The formula
How it works
Estimate the cooling capacity in BTUs you need to air-condition a room. Enter the room size, how many people use it, its sun exposure and whether it is a kitchen to get a recommended BTU rating.
FAQ
What is a BTU?
A BTU (British thermal unit) measures heat energy; air conditioners are rated by how many BTUs of heat they remove per hour. A bigger room, more sun or more people all add heat, so you need more BTUs to keep it comfortable. Air-conditioner capacity is often also quoted in “tons”, where one ton equals 12,000 BTU.
What if I pick a unit that is too big?
An oversized air conditioner cools the air fast but shuts off before it removes humidity, leaving the room cold and clammy, and it cycles on and off inefficiently. Matching capacity to the room — as this calculator does — gives steadier temperature, better dehumidifying and lower running costs.
Does ceiling height matter?
This calculator assumes a standard ceiling around 8 feet, since it works from floor area rather than volume. A noticeably higher ceiling means more air to cool, so add roughly 10–20% more capacity for rooms with tall or vaulted ceilings.
Does it account for insulation or window size?
Not directly — it uses the general area-based rule of thumb adjusted only for sun exposure, occupants and kitchen use. Poor insulation, large or single-pane windows, or a top-floor room under a hot roof all add extra heat load beyond what the estimate covers, so lean toward the next size up in those cases.
Can I use this for a room upstairs or under the roof?
Yes, but treat the sun exposure setting generously — top-floor rooms and those under an uninsulated roof pick up significant extra heat, so choosing “sunny” even for a shaded window is often more realistic for these spaces.
Does the calculator tell me which type of air conditioner to buy?
No — it only estimates the BTU capacity you need. You still choose the format — window, portable or mini-split/ductless — based on your space, budget and whether you can install an exterior vent, then look for that BTU rating within the format you pick.
Is a higher-BTU unit always more efficient?
No, BTU capacity and efficiency are separate things. Capacity (BTU) tells you how much heat a unit can remove, while efficiency — shown as SEER or EER — tells you how much electricity it uses to do that; a correctly sized unit with a high efficiency rating cools well and costs less to run than an oversized but inefficient one.
About the BTU calculator
This calculator estimates the cooling capacity, in BTUs per hour, needed to air-condition a room. It starts from the room’s floor area and adjusts for the things that add heat: strong sun, extra people and the appliances of a kitchen. The result is a recommended BTU rating you can use when choosing an air conditioner, so the unit is neither underpowered nor wastefully oversized.
How to use it
Enter the room’s area in square feet, the number of people who regularly use it, its sun exposure, and whether it is a kitchen. The calculator applies the standard base of about 20 BTU per square foot and adds for the other factors. For example, a shaded 300-square-foot living room for two needs roughly 6,000 BTU, while a sunny kitchen of the same size needs considerably more.
The formula
The estimate starts at BTU, the rule of thumb for cooling. Sun exposure adjusts this down about 10% for shade or up 10% for full sun. Each occupant beyond two adds roughly 600 BTU, and a kitchen adds about 4,000 BTU for its appliances. The total is rounded to a sensible figure you can match to an available unit.
Where it is used
Homeowners and renters use it before buying a window or portable air conditioner, to pick a model that will actually cool the space. Installers use similar sizing for split systems. Because both undersizing and oversizing cause problems — poor cooling on one hand, humidity and short-cycling on the other — a quick BTU estimate is a worthwhile first step in any cooling purchase.