One of the most common mistakes when buying an air conditioner is looking only at price or brand and skipping the capacity calculation entirely. The result, almost always, is a unit that struggles to cool the room properly (if undersized) or that uses more power than it should (if oversized). Let’s walk through how to calculate it correctly in a few steps.

The starting point: what is BTU/h

BTU/h stands for British Thermal Unit per Hour, the standard unit indicating an air conditioner’s cooling (or heating) capacity per hour. The higher the value, the more powerful the unit. It’s the number printed on every AC unit for sale — 9,000, 12,000, 18,000 BTU, and so on — and choosing that number correctly has a direct impact on both comfort and running costs.

The quick method: a reference table

If you don’t want to run the numbers, you can start from this rough table, based on room floor area (assuming a standard ceiling height of about 2.7 meters):

  • 9,000 BTU — small to medium-small rooms, 15-25 m²
  • 12,000 BTU — standard rooms, 25-40 m²
  • 18,000 BTU — large spaces or open plans, 40-60 m²
  • 24,000 BTU — very large spaces, over 60 m²

These figures are a good starting point, but they don’t account for your specific room’s characteristics. For a more precise calculation, use the formula below.

The precise calculation: the K-coefficient formula

  1. Calculate the room’s volume, not just its floor area: multiply width × depth × height (all in meters). A 20 m² room with a 2.7 m ceiling has a volume of 54 m³.
  2. Multiply the volume by a K coefficient, representing the watts needed per cubic meter. A common practical value is K = 25 for a single, typical room, or K = 36 for more general calculations or rooms with greater heat dispersion (for example, an entire home). In our example: 54 m³ × 25 = 1,350 W needed.
  3. Convert watts to BTU/h, the unit manufacturers use: 1 watt equals 3.41 BTU/h. So 1,350 W × 3.41 = roughly 4,600 BTU/h.
  4. Round up to the nearest available size, adding a safety margin of about 10% if the room gets a lot of sun, hosts many people, or contains heat-generating electronics (computers, TVs, appliances).

In our example, a 9,000 BTU unit would be the sensible choice, even though the raw calculation suggested a lower figure: it’s better to have a small margin than to end up with a unit that struggles on the hottest days.

Factors that can change the calculation

The K-coefficient calculation remains indicative: it doesn’t replace a professional assessment, but it’s more than enough to guide your purchase. Also factor in the following, which can push you toward a larger size:

  • Sun exposure: a south-facing room or one with large windows needs more power than a shaded room.
  • Thermal insulation: older or poorly insulated walls and windows lose more cold (or heat) to the outside.
  • Number of people and devices: every person and every running electronic device in the room generates heat the AC has to offset.
  • Floor level: top floors, more exposed to sun on the roof, tend to heat up more than middle floors.

In summary

For a quick estimate, use the floor-area table; for a more accurate calculation, multiply the room’s volume by the K coefficient (25 for a single room, 36 for a more conservative estimate) and convert the result to BTU/h by multiplying by 3.41. Always round up to the nearest available size: a small extra margin of power costs almost nothing on your electricity bill, but makes a huge difference on the hottest days of the year.