The AC BTU calculator above sizes a room air conditioner using the cooling capacity chart published by Energy Star, then applies the adjustments that the same guidance sets out for shade, sunlight, occupancy and kitchen use. It reports the raw calculated load, the nearest available unit size you would actually buy, and the equivalent in cooling tons.
Arb Digital builds free calculators for the everyday arithmetic behind buying decisions, and air conditioning is one where the wrong answer is expensive twice — once at the till and again on every electricity bill for the next decade. The numbers here come from published federal guidance rather than a table we invented, and the page is explicit about where that guidance stops and professional load calculation begins.
What This AC BTU Calculator Does
A BTU is a British thermal unit, the energy needed to raise one pound of water by one degree Fahrenheit. Air conditioners are rated in BTU per hour, which is a rate of heat removal rather than a quantity of energy. A ton of cooling is 12,000 BTU/h, a unit that survives from the days when cooling was measured against the heat absorbed by a ton of melting ice over 24 hours.
The calculator starts from the Energy Star room air conditioner cooling capacity chart, which maps floor area to a required capacity in fourteen steps — 100 to 150 square feet needs 5,000 BTU/h, 450 to 550 square feet needs 12,000 BTU/h, and 1,500 to 2,000 square feet needs 30,000 BTU/h. It then applies the four adjustments that the same page specifies: reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny one, add 600 BTU/h for each person beyond two who regularly occupies the room, and add 4,000 BTU/h if the unit serves a kitchen.
Two things are ours rather than Energy Star's, and both are labelled as such in the form. The ceiling-height adjustment scales the result when your ceiling differs from the eight feet the chart assumes. The climate selector is a judgement call you make, not a published factor, because the Energy Star chart does not vary by region. One boundary worth stating plainly: our electricity bill calculator prices what a unit costs to run once you own it, while this page sizes the unit in the first place.
How to Use It
- Enter the room size. Give the floor area directly, or switch to length and width for a rectangular room. Metric input is converted to square feet before the chart is consulted, because the chart is published in square feet.
- Set the ceiling height. Leave it at 8 feet if that is what you have. A 10-foot ceiling means 25% more air volume to cool, and the tool scales the load accordingly.
- Choose the sun exposure. Heavily shaded means genuinely shaded — north-facing in the northern hemisphere, or behind mature trees. Very sunny means large unshaded glazing facing the afternoon sun.
- Enter occupancy and kitchen status. Count people who are normally in the room at the same time, not everyone who passes through. The kitchen addition is large because cooking appliances dump heat directly into the space.
- Read the unit size, not the raw figure. Room air conditioners are sold in discrete capacities, so the hero number is the next size up from your calculated load — the one you can actually buy.
The Formula / How It's Calculated
The chart lookup comes first. Take the floor area in square feet and find the band it falls into; that band's capacity is the base. Multiply by the ceiling factor, which is your ceiling height divided by eight. Then apply the sun percentage, add 600 BTU/h for every occupant above two, add 4,000 BTU/h if it is a kitchen, apply your climate percentage, and finally apply any sizing margin you have set.
Work the default example. A 400 square-foot room falls in the 400-to-450 band, so the base is 10,000 BTU/h. The ceiling is 8 feet, so the factor is 1.0. Sun exposure is average, so no change. Three people occupy the room, which is one above two, adding 600 BTU/h to give 10,600 BTU/h. It is not a kitchen and no climate adjustment is applied. Room units are sold at 5,000, 6,000, 8,000, 10,000, 12,000 and upwards, so the smallest unit that meets 10,600 is 12,000 BTU/h, which is exactly one ton of cooling.
Change one input and see how much it matters. Make the same room a very sunny kitchen with four occupants: 10,000 × 1.10 = 11,000, plus 1,200 for two extra people, plus 4,000 for the kitchen, giving 16,200 BTU/h and pushing you to an 18,000 BTU/h unit. That is a 50% jump in capacity from a room of identical size, which is why area alone is never a sufficient answer. The US Department of Energy's page on consumer room air conditioners sets out the federal efficiency standards these units must meet, which is the other half of the buying decision.
Why Oversizing Is the Expensive Mistake
The instinct when in doubt is to buy bigger. With air conditioning that instinct is wrong, and the reason is dehumidification. An air conditioner removes moisture by running long enough for water to condense on its cold coil and drain away. An oversized unit hits the thermostat setpoint fast, shuts off, and never runs long enough to strip the humidity out of the air.
The result is a room that is cold and damp rather than cool and dry — the clammy feeling people often mistake for the unit being too weak. The response is usually to drop the setpoint further, which makes the cycling worse. Short cycling also wears the compressor, since starting is the hardest thing a compressor does, and it wastes energy because each start-up draws a surge of current before the system reaches steady operation.
An undersized unit has the opposite failure mode: it runs continuously and never reaches the setpoint on the hottest days. That is genuinely uncomfortable, but it dehumidifies well and it is a milder failure than most people expect. This is why the sizing margin on this calculator defaults to zero. Round up to the next available size because that is all you can buy, not because a margin is desirable.
What the Chart Cannot See
The Energy Star chart is a screening tool, and it works from a single input: floor area. It knows nothing about your insulation, your window area, your air leakage, the colour of your roof, or whether the room above is a conditioned bedroom or an unvented attic at 60°C.
Those factors move the real load substantially. A room with poor loft insulation and large west-facing windows can need half again as much capacity as an identical room that is well insulated and shaded. A room with heavy air leakage brings in outdoor humidity continuously, which is a latent load the chart does not model at all. The insulation calculator and the wall area calculator are useful for understanding the envelope side of this, and improving the envelope is almost always cheaper per unit of comfort than buying more cooling.
For a whole-house system, or for anything where the equipment cost is significant, the proper method is a room-by-room load calculation. ACCA's technical manuals describe Manual J residential load calculation, the ANSI-approved standard that national building codes and most US states and municipalities require for residential system design. A Manual J calculation accounts for construction assemblies, orientation, glazing, infiltration and local design temperatures, and it is what a licensed installer will run. This page is a sanity check that tells you roughly what to expect, not a substitute for that.
Ceiling Height, Volume and Open-Plan Rooms
Floor area is a proxy for air volume, and it stops being a good proxy the moment the ceiling is not eight feet. A 400 square-foot room with a 12-foot ceiling holds half again as much air as the chart assumes, and the extra volume has to be cooled and kept cool. The calculator handles this with a straightforward volume ratio, which is a reasonable first approximation, though it slightly overstates the load because heat gain scales more with surface area than with volume.
Open-plan spaces are harder. A single room unit serving a kitchen-diner that opens into a hallway is effectively cooling all of it, and you must include every square foot that shares air with the space. Conversely, a room that can be closed off with a door is a genuinely separate load. If you are sizing several units for a house, size each one for the space it can actually isolate, and use the square footage calculator to measure irregular rooms accurately rather than estimating.
One more geometric point: a long narrow room is harder to cool evenly than a square one of the same area, because the unit's airflow has to reach the far end. That is a placement and airflow problem rather than a capacity problem, and adding BTUs will not solve it.
BTU, Tons, Watts and Efficiency Ratings
The same capacity gets quoted in three units depending on where you are. In the United States it is BTU per hour for room units and tons for central systems, where one ton is 12,000 BTU/h. In most of the rest of the world it is kilowatts of cooling capacity, where 12,000 BTU/h is roughly 3.52 kW. If you are shopping across markets, the energy converter will move between them.
Do not confuse cooling capacity in kilowatts with electrical input in kilowatts — they are different numbers for the same machine, and the ratio between them is the efficiency. Room units are rated by CEER in the United States, combining cooling output with standby power, while central systems use SEER2. Europe and Australia use EER and the seasonal SEER. A higher figure means less electricity for the same cooling; it says nothing about capacity. To turn a rating into money, feed the input wattage and your run hours into the electricity bill calculator, and if you are considering offsetting the summer load, the solar panel calculator covers the generation side.
Arb Digital builds free, no-signup calculators for construction, energy, statistics and finance. Browse the full library, or get in touch if there is a calculation your team keeps doing by hand.
Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Buying two sizes up "to be safe" — oversized units short cycle, leave the room humid, and cost more to run than a correctly sized one.
- Sizing an open-plan space by the area of one part of it — if the air is shared, every square foot of it is part of the load.
- Ignoring ceiling height entirely — the published chart assumes eight feet, and a vaulted ceiling can add a third to the air volume.
- Confusing cooling kilowatts with electrical kilowatts — a 3.5 kW unit removes 3.5 kW of heat while drawing roughly a third of that from the socket.
- Treating a screening chart as a load calculation — insulation, glazing, orientation and air leakage all move the real answer, and none of them appear in an area-based table.
Related Free Tools From Arb Digital
Measure an irregular room accurately with the square footage calculator, work out wall areas for the envelope with the wall area calculator, plan the thermal envelope with the insulation calculator, price the running cost with the electricity bill calculator, and switch between BTU, kilowatts and joules with the energy converter. The temperature converter handles design temperatures quoted in the other scale. The full free online tools hub holds the rest of the construction set.
Frequently Asked Questions
There is no single figure, because the published guidance uses bands rather than a constant. Energy Star's chart puts a 400 to 450 square-foot room at 10,000 BTU/h and a 1,500 to 2,000 square-foot space at 30,000 BTU/h, which are different rates per square foot. Use the chart bands rather than a multiplier.
The Energy Star chart places 400 to 450 square feet at 10,000 BTU/h before adjustments. Adding one occupant above two brings it to 10,600, and since units are sold in fixed sizes the practical purchase is a 12,000 BTU/h model.
One ton equals 12,000 BTU per hour. The term dates from measuring cooling against the heat absorbed by a ton of ice melting over 24 hours, and it is still the standard unit for central systems in the United States.
No. An oversized unit reaches the setpoint quickly and switches off before it has removed humidity, leaving the room cold and clammy. It also short cycles, which wears the compressor and wastes energy on repeated start-ups.
Yes. The published chart assumes standard eight-foot ceilings. A ten-foot ceiling means 25% more air volume in the same floor area, and this calculator scales the requirement by the ratio of your height to eight feet.
Cooking appliances release heat directly into the room while they run. Energy Star's guidance adds a flat 4,000 BTU per hour where the unit serves a kitchen, which is often more than the adjustment for sunlight and occupancy combined.
The Energy Star chart does not vary by region, so any climate adjustment on this page is your own judgement rather than published guidance. A proper regional answer comes from a load calculation that uses your local design temperatures.
Yes for anything beyond a single room unit. Residential load calculation to ACCA Manual J is required by national building codes and most US states, and it accounts for insulation, glazing, orientation and air leakage that no area-based chart can see.
This calculator is provided for education and planning only. It applies published Energy Star screening guidance and does not replace a Manual J load calculation. Final equipment sizing, electrical work and installation should be carried out by a licensed HVAC professional working to your local code.
