Air Conditioner BTU Calculator

How this is calculated

The base rule is 20 BTU per hour per square foot of floor area for average conditions. Published guidance runs 20 to 25 depending on how the room behaves, and 20 is the usual starting point. THE ENERGY STAR ADJUSTMENTS, which are the useful part because they are specific: Heavily shaded room - reduce capacity by 10 percent. Very sunny room - increase by 10 percent. More than two people regularly in the room - add 600 BTU for each additional person. Kitchen - add 4,000 BTU. That kitchen figure is large and it is not a typo. Cooking puts a great deal of heat into a room, and a unit sized for the floor area alone will simply never keep up. ⚠️ BIGGER IS NOT BETTER, AND THIS IS THE WHOLE POINT OF SIZING. An oversized air conditioner cools the air to the thermostat setting quickly and then shuts off - which sounds efficient and is not. Removing humidity takes RUNNING TIME, and a unit that short-cycles never runs long enough to do it. The result is a room that is cold and clammy rather than comfortable, plus more wear on the compressor from constant starting and stopping. An undersized unit runs constantly and never quite gets there. Both are unpleasant; only one of them is the mistake people actually make. Almost nobody buys an air conditioner too small. UNITS COME IN FIXED SIZES - commonly 5,000, 6,000, 8,000, 10,000, 12,000 BTU and up. Your calculated figure will land between two of them. The conventional advice is to take the nearer one rather than automatically rounding up, and to prefer the smaller of the two when the room is well insulated or shaded. CEILING HEIGHT IS NOT IN THE BASIC RULE, and it matters. The 20-per-square-foot figure assumes something like a standard 8-foot ceiling. A room with a 12-foot ceiling holds half as much air again, and scaling by the height ratio is a reasonable correction the simple rule ignores entirely. WHAT THIS IS NOT: a load calculation. Sizing a whole-house or central system properly means a Manual J calculation, which accounts for insulation levels, window area and orientation, air leakage, local climate design temperatures and much else. This is the room-unit rule of thumb, and it is genuinely adequate for choosing a window or portable unit for one room. It is not adequate for specifying a system. EER OR SEER MATTERS SEPARATELY from size - look for an ENERGY STAR label and an EER of 10 or above. Size decides whether the room is comfortable; efficiency decides what it costs to run.

Common questions

Why is buying a bigger unit a mistake?

Because removing humidity takes RUNNING TIME. An oversized air conditioner cools the air to the thermostat setting quickly and then shuts off - which sounds efficient and is not, because it never runs long enough to pull the moisture out. The result is a room that is cold and clammy rather than comfortable, plus extra wear on the compressor from constant starting and stopping. An undersized unit runs forever and never quite gets there; both are unpleasant, but only oversizing is the mistake people actually make. Almost nobody buys an air conditioner too small.

What are the adjustments based on?

ENERGY STAR's own published guidance, which is unusually specific for a rule of thumb: reduce capacity 10 percent for a heavily shaded room, increase it 10 percent for a very sunny one, add 600 BTU for each person beyond two who is regularly in the room, and add 4,000 BTU for a kitchen. That last figure is large and deliberate - cooking puts a great deal of heat into a room, and a unit sized on floor area alone will never keep up in one.

Why does ceiling height matter if the rule is per square foot?

Because the rule quietly assumes a standard ceiling of about 8 feet, and an air conditioner cools air rather than floor. A room with a 12-foot ceiling holds half as much air again for the same footprint, so the basic figure under-sizes it by roughly that much. This calculator scales by the height ratio, which is a reasonable correction to a rule that otherwise ignores the third dimension entirely - and it is why the room volume is shown as an output.

My answer is between two unit sizes.

It usually will be, because units come in fixed steps - commonly 5,000, 6,000, 8,000, 10,000 and 12,000 BTU. The conventional advice is to take the NEARER size rather than automatically rounding up, and to prefer the smaller of the two when the room is well insulated or well shaded. Rounding up feels like buying safety and is actually buying the short-cycling problem above.

Is this good enough to size a whole house?

No, and it is worth being clear about that. Properly sizing a central or whole-house system means a Manual J load calculation, which accounts for insulation levels, window area and orientation, air leakage, local climate design temperatures and a good deal more. This is the room-unit rule of thumb: genuinely adequate for choosing a window or portable unit for one room, and not adequate for specifying a system. If someone is quoting you a central system without doing a load calculation, that is worth noticing.

What about efficiency ratings?

They are a separate question from size, and both matter. Size decides whether the room is comfortable; efficiency decides what it costs to run. Look for an ENERGY STAR label and an EER of 10 or above. A correctly sized efficient unit beats an oversized efficient one on both counts, which is the point worth taking away - efficiency ratings cannot rescue a unit that is the wrong size for the room.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Air Conditioner BTU Calculator at https://www.bfcbrilliance.com/tools/ac-btu-calculator.

What I entered:
- Room length (ft): ___
- Room width (ft): ___
- Ceiling height (ft): ___
- BTU per square foot (BTU): ___
- Sun exposure: ___
- People beyond two, regularly (people): ___
- Is it a kitchen?: ___

What it calculated:
- BTU needed: ___
- Floor area: ___
- Before any adjustments: ___
- Ceiling height factor: ___
- Sun adjustment: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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