Evaporative Cooler CFM Calculator — Swamp Cooler Sizing

Enter your floor area, ceiling height and climate zone, and the calculator applies the swamp-cooler sizing rule — 2 to 3 CFM per square foot depending on how dry your air is. A 1,600 sq ft single-story house in a hot-dry climate needs about 1,600 × 2.0 = 3,200 CFM; the same house in a more humid region wants 4,000–4,800. The calculator also converts the result into air changes per hour (the 20–40 ACH band evaporative coolers live in) and tells you how much open window area the system needs to push air through. That last number is the one swamp cooler owners most often miss.

The trade shortcut is 2–3 CFM per square foot, stepped by climate: 2.0 for hot-dry deserts, 2.5 for typical inland climates, and 3.0 or more where humidity creeps in. At typical 8 ft ceilings that works out to roughly 15–25 air changes per hour — and the wider published guidance for evaporative cooling stretches the band to 20–40 ACH because these machines move enormous volumes of air compared to any other home equipment. When a sizing chart says a 2,000 sq ft house needs a 4,000–5,000 CFM cooler, it is this same arithmetic. An evaporative cooler sizing guide built on CFM per square foot will always land you in the right cooler class.

Because they cool by trading sensible heat for humidity, and the trade only works while fresh air keeps arriving. A refrigerated AC recirculates a sealed volume through a cold coil and can drop the same air 20 degrees on every pass; an evaporative cooler's supply air is only 10–20 degrees below outdoor temperature to begin with, and each cubic foot gives up its cooling potential in a single visit before exiting out your open windows. The 20–40 ACH rule is what keeps the average room temperature between the supply temperature and the outdoor temperature. Throttle the airflow down and you don't save energy — you just let the house drift toward the swamp outside.

Multiply by the climate factor: 1,500 × 2.0 = 3,000 CFM in a dry climate (a 3,200 CFM class unit), 1,500 × 2.5 = 3,750 (the 4,500 class), and 1,500 × 3.0 = 4,500 at the humid margin. With 9 ft ceilings the room volumes grow, so lean to the upper choice. Practically, the common answer is a 4,200–5,200 CFM whole-house unit — which is why 4,500 and 5,200 CFM down-draft coolers dominate the rooftop landscape in the Mountain West. Undersizing is the single most common swamp cooler mistake: the machine runs constantly and the house never quite gets comfortable.

Fading honesty, zone by zone. Evaporative cooling is governed by wet-bulb depression — the gap between the air's dry-bulb and wet-bulb temperatures. In Phoenix that gap is enormous and coolers shine; in Memphis in August the gap nearly closes and the pad adds humidity the air cannot absorb. Below roughly 50°F dew point, results are good; 50–60°F dew point is marginal and demands the full 3 CFM/sq ft plus wide-open exhaust; above that, a swamp cooler mostly makes the house muggy. Two-stage (indirect-direct) coolers extend the range by pre-cooling air without adding humidity, but they cost several times more.

Plan on 1–2 square feet of open window or vent area per 1,000 CFM of cooler — a 4,500 CFM whole-house unit wants 4.5–9 sq ft of escape routes, typically two or three windows cracked 6–10 inches. The openings do double duty: they let the heat-laden air leave and they cap the static pressure so the blower keeps delivering rated CFM. Close the house tight and the cooler suffocates — airflow collapses, humidity saturates the rooms, and the damp environment starts feeding odors and condensation on cool surfaces. Crack windows on the leeward side of the house and the air self-steers across the living space.

Low velocity, high volume: evaporative ducts move air at just 800–1,200 FPM, because the goal is quiet, gentle delivery rather than chip-transport or throw. A 4,500 CFM cooler at 1,000 FPM needs 4.5 sq ft of duct area — roughly a 29-inch round trunk, which is why whole-house coolers usually dump into a large ceiling diffuser or an unused furnace return plenum instead of long duct runs. Undersized ducts choke these blowers fast; the calculator's duct check flags any size that pushes past 1,200 FPM before you buy the collar.

The CFM-per-square-foot math is identical; what changes is the area you feed it. A window cooler is sized on the single room it serves — a 200 sq ft bedroom in a dry climate is 200 × 2 = 400 CFM, so the popular 500 CFM window class covers it with margin. Whole-house units are sized on the entire conditioned footprint and then mounted on roof or wall to dump into a central plenum. The mistake to avoid is double counting: a window cooler in the bedroom plus a whole-house unit both running just fight each other through the open windows you're paying to keep cracked.

It's a different universe: an evaporative cooler targets 20–40 air changes per hour, while a refrigerated AC system typically conditions its ducted airflow at the equivalent of 5–8 room changes per hour and recirculates it. That is not AC being lazy — the AC re-cools the same air dozens of times, so its CFM per ton (around 400) is optimized for coil contact, not air replacement. The swamp cooler's air makes one pass and leaves. This is also why you never mix the logic: an evap cooler sized at AC's 1 CFM per sq ft would be half the airflow it needs, and an AC sized at 3 CFM per sq ft would be an arctic hurricane.

No — and understanding why teaches you the whole machine. An evaporative cooler continuously adds moisture to the supply air; without exhaust, indoor humidity climbs until the pads can no longer evaporate, cooling stops entirely, and you have built a humidifier. The open windows are not wasted energy, they are the exhaust system — the evaporative equivalent of the return duct an AC depends on. If your climate or your doors-and-windows lifestyle can't accommodate permanent openings, the machine that matches your life is refrigerated AC, not a swamp cooler.