallons per hour on hot afternoons, plus bleed-off, commonly uses 50 to 200 gallons over a full cooling day. Desert heat and hard water push it to the high end.
Does more humidity mean less water use?
Yes. On humid days the air can absorb less water, so evaporation and the temperature drop both fall. The cooler uses less water precisely because it is doing less cooling.
What bleed-off rate should I use?
Soft water can manage with bleed-off around 25% of evaporation, average water with about 50%, and very hard water with 75 to 100%. Too little bleed-off lets scale build up on pads and the pan.
Why is the calculator based on CFM and temperature drop?
Every pound of water that evaporates absorbs about 1,060 BTU, and that heat comes out of the air stream. Knowing airflow and temperature drop gives the heat removed, which fixes the water evaporated.