How the BTU calculator works
A BTU (British thermal unit) per hour is the rate at which an air conditioner removes heat or a heater adds it; 12,000 BTU/h is the old refrigeration "ton" and equals 3.52 kW. For cooling a single room, the calculator reads the capacity straight from the floor-area bands of the US Energy Star room air conditioner chart, then applies the chart's own corrections for sunshine, occupants and cooking.
Cooling BTU/h = Energy Star band for (area in ft² × ceiling height ÷ 8 ft) × sun (±10%) + 600 × (people − 2) + 4,000 for a kitchenThe chart assumes 8 ft ceilings, so a taller room counts as proportionally more floor before the band is picked: a 300 ft² room with 10 ft ceilings is sized like 375 ft². A 380 ft² room with 8 ft ceilings and normal sun lands in the 350–400 ft² band and gets 9,000 BTU/h, exactly as in the chart. Shade takes off 10%, strong sun adds 10%, every regular occupant beyond two adds 600 BTU/h and a kitchen adds 4,000 BTU/h. Metric rooms are converted to square feet (1 m² = 10.764 ft²) for the lookup, and 1 BTU/h = 0.293 W. Beyond 2,500 ft² the chart stops and the calculator scales the last band linearly, which is only a placeholder for a real load calculation. For heating it uses 30 BTU/h per square foot (about 95 W/m²) for an average home in a moderate climate, scaled by ceiling height, down for good insulation or a mild winter and up for poor insulation or severe cold.
Worked example
A 5 m × 4 m bedroom with 2.5 m ceilings has 20 m², which is 215.3 ft². The 2.5 m ceiling is 2.5% higher than 8 ft, so the room counts as 220.7 ft², inside the 150–250 ft² band: 6,000 BTU/h. Large west-facing windows add 10% (6,600), and a third regular occupant adds 600, for 7,200 BTU/h, or 2.11 kW of cooling (about 106 W/m²). That is just over the 7,000 size, so the next standard unit is 8,000 BTU/h (2.3 kW). Used as a kitchen, the same room would need 11,200 BTU/h and a 12,000 BTU/h unit. Heating the same room, averagely insulated in a moderate climate, needs about 6,620 BTU/h, or 1.94 kW, which is roughly 97 W per square metre.
Room air conditioner sizing chart
| Room area | Cooling capacity | In kilowatts |
|---|---|---|
| 100–150 ft² (9–14 m²) | 5,000 BTU/h | 1.5 kW |
| 150–250 ft² (14–23 m²) | 6,000 BTU/h | 1.8 kW |
| 250–300 ft² (23–28 m²) | 7,000 BTU/h | 2.1 kW |
| 300–350 ft² (28–33 m²) | 8,000 BTU/h | 2.3 kW |
| 350–400 ft² (33–37 m²) | 9,000 BTU/h | 2.6 kW |
| 400–450 ft² (37–42 m²) | 10,000 BTU/h | 2.9 kW |
| 450–550 ft² (42–51 m²) | 12,000 BTU/h | 3.5 kW |
| 550–700 ft² (51–65 m²) | 14,000 BTU/h | 4.1 kW |
| 700–1,000 ft² (65–93 m²) | 18,000 BTU/h | 5.3 kW |
| 1,000–1,200 ft² (93–111 m²) | 21,000 BTU/h | 6.2 kW |
| 1,200–1,400 ft² (111–130 m²) | 23,000 BTU/h | 6.7 kW |
| 1,400–1,500 ft² (130–139 m²) | 24,000 BTU/h | 7.0 kW |
| 1,500–2,000 ft² (139–186 m²) | 30,000 BTU/h | 8.8 kW |
| 2,000–2,500 ft² (186–232 m²) | 34,000 BTU/h | 10.0 kW |
The chart assumes 8 ft ceilings and normal sun; rooms under 100 ft² also get the smallest 5,000 BTU/h unit. For a kitchen, Energy Star adds 4,000 BTU/h because of the heat from cooking appliances.
Why the right size matters
- Oversized air conditioners short-cycle. They cool the air quickly and switch off before removing moisture, leaving the room cold and clammy, and they wear out faster.
- Undersized units run flat out. They never reach the set temperature on the hottest days and use more energy per degree of cooling.
- Heat pumps lose capacity in the cold. Check the output at the lowest outdoor temperature in the data sheet, not only the nominal rating.
- Whole-house systems need a proper load calculation. Rules of thumb suit a single room. For central systems, boilers or several rooms, ask an installer for a Manual J (North America) or EN 12831 (Europe) heat-load calculation.
Frequently asked questions
How many BTU do I need for a 12x12 room?
A 12 × 12 ft room is 144 ft², which falls in the Energy Star 100–150 ft² band: 5,000 BTU/h. If the room gets a lot of sun, add 10% (5,500), which calls for a 6,000 BTU/h unit.
How many BTU per square foot for air conditioning?
The Energy Star chart works out at roughly 20–30 BTU/h per ft² for typical single rooms with 8 ft ceilings (about 65–95 W/m²), dropping to around 15 BTU/h per ft² for very large open spaces.
How do I convert BTU/h to kW?
Multiply BTU/h by 0.000293. A 9,000 BTU/h air conditioner is about 2.6 kW, and a 12,000 BTU/h unit is 3.5 kW.
What size heater do I need for a 20 m² room?
In a moderately cold climate with average insulation, about 1.9 kW. A well-insulated room needs closer to 1.4 kW, a poorly insulated one in a cold region 3.5 kW or more.
Is a bigger air conditioner better?
No. A unit much larger than the load cools in short bursts, removes less humidity and cycles more, so choose the next standard size above the calculated figure.
Last reviewed: 2026-09-27 · How we calculate