How many BTU air conditioner should I buy? Calculation by room size

The most common mistake when buying an air conditioner is choosing the capacity by looking only at the room's square meters, or with the thinking 'bigger is better, it will be comfortable'. An undersized air conditioner cannot keep up in the hottest hours and runs at full power all the time. An oversized one cycles on and off often, does not remove enough humidity and creates an unpleasant chill in the room.

In this article we explain how to estimate capacity for residential split air conditioners, which factors change the result and when a professional calculation is needed.

What is BTU?

BTU/h (British Thermal Unit per hour) is the capacity unit that shows how much heat an air conditioner can remove from or deliver to a room in one hour. In technical catalogs the same value is also given in kilowatts (kW); 1 kW corresponds to about 3,412 BTU/h. So a 12,000 BTU/h air conditioner has a cooling capacity of about 3.5 kW.

The BTU value shows how much cooling or heating the air conditioner does, not how much electricity it draws. What determines consumption is the unit's efficiency ratings (SEER and SCOP). Of two air conditioners with the same capacity, the one with higher efficiency uses less electricity.

Rough selection table by square meter

For a room with average insulation, a standard ceiling height (2.6–2.8 m) and no excessive sun, the ranges below are a good starting point. The values in the table are approximate and should be adjusted up or down according to the factors we describe below.

Which factors change the capacity?

Two rooms with the same floor area can be very different in terms of heat load. The right capacity is chosen based on the total heat entering the room. The factors below change this total the most.

01Sun and glazing area

Rooms facing south and west with large windows get a lot of solar heat in the afternoon. In these rooms it is usually necessary to go up one size from the value in the table. In north-facing rooms with small windows, the lower end of the range is often enough.

02Floor and insulation

Attics and top-floor apartments pick up extra heat from the sun-heated roof. Uninsulated exterior walls, old single-glazed windows and high ceilings also increase the load. In an apartment with exterior insulation and thermally insulated glazing, a smaller unit may be enough for the same room.

03Number of people and appliances

Every person and every electrical appliance in the room gives off heat. A crowded living room, an oven and stove running in an open kitchen, or a study with several computers should be added to the calculation separately. In living rooms with open kitchens, the kitchen area should also be included in the total floor area.

04Region and climate

In hot and humid regions such as Antalya, Adana or İzmir, the summer load is clearly higher than in inland areas such as Ankara or Eskişehir. If you will also use the air conditioner for heating, winter conditions are decisive in cold regions such as Erzurum and Kars. In these regions, how well the heating capacity holds up at low outdoor temperatures becomes more important than cooling capacity when choosing a unit.

The heating capacity stated in catalogs is usually measured at mild outdoor conditions. When the outdoor air drops below zero, the capacity of heat pump air conditioners falls; if you are buying for heating in a cold region, always check the manufacturer's low-temperature values.

Example: two identical living rooms, two different results

Let's imagine two 28 m² living rooms in Ankara. The first is on a middle floor, faces north, has thermally insulated windows and the building has exterior insulation. The second is on the top floor, has large west-facing windows and is combined with an open kitchen. According to the table, the starting value for both is 18,000 BTU/h.

In the first living room, 18,000 BTU/h is more than enough, and since the load is low, even a smaller unit could do the job. In the second, once afternoon sun, roof heat and kitchen load combine, a 24,000 BTU/h unit or a solution that cools the room from two points may be needed. As you can see, square meters alone do not decide it.

Why is buying a big air conditioner not a good idea?

An oversized air conditioner cools the room very quickly and stops when it reaches the set temperature. In this short running time the coil cannot condense enough moisture from the air; the room feels cool but humid and clammy. Frequent cycling on and off also wears the compressor and reduces efficiency.

Inverter air conditioners partly reduce this problem by lowering their speed, but there is a lower limit to that too. A greatly oversized inverter unit may still deliver too much to the room even at its lowest speed. We explained the effect of inverter technology on consumption in our article on inverter air conditioner savings.

What else should you look at besides capacity?

The right BTU value is only the first step of a good air conditioner choice. Among units of the same capacity there are clear differences in comfort and running cost; check the points below before buying.

• Efficiency ratings: A unit with a high SEER for cooling and a high SCOP for heating does the same job with less electricity. The class on the energy label is also based on these values.
• Indoor unit location: The air should not blow directly onto where people sit or sleep, and it should sweep the longest side of the room. Positions with obstructions such as curtains, the top of a wardrobe or behind a beam waste capacity.
• Outdoor unit location and pipe length: The outdoor unit should be in a shaded spot with open ventilation; the pipe length between the indoor and outdoor units must not exceed the limit allowed by the manufacturer.
• Sound level: In bedrooms, the noise level (dB) of the indoor unit at its lowest fan speed matters.

When these points are evaluated together at the site survey, problems such as a unit that looks sufficient on paper but falls short in the wrong location are avoided.

When is a professional calculation needed?

For areas larger than 45 m², attics, spaces with large glass facades, apartments where several rooms will be cooled by a single system, and offices, the capacity must be determined with a heat load calculation. In these cases, options such as multiple splits, a multi system or a ducted air conditioner are compared. We covered this comparison in our article on multi air conditioner or split.

At YOİ Mühendislik, during the site survey we examine the room's orientation, glazing area, insulation and use on site, calculate the capacity and decide the indoor and outdoor unit locations together. Because we sell and install all brands, our recommendation is based on the capacity suited to your room, not on a specific product. For details, see our air conditioner sales and installation page.