How to Size a Swamp Cooler: The CFM Formula, Climate Factors and Worked Examples
Size a swamp cooler in five steps: CFM = sq ft x ceiling height x air changes / 60. Climate air change rates, duct loss margins and worked examples by city.
Short answer
Size a swamp cooler by airflow: multiply floor area by ceiling height to get cubic feet, multiply by the air changes per hour your climate needs (about 40 in hot desert, 30 in moderate dry, 20 in mild dry areas), then divide by 60. A 1,500 sq ft home with 8 ft ceilings in Phoenix needs about 8,000 CFM.
Key takeaways
- CFM = floor area x ceiling height x air changes per hour / 60.
- Use 40 air changes per hour in hot desert climates, 30 in moderate dry climates and 20 in mild or coastal-dry areas.
- Shortcut: divide house volume by 1.5, 2 or 3 minutes per air change.
- Manufacturer ratings are often at zero static pressure, so add 10-30% margin for ducted installs.
- Bump up one climate tier for big west-facing glass, poor insulation or dark roofs.
On this page
A swamp cooler is sized by how much air it moves, measured in cubic feet per minute (CFM). The method is simple: work out the volume of the space you want to cool, decide how often that air needs to be replaced for your climate, and convert to CFM. The rest of this page explains why that works, gives the air change rates to use, and walks through the adjustments that separate a cooler that keeps up in August from one that loses the afternoon.
If you just want the number, the cooler size calculator runs the same formula used below.
Why swamp coolers are sized by air changes
A refrigerated air conditioner recirculates indoor air and removes heat with a compressor, so it is sized in tons of cooling capacity. An evaporative cooler works differently. It is a once-through system: it pushes cooled outdoor air in, and that air picks up heat from walls, people, appliances and sunlight as it crosses the house before leaving through open windows or relief vents.
The faster you replace the air, the less each cubic foot warms up before it leaves, and the closer indoor temperature stays to the cooler's supply temperature. That is why sizing is expressed as air changes per hour (ACH): how many times per hour the cooler replaces the full volume of the house. Hotter climates add more heat through the building shell and need faster flushing. How the supply air itself gets cold is explained in how evaporative coolers work.
The sizing formula and climate factors
The division by 60 converts air changes per hour into air per minute. Use the air change rate for your climate:
| Climate | Example cities | Air changes per hour | One full air change every | Shortcut |
|---|---|---|---|---|
| Hot-dry desert | Phoenix, Las Vegas | 40 | 1.5 minutes | Volume / 1.5 |
| Moderate dry | Denver, Albuquerque, Boise | 30 | 2 minutes | Volume / 2 |
| Mild or coastal-dry | Mild inland and coastal-dry areas | 20 | 3 minutes | Volume / 3 |
The shortcut column follows directly from the formula: 40 / 60 is the same as dividing by 1.5, 30 / 60 is dividing by 2, and 20 / 60 is dividing by 3. So a 12,000 cubic foot house needs 8,000 CFM in the desert, 6,000 CFM in a moderate dry climate and 4,000 CFM in a mild one.
Not sure which tier you are in? Look up your city in the climate index, which lists design temperatures for 50 US cities, or read best climates for swamp coolers. If your summers swing between tiers, size for the hotter one.
Step-by-step sizing
- Measure the cooled floor area. Add up the square footage of every room the cooler will supply. Leave out closed-off rooms, unconditioned garages and storage spaces that will not get air.
- Find the ceiling height. Use 8 ft for standard ceilings. For vaulted rooms, use the average height or calculate those rooms separately.
- Calculate volume. Floor area x ceiling height = cubic feet.
- Apply the air change rate. Volume x 40, 30 or 20, then divide by 60.
- Adjust for heat gain. Move up one tier (for example from 30 to 40 ACH) if the house has large unshaded west or south glass, little attic insulation, a dark roof, or many occupants.
- Adjust for duct losses. Divide by a delivery factor (see below) to find the rated CFM to shop for.
- Pick the cooler. Choose the model whose rating meets or modestly exceeds the result, preferably with a two-speed motor.
Worked examples by climate
Worked example 1: Phoenix, 1,800 sq ft, 8 ft ceilings
Volume: 1,800 x 8 = 14,400 cubic feet. Hot-dry desert at 40 ACH: 14,400 x 40 / 60 = 9,600 CFM delivered. With a downdraft unit on a short drop to a central diffuser (delivery factor about 0.9), shop for 9,600 / 0.9 = 10,667 CFM rated. That is above many single residential units, so this home is a candidate for a large unit or two coolers serving the bedroom wing and the living areas separately.
Worked example 2: Denver, 1,400 sq ft, mixed ceilings
1,100 sq ft has 8 ft ceilings (8,800 cubic feet) and a 300 sq ft great room rises from 8 to 14 ft, an average of 11 ft (3,300 cubic feet). Total 12,100 cubic feet. Moderate dry at 30 ACH: 12,100 x 30 / 60 = 6,050 CFM. Attic ducts with several runs (delivery factor about 0.8) mean shopping near 6,050 / 0.8 = 7,563 CFM rated, or improving the ducts so a 6,500 to 7,000 CFM unit can do the job.
Worked example 3: Mild coastal-dry, 1,200 sq ft, 8 ft ceilings
Volume: 9,600 cubic feet. At 20 ACH: 9,600 x 20 / 60 = 3,200 CFM. A through-wall side-draft with almost no duct (delivery factor about 0.95) needs roughly 3,400 CFM rated. Here a large window unit could even cover the main living space.
Rated CFM vs delivered CFM
Manufacturer-rated CFM is often measured at zero static pressure: the blower moving air into open space with nothing in the way. Real installations add resistance from the discharge transition, duct runs, elbows, dampers and grilles. As resistance (static pressure) rises, a blower moves less air. Ducted installs therefore deliver less than the label says.
| Installation | Typical delivered share of zero-static rating | Divide required CFM by |
|---|---|---|
| Through-wall or short drop to one diffuser | About 90-95% | 0.9-0.95 |
| Short, well-sized duct system | About 80-90% | 0.8-0.9 |
| Long attic ducts, several elbows, small registers | About 70-80% or less | 0.7-0.8 |
These factors are planning estimates. If the manufacturer publishes airflow at 0.2 or 0.3 in of static pressure, use that figure instead. The physics behind static pressure and fan curves is in CFM explained, and duct sizing is covered in ductwork for evaporative coolers.
Why airflow and pad efficiency work together
Airflow sets how much heat the cooler can carry out of the house. The sensible cooling it delivers to the space is:
Worked example: the same 9,600 CFM with different pads
On a 105 F Phoenix afternoon with a 70 F wet bulb, an 8 in rigid media cooler at 88% efficiency supplies 105 - 0.88 x 35 = 74.2 F air. If you want to hold 82 F indoors, each cubic foot can absorb 7.8 F of heat: 1.08 x 9,600 x 7.8 = 80,870 BTU/h. An aspen cooler at 75% supplies 78.75 F, leaving only 3.25 F of headroom: 1.08 x 9,600 x 3.25 = 33,696 BTU/h. Same airflow, less than half the useful cooling.
The lesson: on the hottest days, an undersized aspen cooler loses twice. If you cannot add airflow, better media is the next lever. See aspen vs rigid media and check supply temperatures for your weather with the cooling temperature calculator.
Oversizing, undersizing and relief air
An undersized cooler runs on high all afternoon, and indoor temperature climbs steadily from midday onward. It is the most common complaint technicians hear, and the fix is either more airflow, better pads, lower heat gain or a second unit.
Moderate oversizing (up to roughly 20 to 25% over the calculated figure) is usually fine because a two-speed motor lets you run on low through mornings and evenings. Large oversizing wastes money and creates noise, and it pushes more humid air than the house can exhaust unless you open more relief area.
Relief air is part of sizing. A cooler can only deliver its airflow if the same amount of air can leave. A common field rule is roughly 1 to 2 sq ft of open window or relief vent area per 1,000 CFM. The Phoenix example above, at 9,600 CFM, needs on the order of 10 to 19 sq ft of total opening spread across the rooms you want cooled. See venting and relief air.
Pro tip: if a cooler that is correctly sized on paper feels weak, open more windows before blaming the cooler. Pressurized houses with too little relief area are behind a large share of "not enough air" service calls.
When your required airflow is set, the whole-house buying guide covers choosing the unit. For rooms, garages and patios, see sizing portable and outdoor coolers.
Frequently asked questions
What size swamp cooler do I need for 1,000 square feet?
With 8 ft ceilings, 1,000 sq ft is 8,000 cubic feet. That works out to about 5,333 CFM in a hot desert climate (40 air changes per hour), 4,000 CFM in a moderate dry climate (30) and about 2,667 CFM in a mild dry climate (20).
What size swamp cooler do I need for 2,000 square feet?
With 8 ft ceilings, about 10,667 CFM in a hot desert climate, 8,000 CFM in a moderate dry climate and 5,333 CFM in a mild dry climate. Homes at the high end often use two coolers or a large unit with well-sized ducts.
Is it bad to oversize a swamp cooler?
Moderate oversizing is fine with a two-speed motor, because you can run on low most of the time. Large oversizing raises cost and noise and demands more relief air, otherwise indoor humidity builds.
Should I include the garage in my cooler sizing?
Only if the cooler will actually supply air to the garage through a duct or open door. Otherwise size for the conditioned living space and treat the garage separately.
Do I size a swamp cooler by tons like an air conditioner?
No. Evaporative coolers are rated and sized by airflow in CFM, because their cooling depends on air volume and the outdoor wet bulb, not on a fixed refrigeration capacity.
How do I size for vaulted ceilings?
Use the average ceiling height across the room, or calculate the vaulted area's volume separately and add it. A room that rises from 8 ft to 14 ft has an average height of about 11 ft.
Sources and further reading
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- ASHRAE Handbook: HVAC Applications (Evaporative Cooling chapter), ASHRAE
- Evaporative Coolers, Colorado State University Extension
- Climate Data Online, NOAA National Centers for Environmental Information