Find what size air conditioner you need with a clear BTU chart by room size, plus adjustments for sun, ceilings and climate.
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The size of the air conditioner you need is measured in BTUs (British Thermal Units per hour), and the starting point is simple: 20 BTU per square foot of floor space. A 300 square foot room needs roughly 6,000 BTU. A 500 square foot room needs roughly 10,000 BTU. From that baseline you adjust for sun exposure, ceiling height, number of occupants, and local climate. Our best air conditioners guide covers top-rated options across all BTU ranges.
Getting the size right matters more than most buyers realize. An undersized unit runs constantly without reaching the set temperature. An oversized unit cools too fast, shuts off before removing enough humidity, and cycles on and off repeatedly — a pattern called short-cycling that stresses the compressor and leaves the room feeling clammy even when it is cool.
Use the chart below as your starting point, then apply the adjustments that match your specific conditions. If you just want the quick reference, the air conditioner BTU chart is the fastest lookup.
| Room Size (sq ft) | Recommended BTU | Typical Use Case |
|---|---|---|
| 100 to 150 | 5,000 BTU | Small bedroom, home office |
| 150 to 250 | 6,000 BTU | Standard bedroom |
| 250 to 350 | 8,000 BTU | Large bedroom, small living room |
| 350 to 450 | 10,000 BTU | Large bedroom, small open plan |
| 450 to 550 | 12,000 BTU | Large living room, studio apartment |
| 550 to 700 | 14,000 BTU | Large open-plan space |
| 700 to 1,000 | 18,000 to 21,000 BTU | Very large room or combined space |
| 1,000 to 1,200 | 21,000 to 24,000 BTU | Whole-floor open plan or combined rooms |
For spaces above 700 square feet, a single window or portable unit often struggles even at maximum BTU. A mini-split or a central system becomes the more effective solution. See the best air conditioners for large rooms for options built for these spaces.
A room with large south- or west-facing windows that receive direct afternoon sun gains heat faster than a shaded room. Add 10% to the BTU figure for a normally sunny room. For a room with full-wall glass or a conservatory-style exposure, add 15 to 20%. Conversely, a heavily shaded north-facing room with good insulation can subtract 10% from the base figure. This single adjustment is often the biggest correction buyers skip.
An easy way to judge: if the room feels noticeably warm within an hour of direct sun hitting the windows even with blinds drawn, treat it as high sun exposure and add 15%. If you rarely open the blinds or the windows face north or northeast, the base figure is appropriate.
The BTU chart above assumes standard 8-foot ceilings. Ceiling height increases the cubic volume of air that needs to be cooled, so taller ceilings need more capacity. Use these multipliers on your base BTU figure:
A 400 square foot room with 12-foot vaulted ceilings needs roughly 14,000 BTU rather than the 10,000 the square footage alone would suggest. Open loft spaces with exposed roof structures and no insulation directly above can add even more heat load — treat those like vaulted ceilings and add 50%.
People generate heat. The base sizing assumes two regular occupants. Add approximately 600 BTU for each additional person who regularly uses the room. A home office where three or four people work together, or a living room used by a large family, may need 1,200 to 2,400 BTU more than the square footage calculation alone suggests.
Large gatherings in a room — dinner parties, game nights — create temporary heat spikes. For rooms used primarily for entertaining groups, size toward the upper end of the BTU range for the square footage. For a home office with only one person, you can size toward the lower end and subtract 600 BTU if the base chart assumed two occupants.
Buyers in hot-humid climates (Gulf Coast, Florida, coastal Southeast) or hot-dry climates (Desert Southwest) should size toward the upper end of the BTU range for their room or add 10% over the chart figure. The unit needs to handle outdoor temperatures in the high 90s to low 100s on peak days, not just an average summer day. In mild climates (Pacific Coast, high-altitude Mountain West), the chart baseline is usually accurate or can even be reduced slightly for spaces used only during the day.
| Climate Zone | Adjustment | Examples |
|---|---|---|
| Hot-humid | +10 to 15% | Florida, Gulf Coast, coastal Southeast |
| Hot-dry | +10% | Phoenix, Las Vegas, inland California |
| Mixed-humid | Base figure | Mid-Atlantic, Tennessee, Kansas City |
| Cold / marine | Base or -5% | Pacific Northwest, New England summers |
| High altitude | -5 to -10% | Denver, Albuquerque, Salt Lake City |
Here are four concrete scenarios with the full calculation shown.
Scenario A: 10×14 bedroom, 8-ft ceiling, one occupant, average sun, mid-Atlantic climate.
Square footage: 140 sq ft. Base BTU: 140 x 20 = 2,800. Nearest standard: 5,000 BTU. Occupancy: one person so subtract 600 BTU — still round up to 5,000. Result: a 5,000 BTU window unit is the right choice.
Scenario B: 16×25 living room, 9-ft ceiling, three occupants, west-facing windows, hot-humid climate (Tampa, FL).
Square footage: 400 sq ft. Base BTU: 400 x 20 = 8,000. Ceiling adjustment (+12%): 8,960. Sun adjustment (+10%): 9,856. Extra occupant (+600): 10,456. Climate (+12%): 11,711. Round up to 12,000 BTU. The best energy efficient air conditioners lists high-CEER 12,000 BTU options worth prioritizing here given year-round use.
Scenario C: Combined kitchen and family room, 850 sq ft, 10-ft ceiling, four occupants, partial west sun, Phoenix, AZ.
Square footage: 850 sq ft. Base BTU: 850 x 20 = 17,000. Ceiling (+25%): 21,250. Partial sun (+7%): 22,738. Two extra occupants (+1,200): 23,938. Climate (+10%): 26,332. This space needs roughly 24,000 to 30,000 BTU — beyond any window unit. A multi-zone mini-split covering the space properly is the right call. See the best mini-split air conditioners for multi-zone options.
Scenario D: Insulated home office, 200 sq ft, 8-ft ceiling, north-facing, one occupant, Portland, OR.
Square footage: 200 sq ft. Base BTU: 4,000. Heavy shade (-10%): 3,600. One occupant (-600): 3,000. Mild climate (-5%): 2,850. A 5,000 BTU window unit is the smallest available standard size and provides a generous buffer for the rare heat wave. This is a case where slight oversizing is fine because Portland summers are short and mild.
It is tempting to buy a larger unit thinking more BTUs means faster, better cooling. In practice, an oversized unit cools the air temperature quickly but does not run long enough to remove humidity effectively. You end up with a room that feels cold but clammy — the thermostat satisfied before the dehumidification cycle completed. Oversized units also wear out faster because of the constant start-stop cycling. Stick within 15% above your calculated target at most. For a full reference, see the air conditioner BTU chart.
Portable air conditioners are rated in BTU like window units, but their effective output is lower because the compressor is inside the room generating heat that the unit must then remove. A 12,000 BTU portable unit performs more like a 9,000 to 10,000 BTU window unit in practice. If you are buying a portable unit, size up by one tier from what the chart shows. The best portable air conditioners and best window air conditioners guides both note effective vs rated output where it is relevant.
Once your calculated BTU need exceeds 14,000, or if you are cooling multiple connected spaces, a mini-split system is often a better fit than any window or portable unit. Mini-splits deliver the same BTU with much higher efficiency and lower noise. The compressor is outside, the indoor unit is compact, and multi-zone systems can cool two or three rooms from one outdoor unit. See the best mini-split air conditioners for options starting around 9,000 BTU through 36,000 BTU.
Once you have your BTU target, the best air conditioners guide is organized by BTU range to make matching a specific model straightforward. For the condensed reference, keep the air conditioner BTU chart open alongside it.
A 400 square foot room with standard 8-foot ceilings and average sun exposure needs about 10,000 BTU. Add 10 to 15% for significant direct sun, or add 12% for 9-foot ceilings. Most 10,000 to 12,000 BTU window units cover this range well.
No. An oversized AC short-cycles -- it reaches the set temperature quickly but shuts off before removing enough humidity, leaving the room feeling cold and clammy. It also causes more compressor wear. Size within about 10 to 15% of your calculated need rather than jumping to a much larger unit.
Room shape matters less than square footage and ceiling height. An L-shaped room with 450 square feet and 8-foot ceilings needs about 12,000 BTU the same as a rectangular room of the same area. If part of the L gets much more sun than the rest, apply the sun adjustment to the whole figure.
Size up by one tier for a portable unit. A portable rated at 12,000 BTU delivers roughly 9,000 to 10,000 BTU of effective cooling because the compressor inside the room generates heat. If the chart says 10,000 BTU and you are buying a portable, buy a 12,000 BTU model.
Measure the longest wall and the shortest wall of the room in feet, then multiply them together to get square footage. For an L-shaped room, divide it into two rectangles, calculate each, and add them. For open floor plans where air flows freely between rooms, include the total square footage of all connected areas the AC needs to cool.
Often yes. Upper floors accumulate more heat from the roof above and from heat rising through the structure. As a rough guide, add 5 to 10% to the BTU figure for a second-floor room compared to the same-size ground-floor room, especially in climates with hot summers. Attic insulation quality makes a significant difference -- a well-insulated attic above the room can narrow or eliminate this gap.
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