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AC BTU Calculator

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This is the rule of thumb retailers use — 20 BTU/h per square foot with the adjustments ENERGY STAR names — made explicit and adjustable instead of hidden behind a dropdown. It is a starting figure, and the page is clear about where that stops being enough.

Area unit

The rule of thumb assumes 8 ft — anything higher scales the load

Height unit

±10%, as the ENERGY STAR guidance sets it

600 BTU/h for each beyond the first two

Add for a hot climate or poor insulation; subtract for a well-insulated building

How to use the ac btu calculator

  1. 1Enter the floor area of the room, in square feet or square metres.
  2. 2Set the ceiling height. The base figure assumes eight feet, and anything taller scales the load.
  3. 3Choose the sun exposure and say how many people are usually in the room.
  4. 4Tick the kitchen box if it applies, then read the breakdown to see where the figure came from.

Examples

An average bedroom

Input
250 ft², 8 ft ceiling, average sun, 2 people
Result
5,000 BTU/h — 0.42 tons, about 1.47 kW of cooling

A sunny open-plan kitchen

Input
400 ft², 10 ft ceiling, very sunny, 4 people, kitchen
Result
About 16,000 BTU/h once height, sun, occupants and appliances are added

About the ac btu calculator

Where the 20 BTU per square foot comes from

It is the figure ENERGY STAR publishes for sizing a room air conditioner, and it is really about volume rather than area — 20 BTU/h per square foot at an eight-foot ceiling is 2.5 BTU/h per cubic foot. That is why a taller ceiling scales the base figure here rather than being ignored.

The adjustments around it are the same guidance's: ten per cent either way for a very sunny or heavily shaded room, 600 BTU/h for each occupant beyond the first two, and a flat 4,000 for a kitchen. None of them is a precise physical quantity; together they are a reasonable first pass for one room and one unit.

What the rule of thumb cannot see

Insulation is the largest omission. A room with an uninsulated roof directly above it and one under a well-insulated loft have very different loads and identical floor areas, which is what the climate and insulation allowance on this page is for — a way of saying so explicitly rather than leaving it out.

Humidity is the second. An air conditioner does two jobs, lowering temperature and removing moisture, and in a humid climate the second can be the larger part of the work. Manual J separates them into sensible and latent loads for exactly that reason, and the rule of thumb rolls both into one number.

Frequently asked questions

Should I round up to the next size to be safe?
No, and this is the piece of advice worth more than the arithmetic. An oversized unit cools the air to the thermostat setting quickly and shuts off before it has run long enough to condense much moisture out. The room ends up cold and clammy, the compressor short-cycles, and efficiency and equipment life both suffer. Where two sizes are close, the smaller usually performs better.
How accurate is a rule of thumb like this?
It is a starting point and not a load calculation. The proper method is ACCA Manual J, which models the R-value of every wall and the ceiling, the area and orientation of every window, the building's air leakage, the local design temperatures and the humidity load separately from the heat. It commonly lands 20 to 30% away from this figure, in either direction.
Why does a kitchen add so much?
Because an oven and a hob put more heat into a room than everything else in it combined. The standard allowance is 4,000 BTU/h, which is close to the entire load of a small bedroom, and it is why kitchens are the one room where the rule of thumb needs a fixed addition rather than a percentage.
What is a ton of cooling?
Twelve thousand BTU per hour, by definition. The name is literal: it is the rate of cooling produced by a ton of ice melting over twenty-four hours, which is how refrigeration capacity was measured before mechanical systems existed. It survives as the unit American equipment is sold in, while the same machines are rated in kilowatts elsewhere.

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