Sizing is the one decision that cannot be corrected afterwards, and it is the one most often taken by rounding up. The correct capacity comes from a heat gain calculation for that specific room: its volume, its glazing and which way that glazing faces, what is above and below it, and what heat the people and equipment inside it add. Get it wrong in either direction and the room is uncomfortable.
What BTU actually measures
A British thermal unit is a quantity of heat, and a unit's BTU per hour rating is the rate at which it can move heat out of a room. The trade works in kilowatts and the retail market quotes BTU, and they describe the same thing. What neither number tells you is how much heat your room gains, which is the figure the sizing depends on.
What an air conditioning sizing calculator UK sites offer can and cannot know
A sizing calculator asks for floor area and sometimes ceiling height, then applies an assumption per square metre. That is a starting point rather than an answer, because two rooms of identical area can differ enormously in load. A west facing room with full height glazing under an uninsulated flat roof gains several times the heat of the same sized room with a small north window.
Why oversizing is the common mistake
An oversized system cools the air quickly, reaches setpoint and switches off before it has removed any humidity. The room ends up cold and clammy, the compressor short cycles, and the running cost and wear both rise. Undersizing is more obvious because the room never gets there, which is why installers err upward and why it is worth asking to see the calculation.
What size aircon unit do I need? The questions a surveyor should ask
Room dimensions, glazing area and orientation, whether there is shading, what is above and below, wall construction, how many people are normally in the room, and what equipment runs in it. If a firm quotes a capacity without asking most of those, they have sized the room by its area alone and the number is a guess wearing a decimal point.
Other questions people ask
Can I use a rule of thumb per square metre?
As a sanity check, yes; as a specification, no. Rules of thumb assume an average room, and the rooms people want cooled are usually not average. They are the glazed extension, the loft bedroom and the south facing office, all of which gain far more heat than the average the rule was built on.
Does ceiling height matter?
Yes, because you are conditioning a volume rather than a floor. A room with a vaulted ceiling or an original Victorian height holds substantially more air than the same floor area under a standard modern ceiling, and the heat gain through the roof above it is often larger too.
What if one unit has to serve two rooms?
It generally should not. Conditioned air does not travel round corners or through doorways reliably, so a single head serving two rooms cools the one it is in. Where budget forces the compromise, the answer is usually a multi split with a small head in each room rather than one large head in the larger room.
Should the same unit heat the room in winter?
If you want it to, say so before sizing. Heating and cooling loads for the same room are not the same number, and a unit specified only for the cooling peak may be marginal on the coldest days. It is a specification question, not a setting.