Evaporative 'Swamp' Cooler Sizing Calculator

What is The Physics of Swamp Coolers (Evaporative CFM)?

Mechanical Air Conditioners run in a closed loop, constantly re-cooling the same stale air inside the house while condensing moisture out. Evaporative coolers (Swamp Coolers) are the exact opposite. They are 100% fresh-air machines. They pull dry, blistering hot outside air across a wet pad, dropping the sensible temperature drastically while adding humidity. Because they constantly push new air into the house, they require catastrophic volumes of airflow (CFM) to function, and they will completely fail if you do not open the physical windows of the house to let the old air escape.

Mathematical Foundation

Total Building Volume

V = Area \times Height

V

= The total cubic volume of the house in Cubic Feet.

Area

= The square footage footprint of the space being cooled.

Height

= The height of the ceilings (average is 8 ft).

Required Evaporative CFM

CFM = \frac{V}{T_{exchange}}

CFM

= Cubic Feet per Minute required to effectively drop the temperature.

T_{exchange}

= The required Air Exchange Time (in minutes). You must replace all the air in the house every 2 minutes in a desert, or every 3 minutes in a moderate climate.

Laws & Principles

THE WINDOW RULE (EXHAUST): A swamp cooler does not work in a sealed house. It is a giant air pump. To push 6,000 CFM of cold wet air into the house, 6,000 CFM of hot stale air must exit the house. You must provide 2 square feet of open window space for every 1,000 CFM the cooler creates. If you do not, the house pressurizes, airflow immediately stalls, and the house turns into a hot sauna.
THE HUMIDITY CEILING: Evaporative coolers strictly rely on dry heat. If the outdoor Relative Humidity exceeds 40%, the water on the pads physically cannot flash-evaporate. The air absorbs no latent heat, the sensible temperature does not drop, and the machine simply becomes a loud fan blowing damp hot air. They are useless in Florida and mandatory in Arizona.
EXCHANGE RATES BY CLIMATE: In extreme deserts like Phoenix or Death Valley, the sun beats heat through the roof so aggressively that you must replace the entire volume of air inside the house every 2 minutes (T = 2). In a high desert like Reno or a moderate valley, every 3 minutes (T = 3) is sufficient.

Step-by-Step Example Walkthrough

" A homeowner in a scorching Arizona desert wants to install a roof-mounted swamp cooler. The house is 1,800 square feet with standard 8-foot flat ceilings. "

1. Find Total Volume: 1,800 sq ft × 8 ft ceiling = 14,400 Cubic Feet of total air mass.
2. Identify Exchange Limit: Because it is a hot desert, the air must completely turn over every 2 minutes.
3. Calculate CFM: 14,400 Total Volume ÷ 2 Minutes = 7,200 CFM.
4. Calculate Exhaust Requirement: 7.2 (Thousands of CFM) × 2 sq ft = 14.4 square feet of open windows required throughout the house.

Final Result: To keep the house cool, the homeowner must purchase a unit rated for at least 7,200 CFM. Furthermore, they must walk through the house and strategically crack open windows totaling about 14 square feet of open air-space to allow the heat to escape.

Quick Answer: How do you calculate Evaporative Swamp Cooler Size?

To accurately size a Swamp Cooler, you must calculate the total cubic volume of the house and divide it by the required air exchange rate. First, multiply the square footage of your home by the ceiling height (e.g., 1,500 sq ft × 8 ft = 12,000 cubic feet). Next, if you live in a hot desert, divide that volume by 2 minutes. If you live in a moderate dry climate, divide it by 3 minutes. The resulting number (e.g., 12,000 ÷ 2 = 6,000) is the exact Cubic Feet per Minute (CFM) rating you must purchase to effectively cool the home.

The Air-Pump Math

Required CFM = (Square Footage * Ceiling Height) / Minutes per Exchange

Scaling Variables:

Ceiling Height Volumetrics: Vaulted ceilings destroy swamp cooler sizing. A 20-foot vaulted ceiling requires more than double the CFM of a flat 8-foot ceiling, even if the square footage of the floor is identical. Swamp coolers cool the air volume, not the floor.
Climate Divider: The hotter the climate, the smaller the divider. Dividing by 2 (desert) creates a massive CFM requirement. Dividing by 4 (mild plains) cuts the blower size requirement in half.

Typical Climate Exchange Rates

Climate Geography	Exchange Rate	Description
Extreme Heat (Arizona/Death Valley)	1.5 Minutes	Requires absolute maximum turnover due to extreme solar radiation load on roof.
Standard Desert (Nevada/Utah)	2.0 Minutes	The baseline standard for sizing any typical swamp cooler in arid conditions.
Moderate Dry Plains (Colorado)	3.0 Minutes	Allows for smaller blower motors since heat penetration is less aggressive.
High Altitude/Cool (Mountain)	4.0 Minutes	Very small blowers are sufficient to overcome the mild ambient heat gain.

Catastrophic Failures & False Readings

The Closed-Window Sauna Effect

Homeowners accustomed to traditional air conditioning instinctively close all the windows and doors tightly when they turn on their new swamp cooler. This is catastrophic. The swamp cooler will violently pump cold wet air into the house for 10 seconds until the house pressurizes. Once pressurized, the blower fan enters 'stall'—the motor spins but moves zero air. The trapped moisture builds up, and the house turns into an active humid jungle. You must physically open windows to create a draft path for the hot air to be purged.

The Monsoon Death Curve

An evaporative cooler relies on dry, thirsty air. When the air is dry, water evaporates off the cooler pads aggressively, absorbing 1,061 BTUs from the air per pound of water evaporated, plummeting the temperature by 20 to 30 degrees. However, during 'Monsoon Season' in August when desert humidity spikes, the air is already saturated. The water on the pads cannot evaporate. The sensible cooling drops to zero, and the cooler just blows hot, wet air into the house.

Field Design Best Practices & Pro Tips

Do This

✓Use up-ducts for security. Instead of forcing the homeowner to leave windows unlocked and cracked open to exhaust the air, install ceiling 'up-ducts' into the attic. These barometric dampers automatically blow open when the swamp cooler turns on, venting the cold air through the living space, up into the attic (cooling the attic down drastically), and out the roof vents.

Avoid This

✗Never undersize the blower. Swamp cooling is purely a volumetric game. A traditional AC can be slightly undersized and just run longer to dehumidify. A swamp cooler cannot. If you put a 4,000 CFM cooler on a house that requires 7,000 CFM, the air simply will not move fast enough to purge the heat out of the windows, and the house will permanently overheat. Always round up to the next motor size.

Frequently Asked Questions

How do you calculate Swamp Cooler CFM?

Calculate the total cubic volume of the house (Square Footage x Ceiling Height) and divide it by the climate air exchange factor. For a hot desert, divide the volume by 2. For a moderate climate, divide it by 3. The resulting number is the exact CFM your swamp cooler must output.

Why do I have to open windows for an evaporative cooler?

Unlike a refrigerated AC that recirculates inside air, a swamp cooler constantly pumps massive amounts of new, outside air into the house. If you do not open windows, the house pressurizes like a balloon. The fan will stall against the backpressure, no cool air will enter, and the house will turn into a hot sauna.

How many windows should be open for a swamp cooler?

The standard rule is that you must provide 2 square feet of open window or vent space for every 1,000 CFM produced by the cooler. Therefore, a 5,000 CFM swamp cooler requires exactly 10 square feet of total open window space spread throughout the house to exhaust properly without choking the fan.

Does a swamp cooler work in high humidity?

No. Swamp coolers cool the air exclusively through evaporation. As water evaporates into dry air, it consumes heat energy. If the outdoor air is already saturated with humidity (above 40% RH, or during a monsoon), the water cannot evaporate. It will not drop the temperature, and will simply blow damp, muggy air onto you.

Swamp Cooler Sizing Matrix

Volumetric Swamp Cooler Engine

Structural Dimensions

Climate Parameters

Total Blower Power Needed