Absolute What Size Aircon Do I Need? A BTU Guide for Gauteng Homes
Buy an aircon that's too small and it runs flat out all summer without keeping up. Buy one that's too big and you pay more for a room that never feels properly comfortable. Here's how to get it right.
Updated
🔍 R350 call-out to inspect & diagnoseQuick answer
For a typical Gauteng room with a standard 2.4–2.7 m ceiling, use a 9000 BTU split for up to about 15–20 m², 12000 BTU for about 20–30 m², 18000 BTU for about 30–45 m² and 24000 BTU for about 45–60 m². Go one size up for west-facing rooms, big windows, top-floor rooms under a hot roof, high ceilings or open-plan spaces. An inverter unit costs more to buy but less to run. Absolute Repairs supplies and installs split aircons across Johannesburg and Pretoria.
Quick sizing chart
| Aircon size | Room size (approx.) | Typical room |
|---|---|---|
| 9000 BTU | Up to 15–20 m² | Small bedroom, study, home office |
| 12000 BTU | 20–30 m² | Main bedroom, small lounge, office |
| 18000 BTU | 30–45 m² | Lounge, family room, small open-plan area |
| 24000 BTU | 45–60 m² | Large lounge, open-plan living, small shop or office |
| Above 60 m² | Two units, or a cassette or ducted system | Large open-plan, restaurants, offices |
To measure, multiply the room's length by its width. A 4 m × 5 m bedroom is 20 m², which is on the 9000/12000 BTU boundary. Use the checks below to decide.
Go one size up if...
- The room faces west or north-west. Afternoon sun in a Pretoria or Joburg summer adds a lot of heat.
- There's a lot of glass: sliding doors, big windows or a sunroom.
- It's directly under the roof with poor ceiling insulation, or it's a top-floor flat.
- The ceiling is higher than about 2.7 m, or it's a double-volume space.
- It's open-plan and the cool air will flow into a kitchen or passage.
- There are lots of people or equipment: offices, server cupboards, kitchens, gyms.
Inverter or fixed-speed?
Inverter
Slows down instead of switching on and off. Quieter, holds a steadier temperature and uses noticeably less electricity, which suits bedrooms and rooms used daily. Most new units sold are inverters.
Fixed-speed
Cheaper to buy, simpler to repair. Fine for a room used occasionally, but costs more to run if it's on for hours every day.
Most current units use R32 or R410a gas. Older units on R22 are being phased out and are increasingly expensive to regas. If yours is on R22 and failing, replacing it is often the better choice.
What a proper installation includes
- A site visit to check the wall, the outdoor unit position and the electrical supply.
- Indoor unit mounted level, with the drain line falling correctly so it doesn't leak down the wall.
- Copper piping insulated and kept as short as practical, then pressure-tested and vacuumed before the gas is released.
- Outdoor unit on brackets or a stand with airflow around it, out of direct afternoon sun where possible.
- A correctly sized, dedicated circuit and isolator.
- A test run checking cooling, heating and drainage, and a walk-through of the remote.
Poor installation is the cause of many of the aircon problems we repair later, especially water leaking and weak cooling.
Aircon installation in Pretoria and Johannesburg
We supply and install split aircons in Pretoria East, Centurion, Midrand, Sandton, Fourways, Randburg and the West Rand. See aircon installation in Gauteng for more.
Frequently asked questions
What size aircon do I need for a bedroom?
Most bedrooms up to about 20 m² need a 9000 BTU split. A larger main bedroom, or one facing west or under a hot roof, usually needs 12000 BTU.
Is a bigger aircon always better?
No. An oversized unit cools the room quickly then switches off before it has removed the humidity, so the room feels clammy. It also costs more to buy and install.
What size aircon do I need for a 30 m² room?
About 12000 BTU for a well-shaded room, or 18000 BTU if it gets afternoon sun, has big windows or is open-plan.
How much electricity does an inverter aircon use?
Much less than a fixed-speed unit of the same size, because it slows down instead of cycling on and off. The saving is biggest in rooms where the aircon runs for many hours a day.
Can you install an aircon in a complex or estate?
Yes. We'll work with the body corporate or HOA rules on where the outdoor unit can go, and arrange estate access.
Want us to size it for you?
WhatsApp us the room size, which way the windows face and a photo of where the outdoor unit could go. We'll recommend a size and quote for the installation. Monday to Saturday, 07:30–17:30.