Swamp Cooler Sizing
Air Flow
Much of a swamp cooler's ability to cool is based on its air flow. For this reason they are sized by the amount of air flow they produce.
CFM (Cubic Feet per Minute) /
CMM (Cubic Meters per Minute)
To find the right sized unit for the area you want to cool, first look at the cooler's CFM or CMM rating. This number measures the amount of air a swamp cooler can move per minute.
For example, if you're holding an outdoor event in a tent that's 10' x10' x10' (1,000 cubic feet), you'll need a cooler with at least 500 CFM (1,000 cubic feet divided by 2 = 500).
In international measurements:
If you're holding an event in a tent that's 3m x 3m x 3m (27 cubic meters) you'll need a cooler with at least 13.5 CMM (27 cubic meters divided by 2 = 13.5).
A unit with more air flow won't increase the evaporative cooling effect,
but it will increase the amount of breeze you'll feel.
If a manufacturer or retailer only gives you CFM or CMM, you may want to convert it to measurements you're more comfortable with:
1 cubic foot × 35.315 = 1 cubic meter; 1 cubic meter × .0283 = 1 cubic foot.
This is less air flow than the amount needed by an outdoor fan to cool the same sized area. That's because swamp coolers have the additional cooling power of evaporation.
Swamp coolers need fresh air to provide the most cooling. Because they add moisture to the cooled air, if this same air goes through the system again it will pick up even more moisture. If this keeps up, the air will pick up so much moisture it will no longer be able to provide any cooling. At that point you'd be experiencing a hot, muggy mess.
The simple solution is to provide fresh air -- at least 2 square feet for every 100 square feet / .186 square meters for every 9.3 square meters you want to cool. This makes portable coolers well suited to partly enclosed outdoor areas. A tent, cabana or patio with an open side or a shed, trailer or booth with open windows can contain the cooled air and still have plenty of fresh air for the cooling system.
BTUs (British Thermal Units)
The most important measurement when sizing a swamp cooler is its CFM / CMM. But a few manufacturers also measure their coolers' BTU ratings, which can be helpful as a general guideline. BTUs measure the evaporative part of the cooling capacity. Once you find a swamp cooler with the right amount of air flow, see if it has about enough BTUs for the size of area you want cooled.
Since its ability to cool is based on the level of humidity in the air, swamp coolers with more BTUs won't necessarily provide more cooling. They will have the POTENTIAL for more cooling, though. That potential will be more available on hot, dry days than on more humid days.
Air conditioners and some evaporative coolers are measured for BTUs, but this measurement has different importance to each method of cooling. BTUs are the most important measurement of an air conditioner's cooling capacity. But CFM / CMM is the most important measurement of a swamp cooler's cooling capacity.
| AREA TO BE COOLED | CAPACITY NEEDED | |
| Square Feet | Square Meters | BTUs per hour |
| 100 to 150 | 9.3 to 14.0 | 5,000 |
| 150 to 250 | 14.0 to 23.3 | 6,000 |
| 250 to 300 | 23.3 to 27.9 | 7,000 |
| 300 to 350 | 27.9 to 32.5 | 8,000 |
| 350 to 400 | 32.5 to 37.2 | 9,000 |
| 400 to 450 | 37.2 to 41.8 | 10,000 |
| 450 to 550 | 41.8 to 51.1 | 12,000* |
| 550 to 700 | 51.1 to 65.0 | 14,000 |
| 700 to 1,000 | 65.0 to 92.9 | 18,000 |
| 1,000 to 1,200 | 92.9 to 111.5 | 21,000 |
| 1,200 to 1,400 | 111.5 to 130.1 | 23,000 |
| 1,400 to 1,500 | 130.1 to 139.4 | 24,000* |
| 1,500 to 2,000 | 139.4 to 185.8 | 30,000 |
| 2,000 to 2,500 | 185.8 to 232.3 | 34,000 |
BTU ratings are not as precise for evaporative coolers as they are for air conditioners. Swamp coolers need lots of fresh air in the area they are cooling to help balance humidity levels. This is different from an air conditioner that needs an enclosed area to more precisely control the cooling level. Its simpler to estimate the number of BTUs needed to cool an enclosed area that is unaffected by humidity levels than a partially enclosed area whose cooling is based on humidity levels.
Here is a chart showing about how many BTUs are suggested to cool certain sized areas with an air conditioner. It will give you a general starting point with swamp coolers. These numbers will vary by manufacturer and the actual conditions of your area.
Manufacturers often use a kind of shorthand to refer to the sizes of their units. Any unit with a cooling capacity that is a multiple of 12,000 BTUs is said to provide so many "tons" of cooling. A 12,000 BTU unit is called a "1-ton" unit. A 24,000 BTU unit is called a "2-ton" unit, 36,000 BTUs is called a "3-ton" unit and so on.
When you purchase or rent a portable swamp cooler, consider the conditions of the area you’ll be cooling, including the weather and size of area. If you use a sales or rental consultant, this information will help them advise you on the best size and type of unit for your needs.
