Calculating the right aircon BTU for your room: the formula, a size chart, and the Chiang Mai rooms I recommend sizing up for

Choosing the size of an aircon is a decision that stays with your home for many years, so I fully understand why most customers always ask about it before buying. The question I get most often is how many BTU a room of a given size needs. There is already a clear formula for that, and you can do a first calculation yourself quite easily.

But in the jobs I take, most mistakes do not come from multiplying wrongly. They come from not taking the room's particular conditions into account. Homes in Chiang Mai have a wider variety of rooms than many people expect: shophouses that take the full afternoon sun on one side, old wooden houses with uninsulated walls, and upstairs bedrooms under metal roofs that build up heat all afternoon. I will start with the formula and a size chart for you to use as a starting point, then explain which kinds of rooms need extra BTU and by how much, and finish with why choosing a bigger size just in case is not the safe option many people think it is.

What BTU means, in practical terms

BTU is a unit measuring how much heat an aircon can remove from a room per hour. A 12,000 BTU unit is not colder than a 9,000 BTU unit; it can remove more heat in the same amount of time. I often compare it to a water pump for customers: a bigger pump does not make the water cleaner, it just pumps faster.

An aircon's job is to keep removing the heat that flows into the room, fast enough, all the time. Heat gets in many ways: sunlight through glass, heat seeping through the walls and roof, body heat from people, and electrical appliances such as TVs, computers and lights. The more heat flows in, the more BTU the room needs. That is why two rooms of the same size may need aircons of different sizes.

The BTU formula I use to assess a site

The formula used in installation work is to take the room area in square metres and multiply it by a factor based on how the room is used.

• Bedrooms use a factor of about 700–800, because they are used at night with few people and few electrical appliances, so less heat builds up.
• Living rooms, studies and reception rooms use about 800–900, because they are used during the day, people come and go often, and there are more appliances and more sun.
• Calculate the room area as width (metres) times length (metres). If the room is not rectangular, divide it into smaller sections and add them together.

For example, a bedroom 3.5 metres wide and 4 metres long has an area of 14 square metres. Multiplied by 750, that gives 10,500 BTU, which is not a size you can actually buy, because aircons are made in steps: 9,000 / 12,000 / 15,000 / 18,000 / 24,000 BTU. For this room I would recommend 12,000 BTU, because 9,000 BTU falls too far short of what it actually needs.

Room size chart and recommended BTU

If you would rather not calculate it yourself, you can use this chart as a starting point. The figures assume a standard ceiling height of around 2.4–2.6 metres and do not yet include the special conditions I explain in the next section.

Most of the condo and dormitory rooms in Chiang Mai where I install are in the 16–30 square metre range. Studios that combine the sleeping and living areas usually end up with 12,000 BTU for a small room, and 18,000 BTU for a larger room or one that gets the sun. For detached houses where the living room opens onto the kitchen, you have to count the whole combined area, not just the part where the sofa sits.

Which rooms in Chiang Mai need extra BTU

The conditions below are what the basic formula does not cover, and they are the number one reason a room with a correctly calculated BTU still is not cool enough. The formula assumes the room gets no special heat from outside, and many homes in Chiang Mai are not like that.

These conditions can be added together, but should not be stacked beyond what is necessary. For example, an upstairs bedroom of 16 square metres that gets the afternoon sun and sits under the roof: the basic calculation is 16 × 750 = 12,000 BTU. Adding a combined allowance of about 20–25% gives roughly 14,400–15,000 BTU, so I would recommend 15,000 BTU instead of 12,000 BTU. But if the same kind of room were on the ground floor, facing north and shaded by trees, I would tell you honestly that 12,000 BTU is enough, with no need to pay more.

Choosing too big: the problems people rarely consider

The misconception I come across most often is sizing up generously just to be safe. The truth is that an aircon too big for the room causes several clear problems, and is even harder to fix than one that is too small, because the symptom is not a lack of cooling but cooling that feels uncomfortable, and homeowners often do not realise the cause is the size of the unit.

• The compressor cuts out unusually often, because the room reaches the set temperature too quickly, so the unit stops and starts again repeatedly in short cycles.
• The room is cool but your skin still feels sticky, because an aircon needs to run continuously for a while to pull moisture out. When it cuts out early every time, the moisture stays in the room.
• The compressor's lifespan is shortened, because start-up is when it uses the most electricity and wears the most. The more often it starts, the faster it deteriorates.
• The electricity bill does not drop as expected, because the current at start-up is much higher than during normal running, so frequent cycling pushes total consumption above what it should be.
• The room temperature swings up and down, very cold then warm, which disturbs sleep because your body has to keep adjusting all night.

This cool-but-sticky feeling is especially noticeable in Chiang Mai from July to October, the heaviest-rain part of the rainy season, when the air is already very humid. If the aircon is oversized and keeps cutting out early, the moisture stays in the room until it becomes uncomfortable to use. In the jobs I take, homeowners often assume the unit is not powerful enough and turn the temperature down further, which costs more in electricity without fixing the root cause.

Choosing too small: more obvious, but just as damaging

An aircon too small for the room runs almost all the time and hardly ever cuts out, because it cannot remove heat as fast as it flows in. The room never reaches the set temperature, especially on hot-season afternoons in Chiang Mai, when the outside temperature on some days goes above 40 degrees.

• The compressor runs continuously with almost no rest, and its lifespan is shortened because it is working beyond its rating.
• The electricity bill is higher than it should be, because the unit runs at full power all the time with no breaks.
• The room does not reach the set temperature, so the homeowner keeps turning it lower, which does not help when the unit was never powerful enough in the first place.
• On very sunny days, the aircon may only manage 27–28 degrees and stay there.

Calculating BTU from start to finish

1. 1

   Measure the real room area

   Measure the width times the length in metres. If the room connects to another area with no door in between, such as an open kitchen or a stairwell, include that area too, otherwise your figure will be lower than reality.

2. 2

   Multiply by the factor for how the room is used

   Bedrooms use 700–800, and living rooms or studies use 800–900. If you are unsure, use the middle value for now.

3. 3

   Check the room's special conditions

   Consider whether the room gets the afternoon sun, whether it is on the top floor, whether it is a wooden house or has thin walls, and how many people use it at once, then add the percentages for the conditions that match your room.

4. 4

   Choose a size that is actually sold

   Round up to the next size when your figure is well above the smaller size, but do not skip a size, because that turns into the problem of an aircon too big for the room.

5. 5

   Let me assess the site before you decide

   I can check the room's orientation, the ceiling, the pipe run and where the outdoor unit will go, all of which affect both the right size and the installation cost. Getting it clear before you buy saves you having to fix things later.

As for the units themselves, I install Mitsubishi, Daikin, Samsung, LG, Panasonic, Carrier, Haier, Sharp, Toshiba, Gree, Hitachi and Trane. Unit prices depend on the model and on promotions at the time, so feel free to ask. Installation is charged directly by unit size: 9,000–12,000 BTU is 3,000 THB to install, and 13,000–24,000 BTU is 3,500 THB. If an old unit has to be removed first, that is an extra 700 THB. I tell you all the costs from the start, and if I find anything additional on site, I will tell you and ask for your agreement before going ahead.

Summary

• The basic formula is the room area in square metres, times 700–800 for a bedroom and 800–900 for a living room.
• The result is only a starting point. Add an allowance when the room gets the afternoon sun, is under the roof, is a poorly insulated wooden house, is used by many people, or is open to the kitchen.
• An oversized aircon cuts out too often, does not remove enough moisture, leaves the room cool but sticky, and wears the compressor out faster.
• An undersized aircon runs non-stop, costs more in electricity, and cannot reach the set temperature on very sunny days.
• Tackling the source of the heat, such as window film and roof insulation, is better value than moving up an aircon size.