Air conditioner BTU calculator
Air conditioners are sold by cooling capacity in BTU/h (9,000, 12,000, 18,000, 24,000). A simple rule based on floor area, adjusted for sun and occupancy, gets you to the right size for an ordinary room.
Your measurements
Result
- Cooling capacity needed
- 6,120 BTU/h
- Equivalent
- 1.79 kW
- Suitable standard unit
- 9,000 BTU/h
Rule of thumb: ~340 BTU/h per m² (100 W/m²) for 2.5–2.7 m ceilings; add 10–15% for high ceilings, rooms with lots of electronics or kitchens.
How the calculation works
- Base capacity = area × about 340 BTU/h per m² (roughly 100 W/m²).
- Adjust for sun: −10% for a room with little sun, +15% for a very sunny one.
- Add 600 BTU/h for each person beyond two who usually uses the room.
- Power in kW = BTU/h ÷ 3,412; the right unit is the first standard capacity above the need.
Worked example
An 18 m² bedroom with average sun and two people needs about 18 × 340 = 6,120 BTU/h, or about 1.8 kW of cooling — a 9,000 BTU unit is enough.
What the calculator assumes
- The 100 W/m² rule is for ordinary rooms with 2.5–2.7 m ceilings and average insulation.
- 9,000 BTU units have about 2.5–2.6 kW of cooling, 12,000 BTU units about 3.5 kW.
When to adjust the result
- High ceilings, large west-facing windows, top floors under the roof and kitchens need 10–30% more.
- For heating in winter, check the heating capacity at −7 °C on the data sheet, not just the cooling capacity.
Choosing and installing an air conditioner
A unit that is too big cools quickly but switches off before removing humidity, leaving a clammy room; one that is too small runs flat out all day. Rooms with large south- or west-facing windows, top floors and kitchens need more capacity than the floor area alone suggests.
Split units contain refrigerant and must be connected by a certified technician in most countries. Place the indoor unit so air does not blow directly on people, give the outdoor unit free airflow and shade, and plan the condensate drain. A heat-pump model can also heat efficiently in spring and autumn.
Frequently asked questions
What size air conditioner for a 20 m² room?
About 20 × 340 = 6,800 BTU/h by this rule, so a 9,000 BTU unit; choose 12,000 BTU if the room is very sunny or under the roof.
How do I convert BTU to kW?
Divide BTU/h by 3,412. A 12,000 BTU unit is about 3.5 kW of cooling.
Is a portable air conditioner as good as a split?
No — portable units are less efficient and noisier, and their exhaust hose brings warm air back in. A split unit is far more effective for regular use.
What temperature should I set?
Around 24–26°C, or no more than about 6–8°C below the outdoor temperature, for comfort and lower consumption.