Air conditioner size calculator
A component estimate: you can see where every watt comes from.
Your inputs
Your inputs are processed on your device and are not sent to a server.
Example loaded. You can change any value.
There is no figure yet: nothing is calculated until you ask for it.
- Enter your measurements.
- Check the units and the assumptions.
- Press Calculate, or load the example.
Rather see a worked case? Press “Load example” and change any value afterwards.
Next step
You know the capacity you need; what comes next is what it will consume.
Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates
How to use it
- Enter the floor area and the ceiling height.
- Choose the insulation level and your summer climate.
- Say how many people are usually in the room and the window area.
- Set the orientation and shading of those windows.
How it is calculated
The calculator adds four heat contributions and applies a margin. Every factor is visible so you can argue with it:
The specific load per volume is 30 W/m³ with good insulation, 40 with average insulation and 50 with poor insulation. The climate factor is 0.85 for a mild summer, 1.00 for temperate and 1.15 for very hot.
The 120 W per person is the total heat (sensible plus latent) of a seated adult at light activity, the order of magnitude used in everyday HVAC practice.
Solar gain uses 60 W/m² of glazing facing north or permanently shaded, 110 facing south and 150 facing east or west, multiplied by 1.0 unshaded, 0.7 with blinds or curtains and 0.45 with an awning or permanent shade. East and west windows take the low morning and afternoon sun, which is why they count for more than south-facing glass in summer.
Worked example
A 215 sq ft living room with an 8 ft ceiling, average insulation, temperate climate, 2 people and 21 sq ft of unshaded west-facing glazing:
- Volume: 20 m² x 2.44 m = 48.8 m³
- Envelope: 48.8 x 40 x 1.00 = 1,952 W
- People: 2 x 120 = 240 W
- Solar: 1.95 x 150 x 1.0 = 293 W
- Subtotal: 2,485 W, plus 10 % margin = 2,733 W
- That is 9,326 BTU/h, or 0.78 tons
- Commercial size: 12,000 BTU/h (1 ton)
Add blinds to that window and it drops to about 8,300 BTU/h; with good insulation, to 7,500. Both changes together move the unit down a commercial size.
Good to know
- This is a guideline estimate. A professional heat-load calculation considers the real build-up of every wall and roof, thermal bridges, infiltration, humidity and hour-by-hour climate data.
- Oversizing is not free. An oversized unit reaches temperature, stops, and starts again; in those short cycles it dehumidifies badly, so the room feels cold and clammy.
- Insulation dominates. Between good and poor there is a 67 % difference in the envelope load.
- West-facing windows are the enemy. They take the afternoon sun, when the house is already hot. An awning can save more than moving up a size.
- Capacity is not consumption. The BTU/h, frigories or kW on a label measure the heat the unit removes from the room, not the electricity it uses. For that you need the efficiency rating or the rated input power.
Frequently asked questions
What size air conditioner do I need for 200 square feet?
With an 8 ft ceiling, average insulation, a temperate climate, two people and 20 sq ft of west-facing glazing, around 8,800 BTU/h. The nearest commercial size is 9,000 BTU/h, and a 12,000 BTU/h unit would be oversized.
How many BTU per square foot do I need?
The example above works out at roughly 44 BTU/h per square foot, but that figure depends on ceiling height, insulation and sun. Per-area rules are for sanity-checking, not for deciding.
Is it bad to buy a unit with more capacity than needed?
Yes. An oversized unit short-cycles: it cools fast, stops and restarts. It dehumidifies worse, wears faster and costs more.
What matters more, insulation or window size?
In most homes, insulation, because it affects the whole envelope. But a large unshaded west window can add as much heat as several people.
Can I add up rooms to size a whole house?
Not directly. Each room is sized separately, and a central system has to account for diversity: rooms rarely peak at the same moment.
Does this replace a professional calculation?
No. It is a transparent estimate, useful for understanding the order of magnitude and checking a quote, but a real load calculation considers many more variables.