BTU Calculator — Air Conditioner BTU Calculator

Choosing between a window unit, portable AC, and mini-split

Window air conditioners cover roughly 100–500 sq ft and are sold in 5,000–24,000 BTU/hr increments — the right size for a bedroom or a small apartment. Portable units carry the same BTU ratings but lose roughly 10–15% of effective capacity through their exhaust hose because the hose draws hot air back in from adjacent rooms; when shopping a portable unit, size up one step from this calculator's estimate. Mini-splits (ductless heat pumps in cooling mode) cover 600–2,000+ sq ft, are available in 9,000–36,000 BTU/hr, and cool far more efficiently than window units — measured in SEER rather than raw BTU — making them the right tool for rooms the main duct system does not reach.

The shopping workflow: use the BTU estimate to identify your tonnage bracket (divide by 12,000), then compare models on SEER rating within that bracket. A 12,000 BTU/hr window unit with SEER 12 uses 1,000 watts per hour at full load; a SEER 18 model doing the same work uses roughly 667 watts. In a climate with 1,000 annual cooling hours, the SEER 18 unit saves about 333 kWh — worth $40–60 per year at average US rates — which usually pays back the price premium within three seasons.

When to add a safety margin to your cooling estimate

Several room characteristics justify sizing up slightly from the calculated figure. West-facing rooms with large unshaded windows experience late-afternoon sun that the "sunny" multiplier underestimates on peak summer afternoons — add 10–15% in those cases. Rooms above a garage or below an uninsulated roof have extra conductive heat gain that the floor-area method misses; similarly, a room that shares no insulated walls with the outside (interior rooms) can run on slightly less capacity than the formula suggests. High humidity climates — Gulf Coast cities, the Pacific Northwest — put extra demand on the dehumidification stage of cooling, so erring a few hundred BTU/hr above the estimate keeps runtime long enough to wring moisture out.

Conversely, do not size up as a general hedge. An oversized window unit in a 400 sq ft bedroom that would be comfortable at 8,000 BTU/hr will short-cycle when rated at 12,000 — the compressor shuts off in 5–7 minutes, the thermostat stays satisfied while relative humidity climbs, and the unit wears faster. The "biggest one they had in stock" approach costs more to buy, more to run, and more to replace.

Questions

How many BTU to cool a 250 sq ft sunny room in a hot climate?

The engine returns 6,100 BTU/hr — base 5,000 BTU (250 × 20), then × 1.1 for sunny and × 1.1 for hot climate = 6,050, rounded to 6,100. Shop for a 6,000 BTU window unit, which is the closest standard size, and your room should cool comfortably without short-cycling.

Should I buy a portable AC or a window unit for the same BTU?

Window units are more efficient. A portable AC exhausts hot air through a hose that creates a slight negative pressure in the room, drawing warm outside air back in through gaps — effectively fighting itself. Portable units also run warmer compressors than their BTU labels suggest, so in practice their effective cooling is 10–15% lower than the rated figure. For a given BTU need, a window unit will cool the room faster, maintain lower humidity, and use less electricity.

What is SEER and how does it relate to BTU?

SEER (Seasonal Energy Efficiency Ratio) measures how many BTU of cooling a unit delivers per watt-hour of electricity over a full season. A 12,000 BTU/hr unit with SEER 15 draws 800 watts at full load. Higher SEER means the same BTU output at lower wattage — better efficiency. BTU tells you the right size; SEER tells you the running cost once you know the size. Minimum federal SEER for new window units (2023+) is around 12; mini-splits commonly reach SEER 20–30.

More ways to use this calculator

Start with the main btu calculator or compare the other published scenarios.