Are you about to buy an air conditioner and don't know where to start? The first question to ask yourself, even before choosing the model or brand, is how much power you need. And the power of an air conditioner is measured in BTUs.
Choosing the right BTU is the difference between an air conditioner that works well and one that either fails to cool the room, or cools it so quickly that it leaves you feeling humid and with a rising electricity bill. In this guide, we'll explain how to navigate this, room by room, with a simple method and some tips that many people forget.
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What are BTUs and why do they matter?
BTU is an acronym for British Thermal Unit, the international unit of measurement used to indicate the thermal power of an air conditioner. In practice, it expresses how much energy the air conditioner can remove from the air, i.e., how much it can cool in an hour.
The more BTUs an air conditioner has, the more powerful it is. But beware: more powerful doesn't necessarily mean better. An oversized air conditioner for the room cools too quickly, turns off before it has dehumidified the air, and creates those annoying temperature fluctuations that never make you feel truly comfortable. An undersized one, on the other hand, works continuously at maximum without ever reaching the temperature you set, consuming more and wearing out faster.
The right power balances comfort, efficiency, and durability over time.
The basic formula: BTU per square meter
The starting point for calculating BTUs is the room's square footage. The most commonly used formula, confirmed by manufacturers and installers, is simple:
square meters × 340 = necessary BTUs (basic estimate)
Some practical examples:
- Room of approximately 20 sqm: 6,800–7,000 BTU, 9,000 BTU model
- Room of approximately 25–30 sqm: 8,500–10,000 BTU, 9,000–12,000 BTU model
- Room of approximately 35–40 sqm: 12,000–14,000 BTU, 12,000–18,000 BTU model
- Room of approximately 50 sqm: 17,000 BTU, 18,000 BTU model
- Area of 70–100 sqm: 24,000 BTU or multisplit
Keep in mind that commercially available models are standardized to the most common sizes: 9,000, 12,000, 18,000, and 24,000 BTU. When the calculation falls in between, you move up to the next size, never down.
Square footage alone is not enough: factors that change everything
This is the point that many underestimate. The sqm × 340 formula is an excellent starting point, but the actual needs can be very different based on your home's characteristics. Here's what really matters.
Sun exposure. A room facing south or southwest, which gets direct sunlight for many hours, accumulates much more heat than one facing north. In these cases, it's better to choose a larger size than the basic estimate.
Thermal insulation. Well-insulated walls, double glazing, and modern windows retain temperature better and reduce energy needs. Conversely, old walls, dated windows, and drafts increase heat loss and make everything more difficult for the air conditioner. If insulation is poor, add a 20–30% margin to the calculated BTUs.
Ceiling height. The calculation based on square meters assumes standard ceilings of approximately 2.7 meters. If yours are higher, as often happens on the lower floors of historic buildings or in villas, the volume of air to be treated increases, and the requirement rises accordingly.
Large glass surfaces and wide windows. Glass surfaces let in much more solar radiation than a solid wall. The more glass there is, the harder the air conditioner has to work.
People and electronic devices. Each person in a room produces heat, estimated between 300 and 600 BTU. TVs, computers, lights, and other appliances also contribute to raising the internal temperature. In a kitchen or a frequently used living room, this aspect must be taken into account.
How many BTUs for each room in the house
Applying the formula and corrective factors, here is an indicative estimate for the most common rooms in a typical home with 2.7 m ceilings and average insulation:
Bedroom (15–20 sqm): usually 7,000–9,000 BTU are sufficient. It's a room not frequently used during the day, often well-insulated. If it faces south or has a large window, it's better to opt for a 9,000 BTU.
Study or single room (12–18 sqm): 7,000–9,000 BTU are generally sufficient, but if you work remotely with a PC and monitors on for many hours, consider that internal heat production increases.
Living room (25–40 sqm): the most variable room. A standard 30 sqm living room, well-insulated, can be fine with 12,000 BTU, but an open-plan space with an open kitchen, high ceilings, and large windows can easily require 18,000 BTU or a multisplit solution.
Kitchen (10–15 sqm): despite its small size, the kitchen produces a lot of internal heat. A 9,000 BTU model is often recommended, but it depends a lot on how much you use it and how well it's ventilated.
Open-plan or large living area (40–60 sqm): ranging from 18,000 BTU for well-insulated environments, up to 24,000 BTU or a multisplit system for spaces highly exposed to the sun or with high ceilings.
Oversizing or undersizing: what really happens
It's worth understanding the risks of both errors, as they are more serious than they seem.
An undersized air conditioner always works at maximum without ever reaching the desired temperature. It consumes more, wears out faster, and cannot guarantee stable comfort. In summer, in a very hot room, it's frustrating: the air conditioner runs but is never enough.
An oversized air conditioner cools too quickly, turns off before the air has been properly dehumidified, and restarts almost immediately in a continuous cycle of on and off, the so-called "short cycling". The result is cold but humid air, annoying temperature fluctuations, and premature compressor wear.
The right size, neither too small nor too large, is one that works continuously and modulated, especially if it's an inverter model. This is precisely why inverters perform best when correctly sized: they modulate power according to actual needs and maintain stable temperature with low consumption.
Monosplit, dualsplit or multisplit: the choice also depends on BTUs
Once you understand how much power you need for each room, you also have a clearer idea of which configuration to choose.
If you need to air condition only one room, a monosplit is the simplest and most economical solution. Choose the BTU power based on the room's square footage and characteristics.
If there are two or three rooms to air condition, a dualsplit or trialsplit system allows you to connect multiple indoor units to a single outdoor unit. The advantage is that you can choose different sizes for each room, for example, 12,000 BTU for the living room and 9,000 for the bedroom, optimizing the result without multiplying the outdoor units on the balcony or facade.
On Worldclima, you'll find a complete selection of monosplit, dualsplit, and trialsplit air conditioners in all common sizes, for every need and budget.
A final piece of advice before buying
The formula and tables in this guide provide a reliable estimate to help you choose among models. But if you have an unusual situation, very high ceilings, a large open-plan space, an uninsulated attic, an apartment in a historic building, it's worth asking for professional advice before purchasing.
Incorrect sizing is a problem you'll live with for years. The right sizing, on the other hand, means an air conditioner that works well, consumes what it should, and lasts over time.
Do you have doubts about the right power for your home? Contact the Worldclima team: describe the rooms you want to air condition, and we'll help you find the most suitable solution among the models available in the catalog.
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