Air Conditioner BTU Calculator

How to use this calculator

1. Enter the floor area of the room and its ceiling height — an air conditioner cools a volume, not a floor.
2. Pick your climate and how much sun the room takes. A west-facing top-floor room with large windows can need half again as much cooling as a shaded ground-floor room of the same size.
3. Add the number of people normally in the room and any heat-producing appliances.
4. Click Calculate BTU needed for the required capacity and the standard unit size to buy.

How the sizing works

The base load scales with area and with sun exposure, corrected for ceiling height:

The sun factor runs from about 600 BTU per m² for a shaded room to 900 for a west-facing room with large windows on the top floor. The climate factor scales that down for temperate northern Europe and leaves it at full strength in the tropics.

Then come the internal gains, which people routinely forget:

• Each person beyond the first two: +500 BTU/h (a human body at rest is a 100 W heater).
• Each heat-producing appliance — TV, desktop PC, fridge, console: +500 BTU/h.
• A kitchen: +4,000 BTU/h, because the cooker and the fridge fight the air conditioner directly.

Why bigger is not better

An oversized air conditioner is a genuinely bad purchase, and the reason is not just the higher price:

• It short-cycles. It blasts the room to the setpoint, shuts off, and repeats. Compressor start-up is the least efficient moment of operation, so frequent cycling wastes energy.
• It does not dehumidify. Removing moisture takes time on the coil. A unit that reaches temperature in five minutes never runs long enough to dry the air, which is why an oversized unit leaves a room cold and clammy — and why people then set it even colder.
• It costs more to buy and to install.

An undersized unit is worse in the other direction: it runs at 100% permanently, never reaches the setpoint, and both your comfort and your bill suffer. Size it right, then buy the nearest standard rating above the number this calculator gives you — never below.

Standard sizes and what they cover

These are rules of thumb for a normally sunlit room with a 2.6 m ceiling and two occupants. Use the calculator for your actual room — the difference between a shaded bedroom and a sunny open-plan living room of identical size can be an entire unit size.

Frequently asked questions

What size air conditioner do I need for a 20 m² room?

In a temperate climate with normal sun, roughly 10,500 BTU/h — so a 12,000 BTU unit. In a tropical climate the same room needs about 14,000 BTU/h, which means an 18,000 BTU unit. Climate changes the answer by a whole size class.

How many BTU per square metre?

Roughly 450–700 BTU/m² in temperate Europe and 600–900 BTU/m² in the tropics, depending on sun exposure. In imperial terms the old rule of 20 BTU per square foot is a reasonable starting point for a US climate.

Is it bad to buy a bigger air conditioner than I need?

Yes. An oversized unit short-cycles, which wastes energy on repeated start-ups, and it never runs long enough to dehumidify — so the room ends up cold and damp rather than comfortable.

Does ceiling height matter?

Yes, because you are cooling a volume. A 3.5 m ceiling instead of 2.6 m adds about 35% to the load, which is often enough to push you into the next unit size.

How much does a person add?

About 500 BTU/h at rest — a human body dissipates roughly 100 W of heat. In a room that regularly holds six people, that alone is 2,000 BTU/h beyond the base load.

Do I need more BTU for a kitchen?

Yes — add about 4,000 BTU/h. The cooker, the oven and the fridge's condenser all dump heat directly into the room the air conditioner is trying to cool.