Air Conditioner Size Calculator
Add up the heat a room gains and turn it into watts and BTU per hour, so you can match a unit to the room rather than guess.
Results
What this tool does
An air conditioner has to remove heat at least as fast as the room gains it. That heat comes from three places: through the walls, windows and roof; from the people in the room; and from everything plugged in. This page adds them up and converts the total to BTU per hour, which is how units are sold. The figure that matters most is the load per square metre, and it is an input rather than an assumption, because it depends on your building and your climate and nothing else can stand in for it.
Formula
load = area × load/m² + people × heat/person + appliances + solar gain ; BTU/h = W × 3.412142
Variables
| Symbol | Meaning | Unit |
|---|---|---|
a | Floor area | m² |
h | Ceiling height | m |
w | Cooling load per square metre | W/m² |
n | People in the room | — |
pw | Heat given off per person | W |
ap | Appliances and lights | W |
s | Extra for sun and glazing | % |
W | Cooling needed | W |
B | The same in BTU per hour | BTU/h |
K | Power | kW |
V | Room volume | m³ |
R | Works out at, per square metre | W/m² |
Worked example
- Floor area20 m²
- Ceiling height2.6 m
- Cooling load per square metre100 W/m²
- People in the room2
- Heat given off per person100 W
- Appliances and lights300 W
- Extra for sun and glazing15 %
- Cooling needed2875 W
- The same in BTU per hour9810 BTU/h
- Power2.88 kW
- Room volume52.0 m³
- Works out at, per square metre144 W/m²
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
Frequently asked questions
Where does the load per square metre come from?
From you, and it is the number that decides the answer. It depends on how well the walls are insulated, how much glass there is, which way the room faces and how hot your summers get, and no single figure covers all of that. A well-insulated north-facing room in a mild climate sits far below a top-floor room with large west-facing windows in a hot one. If you do not know yours, the installer's survey or your country's building guidance is where to get it — not a page that has never seen your house.
Is bigger always safer?
No. A unit that is too large cools the air to the set temperature quickly and then stops, which leaves the room cold and still damp, because removing humidity takes running time the machine never gets. Short cycles also wear the compressor. Matching the load is the point of working it out at all.