FixMeasure
Basics
What size air conditioner do I need? BTU explained
An air conditioner that is too small never cools the room; one that is too big cools in short bursts and leaves the air damp. This guide explains BTU, the usual sizing rules and the factors that change them.
1. What BTU means
Cooling capacity is often given in BTU per hour (BTU/h, usually shortened to BTU). One BTU is the heat needed to raise one pound of water by one degree Fahrenheit. In metric terms, 1 kW ≈ 3,412 BTU/h, so a 9,000 BTU unit is about 2.6 kW and a 12,000 BTU unit about 3.5 kW of cooling. This is the cooling output, not the electricity the unit uses.
2. The basic rule of thumb
Sizing charts published by ENERGY STAR and others step up in bands by floor area. For an average room with a normal ceiling height, they give roughly:
| Room area | Capacity (BTU/h) |
|---|---|
| 9–14 m² (100–150 ft²) | 5,000 |
| 14–23 m² (150–250 ft²) | 6,000 |
| 23–28 m² (250–300 ft²) | 7,000 |
| 28–33 m² (300–350 ft²) | 8,000 |
| 33–37 m² (350–400 ft²) | 9,000 |
| 42–51 m² (450–550 ft²) | 12,000 |
The Air conditioner BTU calculator uses this table and applies the adjustments below.
3. Adjust for your room
| Condition | Adjustment |
|---|---|
| Heavily shaded room | Reduce by 10% |
| Very sunny room | Increase by 10% |
| More than two people regularly | Add 600 BTU/h per extra person |
| Kitchen | Add 4,000 BTU/h |
Also consider ceilings higher than about 2.5 m, top-floor rooms under a hot roof, large windows facing south or west, and poor insulation. All of these increase the load.
4. Why bigger is not better
An oversized unit reaches the set temperature quickly and switches off before it has removed much moisture. The room feels cold but clammy, the unit cycles on and off more often, and it uses more energy. Choose the size that fits the room; modern inverter units adjust their output and handle small sizing differences better than older on/off models.
5. Efficiency labels
Efficiency is shown as a ratio of cooling output to electricity used: EER at a fixed test condition, and SEER over a typical season. Higher numbers mean lower running costs. In the EU, the energy label also shows a class (A+++ to D) and the annual consumption. In heating mode, heat pumps show COP or SCOP.
6. Reduce the load first
Shading windows from outside, insulating the roof and sealing air leaks can cut the cooling load considerably, so a smaller, cheaper unit may be enough. See Insulation materials compared: wool, EPS, XPS, PIR for insulation options. Installation of split systems involves refrigerant and electrical work and should be done by a certified installer.
Common mistakes
- Buying the biggest unit available, which leaves the room cold and damp.
- Ignoring sun, people and kitchen heat when sizing.
- Comparing units by BTU only and not by efficiency (SEER).
- Placing the unit where furniture or curtains block the airflow.
- Installing a split system yourself without the required certification.
Frequently asked questions
How many BTU do I need for a 20 m² room?
About 6,000 BTU/h for an average room, and about 10% more if it is very sunny. Many split units start at 7,000–9,000 BTU, which is fine for a room of that size if the unit is an inverter model.
How do I convert BTU to kW?
Divide by about 3,412. For example, 12,000 BTU/h ≈ 3.5 kW.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cycles on and off and removes less humidity, so the room feels clammy and energy use goes up.
Does a kitchen need a bigger unit?
Yes. Add about 4,000 BTU/h for the heat from cooking appliances.
How can I size an air conditioner quickly?
Use the AC size calculator: enter the room size and conditions and it gives a recommended capacity.