What size aircon do I need?
Air conditioner size is measured in BTU (British Thermal Units) — the higher the BTU, the larger the area it can cool. As a quick rule of thumb, you need roughly 450–500 BTU per square metre. So a 20 m² bedroom needs about 9,000–12,000 BTU, while a 40 m² open-plan living area needs around 18,000–24,000 BTU. Below is a simple sizing chart, the factors that change it, and a link to our calculator for an exact figure.
Aircon size chart by room size (BTU)
| Room size | Recommended BTU | Typical use |
|---|---|---|
| Up to 20 m² | 9,000 BTU | Bedroom, small office |
| 20–30 m² | 12,000 BTU | Large bedroom, study |
| 30–45 m² | 18,000 BTU | Lounge, open-plan |
| 45–60 m² | 24,000 BTU | Large living area |
| 60 m²+ | 30,000–36,000 BTU | Open-plan / commercial |
Want an exact number for your room? Use our free aircon BTU calculator — enter your room dimensions and it tells you the right size in seconds.
What affects the BTU you need?
The square-metre rule is a starting point. Bump up the BTU if your room:
- Gets lots of sun — large north- or west-facing windows add heat load.
- Has high ceilings — more air volume to cool.
- Is a kitchen — appliances generate extra heat (add about 10–20%).
- Is busy — each regular occupant adds to the cooling load.
- Is poorly insulated — heat leaks in faster.
Why correct sizing matters
An undersized aircon runs flat-out and never quite cools the room, wasting electricity and wearing out early. An oversized unit cools too fast, switches off, and short-cycles — leaving the room humid and clammy while shortening its lifespan. The right size cools efficiently, keeps humidity comfortable and lasts longer.
Not sure? We will size it for you
If you would rather have an expert confirm the right unit, Van Biljoens has been sizing and installing air conditioners across Pretoria since 1956. Request a free quote or call 012 330 0421 and we will recommend the perfect size for your space.
Frequently asked questions
What size aircon do I need for a bedroom?
Most standard bedrooms (up to 20 m²) are well served by a 9,000 BTU unit. Larger main bedrooms around 25–30 m² suit a 12,000 BTU unit.
How many BTU do I need per square metre?
About 450–500 BTU per square metre as a guideline. Add more for sunny rooms, high ceilings, kitchens or poor insulation.
Is it better to oversize or undersize an aircon?
Neither — match the BTU to the room. Oversized units short-cycle and leave the room humid; undersized units run constantly and wear out early.
What size aircon for a 40m² lounge?
A 40 m² open-plan lounge typically needs an 18,000–24,000 BTU unit, depending on sun exposure and ceiling height.
How do I calculate the BTU for my room?
Multiply your room area (in m²) by roughly 450–500, then adjust for sun, ceiling height and occupancy. Our online BTU calculator does this for you instantly.
