What Size Air Conditioner Do I Need?
Choosing the right capacity is the most important aircon decision you will make. Too small and it runs constantly without reaching the set temperature. Too large and it short-cycles, wastes energy and leaves the air damp.
Published 2026-06-01 · Van Biljoens Appliance Services & Air Conditioning
Why sizing matters
An undersized air conditioner runs at full load all day, fails to reach your target temperature in peak heat, and wears out faster. An oversized unit reaches the set temperature quickly, switches off, and then the room heats up again in minutes. This short-cycling also prevents the unit from running long enough to dehumidify the air properly — the result is a room that feels cool but clammy.
Correct sizing means the unit runs steadily in the heat of the day at or near full capacity, and at a lower setting in mild conditions. That is the operating range it is designed for.
The basic rule of thumb
A commonly used starting point is approximately 125 watts of cooling capacity per square metre of floor area for a standard residential room in a moderate climate. At this rate, a 20 m² bedroom would need roughly 2 500 W (2.5 kW) of cooling capacity.
BTU is also widely used: 1 kW of cooling = approximately 3 412 BTU/h. A 2.5 kW unit is therefore approximately a 9 000 BTU unit.
- ✓ ~125 W/m² for a shaded, well-insulated room
- ✓ ~150–175 W/m² for a west- or north-facing room with significant sun exposure
- ✓ ~175–200 W/m² for rooms with large glass areas, open-plan spaces, or high occupancy
Factors that increase the required capacity
The simple area rule does not account for every variable. Factors that increase the required capacity include high ceilings (the unit must cool more air volume), significant direct sun through windows, poor insulation, high occupancy, heat-generating equipment such as computers or cooking, and proximity to a kitchen.
On the other hand, a well-insulated room with double-glazed windows and minimal sun exposure may need less than the area rule suggests.
The BTU calculator
Use our online BTU calculator to get a more accurate estimate based on your room dimensions, ceiling height, sun exposure, and window area. It is a useful guide, though nothing replaces a proper site assessment for larger or more complex installations.
- ✓ Measure floor area (length × width)
- ✓ Note ceiling height
- ✓ Note which direction the main windows face
- ✓ Estimate the proportion of wall that is glass
Getting it right
If you are unsure, contact us with your room details and we can advise on an appropriate capacity. It is better to ask before purchasing than to install an incorrectly sized unit and live with the consequences.
Ready to take the next step?
Van Biljoens has been supplying and servicing Pretoria since 1956. We are here to help.