AC BTU Calculator

Background.

Standing in a shop aisle between a 6,000 and an 8,000 BTU window unit, most people pick the bigger one to be safe. It is often the wrong instinct. An air conditioner that is too small runs flat out all afternoon and never quite catches up, but one that is too large chills the air in a few minutes and switches off before it has pulled much moisture out, leaving a room that is cold and clammy at once, and it costs more to buy.

The usual starting point is a rule ENERGY STAR has long published: about 20 BTU per hour for each square foot of floor, then adjusted for the room. Its guidance raises capacity by 10% for a very sunny room and lowers it by 10% for a heavily shaded one, adds 600 BTU per hour for each person beyond two who regularly uses the room, and adds 4,000 BTU per hour if the room is a kitchen. The estimate here follows the same pattern and also scales for ceiling height, so an 18 by 15 foot room with a 9-foot ceiling, some sun and three regular occupants comes to 7,282.5 BTU per hour, rounded up to a 7,500 BTU unit.

It is a buying estimate for a single room, not a load calculation. Insulation, air leakage, window area and glazing, which way the walls face, your climate and how humid the air is all change the true cooling load, and none of them is modelled. For central air or a whole house, a professional Manual J calculation is the tool; for choosing a window, through-wall or portable unit, a clear rule of thumb applied carefully gets most rooms close.

What is ac btu calculator?

A BTU, or British thermal unit, is the heat needed to warm one pound of water by one degree Fahrenheit, about 1,055 joules. Air conditioners are rated in BTU per hour, the rate at which they can remove heat from a room under standard test conditions; that rate is the unit's cooling capacity. One BTU per hour is about 0.293 watts, so a 7,500 BTU unit removes heat at roughly 2.2 kilowatts, and 12,000 BTU per hour is the 'ton' used for larger systems, the cooling once supplied by melting a ton of ice over a day.

The heat a unit has to remove comes in two forms. Sensible heat raises the temperature — sun through the glass, warm walls, people, appliances. Latent heat is the moisture in the air, which the unit removes by condensing it on its cold coil. A correctly sized unit runs long enough to do both; a much larger one satisfies the thermostat on sensible heat alone and leaves the humidity behind, which is why the right size is a target rather than a minimum to exceed.

Room units are generally sold from about 5,000 to 24,000 BTU per hour. A room that needs more than the top of that range is better served by a split system or more than one unit.

How to use this calculator.

1. 01Measure the room's length and width in feet, wall to wall. For an L-shaped room, work out the total floor area and enter two dimensions that multiply to it — 20 × 13.5 for 270 square feet, for example.
2. 02Enter the ceiling height. The calculation treats 8 feet as standard, so a taller ceiling raises the estimate in proportion and a lower one reduces it.
3. 03Keep the base at 20 BTU per square foot unless you have a reason to change it — raise it for a top-floor room under a hot roof. For a heavily shaded room enter −10 in the sun field, ENERGY STAR's 10% reduction.
4. 04Set the sun adjustment as a percentage (10 for a very sunny room, 0 for an ordinary one, −10 for a heavily shaded one), enter how many people regularly use the room, and add appliance heat in BTU per hour: 4,000 for a kitchen, or the wattage of other equipment × 3.412.
5. 05Choose a rounding increment that matches the sizes on sale, usually 500 or 1,000, then read the raw estimate and the rounded size and shop for a unit at the rounded figure, not far above it.

The formula.

BTU/h = A × b × (h ÷ 8) × (1 + s ÷ 100) + O + E. A is the floor area in square feet, b the base BTU per square foot, h the ceiling height in feet, s the sun adjustment as a percentage, O the occupant heat and E the appliance heat. O = max(0, people − 2) × the extra BTU per additional occupant, so the first two people are already assumed in the base rule and only the third and later add heat. The result is then rounded up — never down — to the next multiple of the chosen increment.

The order of operations matters. The ceiling factor and the sun percentage multiply the area-based load, while occupants and appliances are added afterwards at their full values. A 20% sun adjustment therefore grows a large room's estimate by far more than a small room's, but a kitchen's 4,000 BTU is the same 4,000 whatever the room size.

Two simplifications are worth knowing. Scaling the whole load by h ÷ 8 assumes heat gain grows in step with ceiling height; in reality much of the load comes through windows and the roof, which do not grow with the walls, so a 12-foot ceiling's factor of 1.5 can overstate a tall room with little glass. And the base rule bundles the building's insulation, airtightness and climate into one number per square foot, which is why it can be adjusted. What changes the answer most in practice is the area and the base rate: every extra 50 square feet at 20 BTU adds 1,000 BTU per hour before any multiplier.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Department of Energy (n.d.). "Room Air Conditioners." Energy Saver.
2. [2]ENERGY STAR (n.d.). "Room Air Conditioners." U.S. Environmental Protection Agency and U.S. Department of Energy.
3. [3]Air-Conditioning, Heating, and Refrigeration Institute (2020). "AHRI Standard 210/240: Performance Rating of Unitary Air-conditioning and Air-source Heat Pump Equipment." AHRI.
4. [4]National Institute of Standards and Technology (2026). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.
5. [5]National Institute of Standards and Technology (2008). "Guide for the Use of the International System of Units (SI), NIST Special Publication 811." U.S. Department of Commerce.