BTU Calculator
Size a room air conditioner from room area, sun, occupants and kitchen heat, using ENERGY STAR's chart for temperate rooms and installer practice for the tropics. Outputs BTU/h, tons and kW.
BTU Calculator
How to use the BTU Calculator
Measure the room
Enter the length and width of the room you want to cool. ENERGY STAR's advice is to measure in feet and convert inches to fractions of a foot (12 ft 6 in is 12.5 ft), and to split an odd-shaped room into rectangles and triangles and add them up. Switch to metres if you prefer; the calculator converts.
Pick the climate
Temperate sizes the room from ENERGY STAR's room air conditioner chart, which runs from 100 to 2,500 sq ft and assumes an 8-foot ceiling. Tropical uses the 600 BTU/h per m² that installers in Malaysia and Singapore work to, the same figure as our air-conditioner sizing and running-cost calculator, and gives the answer as a horsepower class.
Adjust for sun, people and cooking
These are ENERGY STAR's own four adjustments: reduce capacity by 10% if the room is heavily shaded, increase it by 10% if it is very sunny, add 600 BTU for each person beyond two who regularly uses the room, and add 4,000 BTU if the unit is in a kitchen.
Size to the room, not bigger
ENERGY STAR is blunt that bigger is not better: an oversized unit costs more, wastes energy and cools the room before it has removed the humidity, leaving it damp and clammy. Choose the size the room needs. For a whole home, or anything off the chart, ask an HVAC contractor for a load calculation.
Sizing a room air conditioner — a chart, not a rate
A room air conditioner is rated in British thermal units per hour (BTU/h), the heat it can remove from the room each hour. The U.S. Energy Information Administration puts one kilowatt-hour at 3,412 BTU, so a 12,000 BTU/h unit removes heat at about 3.5 kW. Getting the number right matters in both directions. Too small and the unit cannot keep up on hot days. Too large and, in ENERGY STAR's words, it will cool the room before it has a chance to remove the humidity, so the room feels damp and clammy, and it costs more to buy and run.
Why the chart and not "20 BTU per square foot"
ENERGY STAR publishes a table, not a rate, and the rate implied by that table falls as rooms grow: a 144 sq ft bedroom sits in the 100–150 sq ft row at 5,000 BTU/h, about 35 BTU per sq ft, while 700–1,000 sq ft is 18,000 BTU/h, under 26. Flattening the table into one rate undersizes small rooms. Correction, 14 October 2026. Until this date the temperate setting here did exactly that, at 20 BTU/ft², and sized a 12 × 12 ft room at 2,880 BTU/h (shown rounded up to 3,000) against the chart's 5,000. It also applied a high-ceiling +25%, an insulation ±10% and a "hot desert" rate that came from no published source; all three are gone. ENERGY STAR says only that its chart assumes an 8-foot ceiling and that a higher ceiling may call for a larger unit.
A 12 × 12 ft bedroom is 5,000 BTU/h on ENERGY STAR's chart, not the 2,880 a flat 20 BTU per square foot gives. Small rooms are where a single rate goes wrong.
Tropical rooms are sized differently
ENERGY STAR's chart is written for the United States. In Singapore and Malaysia a single-room split unit typically cools an uninsulated masonry room in hot, humid air all year, and there is no published equivalent of the chart. What exists is installer practice, and this calculator uses the conservative end of it, 600 BTU/h per m², the same figure and the same code as our Air-Conditioner Sizing & Running Cost Calculator, so the two cannot disagree. Treat it as a starting point for a conversation with an installer, not a specification.
Correction, 29 July 2026. Until this date the tropical base rate here was 25 BTU/ft² — the temperate rule plus 25% — which returned 5,000 BTU for a 14 m² master bedroom while the section below this one told you that room needs 1 HP ≈ 9,000 BTU. The page argued against itself, and our own Air-Conditioner Sizing & Running Cost Calculator returned 9,000 for the same room: a 2.23× disagreement between two tools on this site. The tropical rate is now 600 BTU/h·m², the conservative end of what installers across Malaysia and Singapore actually fit, and the two tools now agree exactly at every room size. The reasoning and sources are in our guide, House calculations for a tropical climate.
Horsepower classes
Split units across Southeast Asia are usually sold by "horsepower" class rather than by BTU. The tropical result above is given as the class whose capacity covers the room, and the result panel lists the classes the calculator uses. The label alone is a loose guide, so check the cooling capacity on the unit's specification sheet before you buy.
ENERGY STAR sizes a room from a table, and the rate falls as rooms grow
ENERGY STAR's chart: 100–150 sq ft needs 5,000 BTU/h; 2,000–2,500 sq ft needs 34,000.
The chart assumes an 8-foot ceiling.
Heavily shaded: −10%. Very sunny: +10% (ENERGY STAR).
Each regular occupant beyond two: +600 BTU/h (ENERGY STAR).
A unit in a kitchen: +4,000 BTU/h (ENERGY STAR).
An oversized unit cools before it dehumidifies, leaving the room damp and clammy.
1 kWh = 3,412 BTU (EIA), so 12,000 BTU/h is about 3.5 kW of cooling.
A flat 20 BTU/ft² sizes a 12 × 12 ft room at 2,880 BTU/h; the chart says 5,000.
Tropical rooms here are sized at 600 BTU/h per m², installer practice rather than a published standard.
Past 2,500 sq ft the chart stops; size a space that big with a load calculation.
Frequently Asked Questions
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Because bigger is not better. ENERGY STAR says an oversized room air conditioner costs more to buy, wastes energy and does not cool better: it brings the temperature down before it has had time to remove the humidity, so the room ends up cool but damp and clammy. The right size is the one the room needs, which is what the chart and its adjustments give you.
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One kilowatt-hour is 3,412 BTU (U.S. Energy Information Administration), so divide BTU/h by 3,412 for kilowatts of cooling: 12,000 BTU/h is about 3.5 kW. US contractors also quote "tons", where one ton is 12,000 BTU/h. Southeast Asian split units are usually labelled by horsepower class; check the BTU or kW figure on the specification sheet, because the class is only a loose label.
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ENERGY STAR's chart is written for US rooms. A tropical single-room split usually cools an uninsulated masonry room in hot, humid air all year, and there is no published chart for it. This calculator uses 600 BTU/h per m², the conservative end of installer practice in Malaysia and Singapore, and the same figure as our air-conditioner sizing and running-cost calculator. Our guide on house calculations for a tropical climate explains where it comes from.
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ENERGY STAR's chart is calculated for an 8-foot ceiling and says that with a higher ceiling you may want a larger unit, without giving a figure. This calculator used to add 25% for a high ceiling; that number had no source, so it is gone. If your ceiling is well above 8 feet, size up with an installer's advice rather than a fixed percentage.
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From 100 sq ft (about 9 m²) to 2,500 sq ft (about 232 m²). Below 100 sq ft the calculator uses the smallest row, 5,000 BTU/h, and says so. At 2,500 sq ft and above it gives no number: a space that size needs a proper load calculation from an HVAC contractor, who will account for windows, insulation and ducts that a room chart cannot.
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No. ENERGY STAR's rule is to add 600 BTU for each person beyond two who regularly occupies the room. Two people are already in the chart's figures, so a bedroom for two needs no adjustment and a living room used by five adds 1,800 BTU/h.
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No. It sizes cooling capacity for a room. Heating needs depend on how cold your winters get and how well the home holds heat, which a room cooling chart does not cover; for a heat pump that will also heat, ask an installer for a heating load calculation.
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No. All calculations run entirely in your browser. Nothing is sent to a server, stored, or shared, and closing the tab leaves nothing behind.
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Measure the whole open area as one room, splitting an L-shape into rectangles as ENERGY STAR suggests, and tick the kitchen box if the cooking area is part of it. If the space is above 2,500 sq ft, or one unit cannot reach every corner, consider more than one unit and ask an installer.
Method & sources
How it computes
Temperate: ENERGY STAR room air conditioner chart (100-2,500 sq ft, 8 ft ceiling) x 0.9 heavily shaded or 1.1 very sunny, +600 BTU/h per occupant beyond two, +4,000 in a kitchen; off the chart at 2,500 sq ft. Tropical: SustainabilityKernel.aircon.size (600 BTU/h per m2, installer practice) + kitchen 4,000. kW = BTU/h / 3,412 (EIA). EnergyKernel.roomAcEnergyStar, tested.
What this tool implements
- Bug fixed: the temperate setting used a flat 20 BTU/ft2, which ENERGY STAR's chart contradicts for small rooms, as the page's own prose said: a 12 x 12 ft room came out at 2,880 BTU/h (shown as 3,000) against the chart's 5,000. It now uses the chart and ENERGY STAR's four adjustments, verbatim.
- Removed as unsourced: the 'hot desert' climate (30 BTU/ft2), the insulation +/-10% and the high-ceiling +25% (ENERGY STAR says only that its chart assumes an 8 ft ceiling), the 'always round up' rule (ENERGY STAR warns against oversizing), inverter savings of 30-40% with an 18-24 month payback, 'accurate within 10-15%', SEER rules of thumb and brand lists.
- The tropical path is unchanged in substance and now calls the same kernel function as the aircon sizing calculator; its 600 BTU/h per m2 is described as Malaysian/Singaporean installer practice, not a published standard. The calculator sizes cooling only; the claim that it also sizes heat-pump heating is removed.
Sources
- ENERGY STAR (U.S. EPA). Room Air Conditioners: properly sized room air conditioners, cooling capacity chart (100 to 2,500 sq ft, 5,000 to 34,000 BTU/h; 8 ft ceiling) and adjustments for shade, sun, occupants and kitchens. https://www.energystar.gov/products/room_air_conditioners
- U.S. Energy Information Administration. British thermal units (Btu): 1 kilowatthour = 3,412 Btu. https://www.eia.gov/energyexplained/units-and-calculators/british…
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