Swamp cooler sizing calculator

How to size a swamp cooler

Swamp cooler sizing uses CFM (cubic feet per minute of air moved) instead of BTU. The standard formula is the room's volume times the number of times you want to replace that air per hour, divided by 60 minutes:

CFM = (floor area in sq ft) × (ceiling height in feet) × (air changes per hour) ÷ 60

Air changes per hour scales with how hot your climate gets. Phoenix-tier heat (regular 105 degrees and above) needs 40 air changes per hour because the cooler has to keep ahead of constant outdoor heat soaking back into the house. Moderate dry climates (Denver, Albuquerque, eastern California) use 30. Mild dry climates at elevation use about 22. The simplified shortcut most competitors publish (sq ft times ceiling height divided by 2) bakes in a flat 30 air changes per hour and ignores climate severity, which is why it under-sizes units in Phoenix and over-sizes them in Boise.

Two adjustments apply on top. A well-sealed newer home (built after 2010 with modern weather-stripping and dual-pane windows) needs about 12 percent less CFM because less air infiltrates through the building envelope. An older leaky home (built before 1980, single-pane windows, drafty doors) needs about 12 percent more. The calculator above handles both adjustments automatically.

Will a swamp cooler actually cool your house?

The most important question on this page has nothing to do with the math above. Swamp coolers only work where the summer air is dry. The rule of thumb: if your average summer humidity stays below 40 percent, a swamp cooler pulls room temperatures down 20 to 30 degrees and feels like real air conditioning. Between 40 and 60 percent humidity, the temperature drop shrinks to 10 to 15 degrees. Above 60 percent humidity, you get almost no cooling, and the unit basically adds moisture to an already-damp house. Easy field test: if you can see dew on the grass most summer mornings, a swamp cooler will disappoint you.

Climates where swamp coolers work well: Phoenix, Tucson, Las Vegas, Albuquerque, Denver, El Paso, Reno, Bakersfield, Boise. Marginal climates where they work part of the season: Salt Lake City and Sacramento (delta breezes and monsoon humidity reduce effectiveness for weeks at a time). Climates where they do not work: anything east of central Texas, the Gulf Coast, the Southeast, the Pacific Northwest during summer rain, or Midwest summers. Even Arizona homeowners lose 6 to 8 weeks of effective swamp cooling during the July to September monsoon season, which is when most Phoenix households keep a small window AC for the wet weeks.

The dew point calculator will tell you exactly how dry your local air runs. A summer dew point under 55 degrees means swamp cooling will perform well; over 65 degrees means refrigerant AC is the only real option.

Standard swamp cooler sizes by home square footage

Most residential swamp coolers ship in five common sizes. The price ranges below are unit-only and full-install for current US contractor quotes:

• 3,000 CFM: up to 800 sq ft, single-room or small bungalow. Unit $400-$700, installed $800-$1,500.
• 4,500 CFM: 1,000-1,400 sq ft, 2-bedroom home. Unit $500-$900, installed $1,200-$2,000.
• 6,500 CFM: 1,500-2,200 sq ft, average single-family home. Unit $700-$1,200, installed $1,500-$2,500.
• 8,500 CFM: 2,200-3,000 sq ft, larger home. Unit $900-$1,800, installed $2,000-$3,500.
• Whole-house ducted (10,000+ CFM): any larger size with ductwork. $2,500-$5,000+ installed depending on duct work needed.

Major residential brands at each tier: Phoenix Manufacturing, Mastercool, Bonaire, Champion, and Aerocool. All five sell at hardware stores and online. Phoenix and Mastercool are the volume builders in the Southwest. Quality differences mostly come down to motor build quality (look for sealed bearings, two-speed minimum) and pump reliability rather than the cooling pads themselves.

How much water a swamp cooler actually uses

The cost most homeowners forget when comparing a swamp cooler to refrigerant AC is the water bill. A typical residential swamp cooler uses 3 to 15 gallons per hour depending on its size and how dry the air is. A 4,500 CFM unit running on a hot dry day uses about 7 gallons per hour. Over a typical 120-day cooling season at 8 hours per day, that adds up to about 6,700 gallons of water for the season.

At Phoenix water rates (about $4 per 1,000 gallons), that is roughly $25 for the whole summer. At Denver water rates (about $9 per 1,000 gallons), about $60. Either number is small compared to the $300 to $800 in electricity savings versus running central AC for the same season. The water cost is a real line item but does not change the overall economic case.

Swamp cooler vs central AC operating cost

The reason swamp coolers are still popular in the dry Southwest, despite being objectively worse at cooling than refrigerant AC, is they use about one fourth the electricity for the same hours of operation. Approximate monthly operating costs in summer:

• Central AC (3-ton, 14 SEER2), 1,800 sq ft home in Phoenix: $200-$350 per month.
• Whole-house swamp cooler, same home: $20-$45 per month for electricity, plus $5-$15 in water.
• Window AC unit, single room: $30-$80 per month.
• Portable swamp cooler, single room: $5-$15 per month.

The math is even better in shoulder seasons (May, September, October) when a swamp cooler can hit setpoint while central AC would have to run constantly. For Phoenix and Tucson homeowners, running a swamp cooler May through June and September through October, then switching to AC only during the July-August monsoon, can cut summer cooling costs in half. If you are choosing between swamp cooler and refrigerant AC for a hot dry climate, the AC tonnage calculator sizes the alternative refrigerant system.

Pad type: aspen fiber vs rigid media

The cooling pads inside a swamp cooler come in two main types. Each has tradeoffs:

• Aspen fiber pads: the standard. $5-$15 per pad. Replace every cooling season. About 75 percent efficient at converting dry air to cool air. Cheap, easy to find at any hardware store, smell faintly of wood for the first few hours.
• Rigid media pads (CELdek and similar): $40-$100 per pad. Last 5 to 7 years. About 85 percent efficient, which means the cooler can run smaller for the same cooling output. Worth the premium if you cool for more than 100 days a year.

Some homeowners run aspen pads as long as possible, rinse them weekly, and treat them as a yearly consumable. Others spend the upfront cost on rigid media and forget about pad maintenance for half a decade. Both approaches work; the rigid media just shifts the math so a smaller (cheaper) unit delivers the same cooling.

Common questions about sizing a swamp cooler

What size swamp cooler do I need for a 1,500 sq ft house?

For a 1,500 sq ft home with standard 8-foot ceilings in a hot dry climate (40 air changes per hour), you need about 8,000 CFM. The next stocked size up is an 8,500 CFM unit. In a moderate dry climate (30 air changes per hour), the same home only needs about 6,000 CFM, which rounds up to a 6,500 CFM unit. Use the calculator above to dial in your specific climate and ceiling height.

How much water does a swamp cooler use per hour?

Between 3 and 15 gallons per hour depending on unit size and outdoor dryness. A 3,000 CFM unit uses about 4-5 gallons per hour, a 6,500 CFM unit uses about 8-10 gallons per hour, and a whole-house 10,000 CFM ducted system can use 12-15 gallons per hour on a hot dry day. Drier air evaporates more water per pass, so usage scales with how parched your climate is.

Will a swamp cooler work in humidity?

Not really. Above 60 percent relative humidity, swamp coolers deliver almost no cooling and start to make a damp house feel worse. The unit needs dry incoming air to evaporate water and absorb heat. In humid climates, refrigerant AC is the only option for actual cooling. The dew point calculator will tell you whether your climate has the kind of dry air evaporative cooling actually needs.

How long do swamp cooler pads last?

Aspen fiber pads last one cooling season and should be replaced every spring. Rigid media (CELdek) pads last 5 to 7 years. Both pad types can fail early if hard water leaves mineral deposits that calcify the fibers; a bleed-off line or a quarterly drain-and-flush slows that down.

Is a swamp cooler cheaper than AC?

To buy, yes: a whole-house swamp cooler runs $2,000 to $5,000 installed vs $5,000 to $10,000 for a comparable central AC system. To run, much cheaper: about one fourth the electricity for the same hours, even adding water cost. The catch is the climate restriction. If your humidity is too high for evaporative cooling, the cost difference does not matter because the swamp cooler simply will not cool your house.

How many CFM do I need for a 1,000 sq ft home?

About 5,300 CFM for a hot dry climate at 8-foot ceilings, which rounds up to a 6,500 CFM unit. About 4,000 CFM for a moderate dry climate, which rounds up to a 4,500 CFM unit. The 1,000 to 1,400 sq ft range sits right in the sweet spot for the 4,500 CFM tier from any of the major brands.

Can a swamp cooler cool a whole house?

Yes, with a ducted system. A whole-house swamp cooler (10,000 CFM and up) mounts on the roof and pushes cooled air through standard HVAC ducts to every room, exactly like central AC. The install runs $2,500 to $5,000-plus depending on whether you have existing ductwork and whether your roof can support the unit. Without ductwork, you are looking at one or two smaller units (window or down-draft style) that cool the rooms they sit in plus maybe one adjacent space through open doorways.

If you have determined that your climate is too humid for a swamp cooler, the AC tonnage calculator sizes the refrigerant alternative. For single-room cooling needs, the BTU sizing calculator handles window and portable AC sizing. If your humidity is the main problem rather than heat, the dehumidifier sizing calculator covers that path.