Sizing Portable and Outdoor Evaporative Coolers for Rooms, Garages and Patios

Portable evaporative coolers are sized two different ways depending on where they run. Inside a room, garage or shop, you have a defined volume of air to replace, so you use the same air-change formula as a whole-house cooler. Outdoors, there is no volume to fill and the breeze carries cooled air away, so you size for the zone where people actually sit or work. This page covers both, with worked examples and the water numbers that decide whether a tank is practical.

For choosing features and pad types, see the portable evaporative cooler buying guide.

Enclosed spaces: use the air-change formula

Use 40 air changes per hour in hot-dry desert climates such as Phoenix and Las Vegas (a full change every 1.5 minutes), 30 in moderate dry climates such as Denver, Albuquerque and Boise (every 2 minutes), and 20 in mild or coastal-dry areas (every 3 minutes). These are the same rates used in how to size a swamp cooler and the cooler size calculator.

Two things follow from the table. Residential portables, which top out around 1,000 to 1,500 CFM, are well matched to bedrooms and small living rooms but cannot fully serve a 2-car garage in the desert. And shops and large garages are firmly in commercial portable territory.

Adjustments for portable ratings and heat load

Add margin for optimistic ratings

Portable airflow ratings rarely reflect the resistance of wet pads, grilles and louvers, and coverage claims in square feet are not standardized. As a planning rule, buy about 25% more rated CFM than your calculation (divide the result by 0.8). For the primary bedroom above in a hot desert, that is 1,195 / 0.8 = about 1,500 CFM rated. Why ratings and real airflow differ is explained in CFM explained.

Move up a tier for hot spaces

Garages and metal buildings often gain far more heat than a house: uninsulated roll-up doors in afternoon sun, metal roofs, a hot car engine, welders or compressors. For those spaces, use the hot-desert rate of 40 air changes per hour even in a moderate dry climate, and consider more airflow if the building is uninsulated. Garage specifics are in swamp cooler for garage.

Relief air and placement indoors

Portables need an exit path for the humid air they produce, exactly like fixed coolers. A common field rule is roughly 1 to 2 sq ft of opening per 1,000 CFM. Place the opening on the far side of the space so cooled air sweeps through the area you use.

Placement matters as much as size:

• Pull outdoor air if you can. A portable placed near an open window, facing in, draws drier outdoor air into its pads instead of re-processing increasingly humid room air.
• Aim at people, not walls. Moving air adds a wind-chill effect. A unit pointed at the bench or bed feels far more effective than one aimed at the center of the room.
• Keep the intake clear. Leave at least a foot or two behind the pads.

The principles are the same as in venting and relief air.

Outdoor spaces: size by the cooling zone

On a patio, at an outdoor event or in an open-sided barn, the cooled air does not accumulate. It leaves the unit as a stream that spreads, slows and mixes with the surrounding air. What you can cool is the zone that stream reaches before it warms back up. That changes the sizing questions:

1. How far is the farthest person? Choose a unit whose listed throw distance comfortably exceeds it. Residential units typically give a noticeable effect within roughly 10 to 20 ft; large commercial portables reach much farther.
2. How wide is the area? A single stream covers a band roughly as wide as the unit near the outlet and spreading with distance. For wide seating, use oscillating louvers or more than one unit.
3. Which way is the wind? Place the cooler upwind of the area so the breeze carries cool air toward people instead of away. In a steady breeze, an unfavorable placement can erase most of the effect.
4. Is the space partly enclosed? Covered patios with two or three walls hold cool air better. Treat a nearly enclosed space as a room and use the air-change formula.

More on patio setups is in outdoor patio evaporative coolers. For warehouses, loading docks and large shops, see commercial and warehouse evaporative cooling.

Water use: tanks vs hose hookups

Bigger airflow means more evaporation. You can estimate it with the sensible cooling formula and the roughly 1,060 BTU absorbed by each pound of water evaporated:

For a 1,000 CFM residential portable with the same 20 F drop the figure is about 2.4 gallons per hour at full rating, and somewhat less in practice. Run your own numbers with the water usage calculator.

Common sizing mistakes

• Trusting square-foot coverage claims. They are not based on a common test. Calculate CFM for the actual space.
• Forgetting the ceiling. A garage or shop with 10 to 14 ft ceilings holds 25 to 75% more air than an 8 ft room of the same floor area.
• Running in a sealed room. Without relief air, humidity climbs and the cooler's output fades, which looks like an undersized unit but is not.
• Buying for the worst corner. One portable cannot reach behind shelving or around corners. Two smaller units aimed at two work zones often beat one large unit.
• Ignoring humidity. On humid days the supply air is barely cooler than outdoor air, no matter the CFM.

When a portable is the wrong answer

If your calculation lands well above what portables deliver for the space, or you find yourself planning three units for one room, step up to a fixed solution. A window unit gives 2,000 to 4,500 CFM from a single opening (see window evaporative coolers), and a whole-house cooler covers the entire home. In humid weather no amount of airflow will help much; check your conditions with the cooling temperature calculator before buying.