An air conditioner BTU chart matching BTU to room size in square feet, with adjustments for sun, ceilings, climate and occupancy.
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The standard starting point is 20 BTU per square foot of room area. A 150 square foot bedroom needs about 3,000 BTU. A 300 square foot living room needs roughly 6,000 BTU. A 500 square foot open-plan space needs around 10,000 BTU. Those are the baseline numbers before any adjustments for sunlight, ceiling height, climate, or how many people regularly occupy the room.
Getting BTU wrong in either direction costs you. An undersized unit runs constantly, never fully cools the room, and wears out faster. An oversized unit cools the air temperature too quickly without running long enough to remove humidity, leaving the room feeling damp and clammy even at the set temperature. Matching BTU to your specific room conditions is worth a few minutes of math. For top-rated window and portable options at the right BTU ranges, see the best air conditioners overview and the best window air conditioners page.
This guide provides a full BTU-by-room-size chart, explains every standard adjustment factor with specific numbers, and works through examples so you can calculate the right BTU for any room in your home.
| Room size (sq ft) | BTU needed (baseline) | Typical room example |
|---|---|---|
| 100 to 150 sq ft | 5,000 BTU | Small bedroom, home office |
| 150 to 250 sq ft | 6,000 BTU | Standard bedroom |
| 250 to 300 sq ft | 7,000 BTU | Large bedroom, small studio |
| 300 to 350 sq ft | 8,000 BTU | Master bedroom, small living room |
| 350 to 400 sq ft | 9,000 BTU | Large bedroom, studio apartment |
| 400 to 450 sq ft | 10,000 BTU | Open kitchen-dining, medium living room |
| 450 to 550 sq ft | 12,000 BTU | Large living room, small open plan |
| 550 to 700 sq ft | 14,000 BTU | Open-plan living-dining |
| 700 to 1,000 sq ft | 18,000 BTU | Large open plan, loft |
| 1,000 to 1,200 sq ft | 21,000 BTU | Large apartment, small home |
These baseline numbers assume a standard eight-foot ceiling, moderate sun exposure, average insulation, and two to three occupants. Use the adjustment factors below to modify the starting number for your actual conditions.
Sun exposure is the most impactful single adjustment. A room with large windows on a south- or west-facing wall in direct afternoon sun can receive solar heat gain that adds the equivalent of several hundred square feet to the cooling load. The Energy Star guideline is to increase BTU by 10 percent for heavily sunlit rooms and decrease by 10 percent for shaded rooms.
A practical definition of “heavily sunlit” is a room where direct sunlight enters through windows for more than three hours during peak afternoon hours (noon to 4 PM) and the windows lack heavy curtains or blinds. If that describes your room, add 10 percent to the baseline BTU. If the room faces north, is shaded by a roof overhang or large trees, or has blackout curtains, subtract 10 percent from the baseline.
For rooms on the top floor directly under a roof, solar heat gain through the ceiling adds further load, especially if the attic is not well insulated. Top-floor rooms may need 15 to 20 percent more BTU than the baseline for the same square footage on a lower floor with similar window exposure.
The 20 BTU-per-square-foot baseline assumes an eight-foot ceiling. For higher ceilings, the volume of air being cooled increases, which requires more BTU. The adjustment is straightforward: multiply the baseline BTU by the ratio of your actual ceiling height to eight feet.
A 300 square foot room with a ten-foot ceiling needs more BTU than the same footprint with an eight-foot ceiling. The ratio is 10 divided by 8, or 1.25. So the 7,000 BTU baseline becomes 7,000 multiplied by 1.25, or 8,750 BTU — round up to 9,000 BTU. For rooms with vaulted ceilings, use the average ceiling height across the room rather than the peak height, since the highest point is above occupied space.
Each person in a room generates approximately 600 BTU per hour of heat load through body heat alone. For rooms with more than two regular occupants, add 600 BTU per additional person beyond two. A home office shared by four people has an additional 1,200 BTU load above the standard two-person baseline.
This adjustment matters most for spaces used for gatherings: a dining room that seats eight at dinner, a home gym used by multiple people, or a basement playroom for children. A 400 square foot room with eight people in it during dinner has a heat load closer to a 700 square foot room for baseline purposes, accounting for the additional body heat.
Kitchens have significantly higher BTU requirements than the square footage alone suggests because cooking appliances generate substantial heat. For a kitchen where cooking happens regularly, add 4,000 BTU to the baseline calculated from square footage. This accounts for a standard range or cooktop in regular use. A kitchen with a commercial-style range or one used for extended heavy cooking may need even more.
An open-plan kitchen-living-dining area is treated as a single zone for BTU calculation. Add up the square footage of the entire open area, then apply the kitchen adjustment as a flat addition, then apply sunlight and ceiling adjustments. The result is higher than either room would require independently, which reflects the reality that cooking in an open plan space heats the entire zone.
The 20 BTU-per-square-foot baseline was developed for moderate US climates. In consistently hot climates — the American South, Desert Southwest, and similar regions where temperatures regularly exceed 95 degrees Fahrenheit for weeks at a time — increase the baseline by 10 to 20 percent. In milder Northern climates where temperatures over 90 degrees are uncommon and summers are short, the baseline is usually sufficient without upward adjustment.
Humidity also affects perceived cooling load. High-humidity climates (the Southeast, the Gulf Coast) benefit from units with stronger dehumidification capability in addition to BTU capacity. All air conditioners dehumidify as a byproduct of cooling, but some are more effective at humidity removal than others. In high-humidity regions, removing humidity is as important to comfort as lowering temperature.
Start with the baseline: 380 square feet at 20 BTU per square foot = 7,600 BTU, round to the nearest standard unit size of 8,000 BTU. The room has large west-facing windows with no curtains, so apply the sunlight adjustment: 8,000 plus 10 percent = 8,800 BTU. Ceiling height is nine feet: 8,800 multiplied by 9/8 = 9,900 BTU, round to 10,000 BTU. The room is typically occupied by one person, which is below the two-person baseline, so no upward occupancy adjustment. This bedroom needs a 10,000 BTU unit.
Without the adjustments, the baseline of 8,000 to 9,000 BTU might seem adequate, but the combination of heavy afternoon sun and the slightly higher ceiling brings the real requirement to 10,000 BTU. A unit sized at 8,000 BTU in this room would struggle on the hottest afternoon hours and run nearly continuously. The extra 2,000 BTU gives it the headroom it needs.
Window air conditioners are significantly more energy-efficient than portable units at the same rated BTU because they exhaust hot air directly outside. Portable air conditioners use a hose vented through a window, but the mechanics mean they also pull some warm air from the room into the exhaust stream, reducing effective cooling capacity. A portable air conditioner rated at 10,000 BTU may deliver the effective cooling of a 7,000 to 8,000 BTU window unit in practice.
When sizing a portable air conditioner, many HVAC guides recommend buying 15 to 25 percent more BTU than a window unit for the same room to compensate for this efficiency loss. If the room calculation says 10,000 BTU for a window unit, plan for 12,000 to 14,000 BTU for a portable. For portable options sized correctly, see the best portable air conditioners page. For a broader look at sizing questions and unit types, the what size air conditioner do I need guide covers the full sizing methodology including ductless mini-split options.
Buying by room label instead of square footage: “bedroom-size” or “living room-size” are marketing descriptions, not engineering measurements. Always calculate square footage and apply the chart. Ignoring sun exposure: this single factor can shift the correct BTU by 1,000 to 2,000 BTU in a typical bedroom, which often means the difference between one standard unit size and the next. Assuming bigger is always better: an oversized unit short-cycles (turns on and off frequently without running a full cycle) and fails to remove humidity properly. In high-humidity climates, an oversized unit may feel uncomfortable even when the thermostat reads the correct temperature.
For a comprehensive guide to unit types, features, and energy ratings in addition to BTU sizing, see the air conditioner buying guide. For a comparison of the strongest models at each BTU range, the best air conditioners overview and the best window air conditioners page both organize picks by room size and BTU to simplify the final selection.
The BTU chart and calculation method in this guide applies to single-room air conditioners: window units and portable units that cool one room at a time. For central air conditioning systems and multi-zone mini-split systems, BTU sizing is calculated across the entire home by an HVAC professional using Manual J calculations that account for insulation levels, window area, duct efficiency, and local climate data. The 20 BTU per square foot rule is a rough starting point for whole-home estimates but is not accurate enough for sizing central systems.
If you are sizing a ductless mini-split for a single room, the same per-room calculation in this guide applies. A 12,000 BTU mini-split head unit (one BTU unit per room) is the most common size for a standard bedroom or home office. Multi-zone mini-split systems connect multiple indoor head units to a single outdoor compressor, with each head sized independently for its room. For ductless options, the best air conditioners page covers mini-split options alongside window and portable units.
BTU tells you the cooling capacity. EER (Energy Efficiency Ratio) and CEER (Combined Energy Efficiency Ratio) tell you how efficiently the unit delivers that cooling. A higher EER or CEER means the unit uses less electricity per BTU of cooling. EER is measured at a single test condition (95 degrees Fahrenheit outside, 80 degrees inside); CEER is a more comprehensive measure that includes standby power draw, making it a better real-world predictor of annual operating cost.
For equivalent BTU units, a higher-EER model will cost less to run each month. The difference in annual electricity costs between a low-EER and high-EER 10,000 BTU window unit can reach $30 to $60 per year depending on local electricity rates and usage hours. Over five to ten years of ownership, a high-EER unit often pays back the purchase price premium in electricity savings alone. Look for units with an EER of 10 or higher for reasonable efficiency; units above 12 EER are considered high-efficiency.
Window air conditioner performance is significantly affected by how well the unit is sealed in the window opening. Gaps between the unit and the window frame allow hot outside air to leak in, reducing effective cooling and forcing the unit to run harder. Most window units include foam side panels to fill the accordion gaps, but a proper seal also requires weatherstripping along the bottom and top edges of the unit where it meets the window sash. An improperly sealed window installation can reduce effective cooling capacity by 10 to 20 percent of the rated BTU.
Window orientation matters for installation. A north-facing window receives less direct sun than a south- or west-facing one, reducing the heat load on the unit from the window itself. For a given room, installing the window AC unit in a north-facing window rather than a west-facing one can slightly reduce the effective load the unit needs to handle, even with the same room square footage. This is a minor factor compared to room size and ceiling height, but worth noting if you have a choice of window to install the unit in. For guidance on choosing the right model at your calculated BTU, the best window air conditioners and best portable air conditioners pages are organized by BTU range to help match each room to the right unit size.
The standard baseline is 20 BTU per square foot of room area. A 300 square foot room needs approximately 6,000 BTU to start. Adjust upward by 10 percent for heavily sunlit rooms, by the ceiling height ratio for ceilings over eight feet, and by 600 BTU per occupant beyond two people.
An oversized unit cools the air temperature too quickly and shuts off before running long enough to remove humidity. The room feels cool but damp and clammy. The unit also short-cycles frequently, which increases wear on the compressor and reduces the unit's lifespan.
An undersized unit runs continuously and never fully cools the room on hot days. It will also run at maximum capacity constantly, which increases energy consumption and accelerates wear. On the hottest days of the year the room will remain above the set temperature regardless of how long the unit runs.
Yes. The Energy Star adjustment is to add 10 percent to the baseline BTU for heavily sunlit rooms -- those with large windows receiving direct afternoon sun for three or more hours. For shaded rooms, you can subtract 10 percent from the baseline.
Yes, typically. Portable units exhaust hot air through a hose and pull some room air into the exhaust stream, reducing effective cooling capacity. A portable rated at 10,000 BTU often delivers the cooling effect of a 7,000 to 8,000 BTU window unit. When sizing a portable, buy 15 to 25 percent more BTU than the room calculation for a window unit would indicate.
Treat the entire open area as a single zone. Measure the total square footage of all connected open spaces (kitchen, dining, living), apply 20 BTU per square foot, then add 4,000 BTU if cooking appliances are used in the space, and apply sunlight and ceiling adjustments as usual. Do not calculate each section separately and add the BTU, as that would double-count the shared air volume.
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