Short answer: for a room of 40 to 50 square metres in normal conditions an 18,000 BTU air conditioner is enough. Go up to 24,000 BTU if the room has large south- or west-facing windows, is on the top floor, in an uninsulated building, has a ceiling above 2.7 m or an open kitchen, or if the air conditioner will be the main heating in winter.
Below is the calculation for 40, 45 and 50 square metres, exactly when 18,000 BTU is not enough and what both sizes cost with installation. The general rules for all room sizes are in the guide how many BTU do I need.
The calculation: 350 BTU per square metre
The starting point is a simple formula: the floor area in square metres times 350 BTU. It applies to a 2.7 m ceiling, normal insulation and moderate sun.
- 40 m² × 350 = 14,000 BTU → 18,000 BTU air conditioner (12,000 BTU is for 25 – 35 m² and is not enough here);
- 45 m² × 350 = 15,750 BTU → 18,000 BTU air conditioner;
- 50 m² × 350 = 17,500 BTU → 18,000 BTU air conditioner, but with no reserve.
That is why an 18,000 BTU air conditioner is for 35 – 50 m², and a 24,000 BTU air conditioner for 50 – 70 m². A 50 square metre room sits right on the boundary: whether you need the larger size is decided by the conditions, not by the area itself.
When 18,000 BTU is not enough
Corrections are added to the base calculation. We use the same ones as in the BTU guide:
- south- or west-facing windows: +10 – 15 %;
- uninsulated building (old panel block, no insulation): +15 – 20 %;
- ceiling 3 m or higher: × 1.10 – 1.15;
- coastal location such as Varna, with a hot and humid summer: +5 – 10 %;
- open kitchen into the living room: +10 – 15 %.
Here is what happens in a few typical cases:
| Room | Calculation | BTU needed | Size |
|---|---|---|---|
| 40 m², normal conditions | 40 × 350 | 14,000 | 18,000 BTU |
| 40 m², south-facing windows | 14,000 × 1.15 | 16,100 | 18,000 BTU |
| 45 m², normal conditions | 45 × 350 | 15,750 | 18,000 BTU |
| 50 m², normal conditions | 50 × 350 | 17,500 | 18,000 BTU |
| 50 m², south-facing windows | 17,500 × 1.15 | 20,125 | 24,000 BTU |
| 45 m², south and uninsulated building | 15,750 × 1.15 × 1.20 | 21,735 | 24,000 BTU |
Up to 45 square metres, 18,000 BTU covers almost every case. At 50 square metres, a sunny aspect alone is enough to push the need above 18,000 BTU. The catalogue also has intermediate 21,000 BTU models, but they are usually more expensive than the most affordable 24,000 BTU units, so in practice the choice is between the two standard sizes.
Heating: why winter tips the choice towards 24,000 BTU
If the air conditioner will be the main heating of the living room, besides the BTU on the label also look at the heating capacity at low temperatures. As the outdoor temperature drops, an air conditioner delivers less heat. In a large room, spare capacity means the unit will reach the set temperature on a cold day too, without running flat out all the time.
For 45 – 50 square metres heated only by the air conditioner, 24,000 BTU is the safer choice. If you have other heating in winter, 18,000 BTU is enough. Which models heat efficiently is covered in the article air conditioner for heating.
Does a larger air conditioner use more electricity
With inverter models the difference is smaller than most people expect. The inverter adjusts the compressor speed and, once the room reaches the set temperature, runs at low power. Reference figures from the article how much electricity an air conditioner uses at 8 hours a day and 0.15 €/kWh:
- 18,000 BTU (class A+++): 0.8 – 1.3 kW, about 29 – 45 € a month in summer;
- 24,000 BTU: 0.8 – 1.4 kW, about 30 – 50 € a month.
The problem is the opposite case. A small air conditioner in a large room runs flat out all the time, uses more and wears out faster. One that is too large for the room switches on and off often and dehumidifies poorly. For 40 – 50 square metres both sizes are within reasonable limits.
How much an air conditioner for 50 square metres costs
As of 15.10.2026 the category inverter air conditioners has 87 models of 18,000 BTU in stock with prices from 540 € and 41 models of 24,000 BTU with prices from 735.01 €. Examples:
| Model | BTU | Heats down to | SCOP | Price |
|---|---|---|---|---|
| TechPoint TCP H18 MAXCOOL | 18,000 | −15 °C | 4.0 | 540 € |
| Nippon KFR 18 GLORY PRO | 18,000 | −25 °C | 4.6 | 799.99 € |
| Nippon NPC 18 T-PRO NORDIC | 18,000 | −25 °C | 4.6 | 800 € |
| TechPoint NPT 24 X-PRO NORDIC | 24,000 | −20 °C | 4.0 | 735.01 € |
| Nippon NPC-24T-PRO NORDIC | 24,000 | −25 °C | 4.7 | 1,039.99 € |
All models by size are on the pages 18,000 BTU air conditioners and 24,000 BTU air conditioners, with filters by brand and price. Standard installation of a residential 16,000 – 24,000 BTU air conditioner costs 180 € with us when the unit is bought from Svit Komfortu. What that price includes is on the page air conditioner installation in Varna.
One air conditioner or two
One powerful unit works well when the room is a single space and the air can reach every corner. If the 50 square metres are a living room with an alcove, an L-shape or two rooms with a shared door, one air conditioner will cool only half of it well. Then two smaller units or a multi-split system with one outdoor unit give a more even temperature. This option is covered in detail in the article multi-split systems.
If the room is over 55 – 60 square metres, see the article air conditioner for 60 m².
How to choose in a minute
- Multiply the area by 350.
- If you have large south- or west-facing windows, a top floor or an old building, add 15 – 20 %.
- If the result is up to 18,000, take an 18,000 BTU unit. If it is above 18,000, a 24,000 BTU one.
- If the air conditioner will be the main heating in 45 – 50 square metres, prefer 24,000 BTU.
If you are not sure, use the BTU calculator or call us: the site visit in Varna is free. Ready-made offers with price and installation are on the page air conditioner Varna.
Frequently asked questions
What air conditioner for 50 square metres?
In normal conditions 18,000 BTU. With south- or west-facing windows, a top floor, an uninsulated building or if the air conditioner is the main heating, 24,000 BTU.
What air conditioner for 40 square metres?
18,000 BTU. The calculation is 40 × 350 = 14,000 BTU, and 12,000 BTU is for rooms of 25 – 35 m².
Is a 12,000 BTU air conditioner enough for 40 square metres?
No. It will run flat out all the time, use more electricity and on a hot day will not reach the set temperature.
How many square metres does an 18,000 BTU air conditioner cool?
Roughly 35 – 50 m² with a 2.7 m ceiling and normal insulation.
How many square metres does a 24,000 BTU air conditioner cool?
Roughly 50 – 70 m².
How much electricity does an air conditioner for 50 square metres use?
An 18,000 BTU class A+++ inverter uses about 0.8 – 1.3 kW, or about 29 – 45 € a month at 8 hours a day and 0.15 €/kWh. For 24,000 BTU the reference is 0.8 – 1.4 kW and 30 – 50 € a month.
How much does an air conditioner for 50 square metres cost with installation?
As of 15.10.2026, 18,000 BTU inverter air conditioners start from 540 € and 24,000 BTU from 735.01 €. Standard installation for a 16,000 – 24,000 BTU unit is 180 €.
