What air conditioning capacity do I need – calculation based on area and room type

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What cooling capacity do I need – complete calculation according to area, room type and real conditions

One of the most common mistakes we see in orders from ordinary households as well as professional clients is underestimating or, conversely, overdimensioning the cooling capacity of an air conditioner. The customer looks at the room area, multiplies it by some number they found on the internet, and orders a unit. The result? Either the air conditioner cannot cool the space even on a tropical day, or it runs in short cycles, doesn't dehumidify the air and consumes unnecessarily high amounts of electricity. Yet the correct calculation of capacity is not rocket science – it only requires considering several specific parameters at once, not just the floor area.

In this article, we will go through the entire calculation process from the basics to advanced correction factors, show real-life examples and explain why the same 20 m² room may require 2.0 kW and 3.5 kW of capacity depending on the conditions. If you are also interested in the unit selection and the comparison of split and multi-split solutions, see the article Split vs. multi-split air conditioning – which one is more cost-effective in our Knowledge Center.

Why floor area alone is not enough

The rule of "100 W per square meter" is so deeply rooted in sales practice that people consider it a physical law. It is not. It is a rough estimate that works well only for average conditions in Central European climate, standard ceiling height of 2.5 m, standard window orientation and average heat gains from people and equipment. As soon as any of these change, the number can be completely out of reality.

In practice, it looks like this: a server room of 15 m² with eight server racks may require 8–12 kW of cooling, while a bedroom of the same area in a well-insulated northern apartment will be satisfied with 1.5 kW. Or an office with a glass curtain wall facing south at 30 m² requires at least 4.5–5 kW, while a similarly sized room in the basement of the same house doesn't need air conditioning almost at all.

Therefore, the real calculation of cooling capacity (in technical language, it is called cooling load or "cooling demand") must take into account several heat sources at once.

Basic calculation: from area to capacity step by step

The simplest way to reach the capacity is the step-by-step calculation method, where each room factor either increases or decreases the basic estimate. The process is as follows:

Step 1 – Basic cooling load according to area

As a starting point, we use the standardized value of 80 W/m² for well-insulated spaces and 100 W/m² for standard apartment spaces with standard insulation. This basic estimate assumes a ceiling height of 2.5 m, which is the most common case in apartment buildings.

Example: Living room 25 m², standard insulation → 25 × 100 = 2 500 W = 2.5 kW basic load.

Step 2 – Correction for ceiling height

If the ceiling height is different from 2.5 m, the air volume in the room changes and the basic calculation needs to be adjusted. The correction factor is simple: if the ceiling is higher, the air volume is greater and the air conditioner has to work more.

Ceiling 2.3 m: correction factor 0.92 (reducing capacity)
Ceiling 2.5 m: factor 1.0 (basic condition)
Ceiling 2.8 m: factor 1.12
Ceiling 3.0 m: factor 1.20
Ceiling 3.5 m: factor 1.40
Ceiling 4.0 m and more (halls, commercial spaces): factor 1.60 and higher

Example: The same living room 25 m², but with a suspended ceiling at 3.2 m height → 2 500 × 1.28 = 3 200 W.

Step 3 – Correction for orientation and glazing

Solar gain through windows is one of the most significant factors that laymen underestimate. The distinction is as follows:

Northern orientation, small windows: subtract 10 % from the base (–10 %)
Northern or eastern orientation, standard windows: base unchanged (0 %)
Southern orientation, standard windows: +10 %
Southern orientation, large windows (over 30 % of wall area): +20 %
Glazed façade on the south or west: +30–40 %
Roof window or skylight: +25 % for each skylight over 1 m²

Step 4 – Heat gains from people and equipment

Each person at rest produces approximately 80 W of heat, at activity 100–120 W and at physical work up to 200–300 W. Equipment produces heat equal to their power consumption (with the exception of equipment where energy is converted into other forms, e.g. mechanical work).

Desktop computer + monitor: 150–250 W
Laptop: 30–65 W
1U rackmount server: 200–500 W
Laser printer: 400–900 W (during printing)
55" TV: 100–150 W
LED lighting: 10–15 W/m² (for offices)

For residential spaces, the typical allowance for people and equipment is 200–400 W per room. For offices with 4–6 employees and computers, an additional 1 000–2 000 W must be added.

Step 5 – Correction for insulation and building condition

New construction with above-standard insulation (low-energy): reduce the base coefficient to 70–75 W/m²
Standard panel or brick construction: 100 W/m²
Old houses before reconstruction, insufficient insulation: 120–130 W/m²
Attic rooms without roof insulation: +20–30 %
Ground floor room above an unheated basement: slightly reduces the load (cooler base)

Table of approximate capacities by area and room type

The following table combines the above-mentioned factors for the most common types of rooms and provides a quick reference overview. Values are in kW of cooling capacity and assume a standard ceiling height of 2.5 m and climatic conditions in Slovakia (summer maximum around 35 °C).

Room type	Area (m²)	Recommended capacity (kW)	Note
Bedroom – north side	10–14	1.5 – 2.0	Minimum people, small window
Bedroom – south side	12–16	2.0 – 2.5	Higher solar gain
Living room – standard	20–28	2.5 – 3.5	TV, 2–3 people
Living room – large windows / south	25–35	4.0 – 5.0	Glass walls
Kitchen	10–16	2.5 – 3.5	Cooker, oven = +500–1000 W
Office – 4 employees	25–30	4.0 – 5.5	PCs, printers, lighting
Office – open space, 10+ people	60–100	10 – 16	Multisplit or VRF
Server room / IT room	15–20	8 – 20	Depends on hardware performance
Retail space / shop	50–80	7 – 12	Depends on entrance doors, canopies
Attic room	20–30	4.0 – 6.0	Strong roof overheating

Real examples from practice: three typical scenarios

Scenario 1: Family house, living room + kitchen, 38 m²

The customer had a new build from 2019, well insulated, ceiling height 2.7 m, large window (4 m × 2.5 m) on the west. The room is occupied by 2–4 people, TV, audio system. The kitchen is open to the living room.

Calculation: 38 m² × 85 W/m² (new build) = 3 230 W. Ceiling height correction 2.7 m: × 1.08 = 3 488 W. Large west window (strong solar gain in the afternoon): +25 % = 4 360 W. People (3 people × 100 W) + TV + audio: +600 W. Kitchen stove (used 1 hour per day – adjustment): +400 W. Total: 5 360 W + 15 % reserve = 6 164 W.

Recommendation: unit of 6.0 kW or 7.0 kW (e.g. the nearest capacity step). The customer originally wanted to buy a 3.5 kW unit "for the living room 38 m²" according to an online calculator. The difference would be dramatic in a hot summer.

Scenario 2: Office, 4 employees, 32 m², 3rd floor, south

Typical situation in an administrative building. The office has 4 employees, each with a computer and monitor, one laser printer, LED lighting, ceiling height 2.8 m, orientation to the south with a window 3 × 1.5 m, double glazing.

Calculation: 32 m² × 100 W/m² = 3 200 W. Ceiling height correction 2.8 m: × 1.12 = 3 584 W. South window (20 % of wall area): +15 % = 4 122 W. Heat gains: 4 people (4 × 100 W) + 4 PCs (4 × 180 W) + printer (600 W) + lighting (32 m² × 12 W = 384 W) = 2 584 W. Total: 4 122 + 2 584 = 6 706 W + 20 % reserve = 8 047 W.

Recommendation: 8.0–9.0 kW. The customer was surprised – "it's just a small office". But when we showed him that the equipment alone produces 2.5 kW of heat continuously, he understood why the old 3.5 kW air conditioner was insufficient.

Scenario 3: Attic apartment, 2 rooms, total 45 m²

Attic apartment in a brick house from 1960, non-insulated roof (only tiles and rafters), sloped ceilings 2.2–3.5 m (average height approx. 2.6 m). Two rooms without direct communication – bedroom 18 m², study 27 m².

Here we considered whether to use one or two units. For an attic without roof insulation, the coefficient is 130–140 W/m²: 45 × 135 = 6 075 W base. Average height: correction ~1.04 = 6 318 W. Solar gains through the roof (intensive): +30 % = 8 213 W. Study with 2 computers: +400 W. Total: 8 613 W + 15 % reserve = 9 905 W.

Recommendation: multisplit solution 2 × 3.5 kW or 4.0 + 5.0 kW according to room distribution. An alternative was one 10 kW unit in the hallway, but air circulation would be insufficient. More on this type of issue in the article Split vs. multisplit air conditioning – which is more cost-effective.

Special cases: when standard calculations are not enough

Server rooms and technical rooms

This is a category where the basis is always measuring the actual power consumption of equipment, not area. Modern servers can have a power consumption of 200 W to 2 000 W per server rack. The correct procedure is: sum all the power consumption of the equipment (from labels or measurements), assume 90–95 % conversion of power to heat, and add 20 % reserve for peak load and future growth.

Example: server room with 6 servers at 500 W power consumption + network equipment 300 W = 3 300 W heat output. Required cooling capacity: 3 300 × 1.10 (room insulation loss) + 20 % reserve = 4 356 W. Minimum 5 kW cooling. For server rooms, redundant solutions are also recommended (two units each at 100 % capacity), which is a topic for the article How to choose air conditioning for professional use – what to focus on.

Restaurant areas and kitchens

A professional restaurant kitchen is an extreme case. A stove with a power consumption of 10–20 kW produces a huge amount of heat, although most is removed by the extractor. Even after subtracting the extractor, 30–50 % of the heat output from the appliances remains in the room. For restaurants, therefore, the performance is calculated separately for the kitchen (special calculation) and for the restaurant area (area × coefficient + customers × 120 W/person + heat gains from the kitchen).

High-traffic areas (shops, waiting rooms)

The key factor here is the frequency of door openings and the number of customers during peak times. Every opening of the door to a 35 °C exterior in a cooled room = immediate thermal load. For street-facing commercial spaces, a correction of +15–25 % is commonly calculated compared to static conditions. Entrance air curtains partially solve this problem, but do not eliminate it completely.

Performance levels of air conditioners and what it means in practice

Air conditioners are manufactured in standardized performance levels, historically linked to the British unit BTU (British Thermal Unit). Therefore, you will encounter designations such as 9 000 BTU, 12 000 BTU, 18 000 BTU, etc. The conversion is: 1 kW ≈ 3 412 BTU/h.

BTU/h	kW (cooling capacity)	Typical use
7 000 BTU	2.0 kW	Small bedroom up to 12 m² (north)
9 000 BTU	2.6 kW	Bedroom 12–18 m², children's room
12 000 BTU	3.5 kW	Living room 20–28 m² (standard conditions)
18 000 BTU	5.2 kW	Living room 30–40 m², office 25 m²
24 000 BTU	7.0 kW	Large living room, office 40+ m²
36 000 BTU	10.5 kW	Larger commercial spaces, shops

When selecting a specific unit, it is important to know that the performance listed in the catalog is always the maximum performance under standard testing conditions (outside temperature 35 °C, inside 27 °C). In real conditions, when it is 38 °C outside or, conversely, when heating at –10 °C, the actual performance may be 10–20 % lower. Therefore, a 15–20 % performance reserve when dimensioning is always justified.

Inverter vs. non-inverter unit: impact on dimensioning

The older type of air conditioning (fixed compressor, on/off regulation) either runs at full power or stops. This type is more demanding in terms of dimensioning accuracy: a unit that is too powerful will run short cycles, will not dehumidify the air and will consume more electricity. A unit that is too weak simply won't be sufficient.

Modern inverter air conditioners have a variable compressor that modulates power smoothly – typically from 30 % to 110 % of the rated power. This means that even a slightly oversized unit can perform its job efficiently: it simply reduces the compressor operation to a lower power and maintains the temperature gently. Despite this, it is true that significant overdimensioning (e.g., 10 kW for 20 m² of a standard bedroom) is not good even with inverter regulation – the unit is more expensive, larger, noisier at minimum power and has higher consumption even when idle.

Optimal practice: choose a power so that the air conditioner runs at 80–90 % power on a summer day of 35 °C. This ensures sufficient reserve and at the same time efficient operation.

Accessories and protection of the outdoor unit: impact on performance in practice

The outdoor unit must have an unobstructed airflow – this is the basic condition for proper functioning. Any obstacles or contamination of the condenser reduce the cooling capacity, which paradoxically means that a well-dimensioned air conditioner with a neglected outdoor unit may behave like an undersized one.

In practice, these are the main situations:

Dusty condenser (pollen, dust, willow fluff) – performance drop of 10–20 %
Outdoor unit in a closed niche without sufficient airflow – overheating, unit protections limit performance
Rainwater flowing directly onto the electrical cabinet of the outdoor unit – faults, short-circuiting of electronics
Insect nests in the electrical part of the outdoor unit – short-circuits, fire in electronics

For the protection of the outdoor unit, there are accessories specifically designed for IVAR.2.0 units: RAIN PROTECTION KIT for IVAR.2.0 air conditioners effectively drains rainwater and protects sensitive electronics from corrosion and short-circuits. For protection against insects that like to nest in the warm electrical cabinet part, the INSECT PROTECTION KIT for IVAR.2.0 air conditioners is intended – fine mesh prevents insects from entering without restricting the airflow through the condenser.

To preserve the aesthetic and functional appearance of the outdoor unit, there are also cover parts: Bottom cover for IVAR.2.0 protects the lower part of the unit from mechanical damage and adverse weather. For the smaller version, the Bottom cover IVAR.2.0 9HP MINI is intended. More about this accessory and its function can be found in the article Protection of the outdoor unit against rain and insects – why and how to do it and Accessories for IVAR.2.0 air conditioners – overview of parts and their functions.

When and why overdimensioning is harmful

Commonly, it seems that a more powerful air conditioner = better. In reality, the opposite is true for non-inverter units and partially also for inverter units in the case of significant overdimensioning. Problems with an oversized air conditioner:

Short cycling: The air conditioner reaches the temperature too quickly, turns off, the space warms up again, and turns on again. This cycle dramatically shortens the lifespan of the compressor.
Insufficient dehumidification: Dehumidification of the air occurs during continuous operation – condensation forms on the evaporator and drips off. In a short cycle, there is not enough time for sufficient moisture to condense and the air is cold but humid. People then feel "humid cold," which is unpleasant.
Higher electricity consumption: Starting the compressor consumes significantly more electricity than steady operation. Frequent starts = higher bill.
Higher purchase price: A more powerful unit costs more, even if you never use its full power.

Ideal state: during maximum summer temperatures, the air conditioner should run at 80–95 % of its power almost continuously (with short breaks), not in short cycles.

Air conditioning and heating: performance when heating

Modern air conditioners are air-to-air heat pumps. This means that in winter they can also heat the room, and with significantly higher efficiency than direct electric heating. Different conditions apply for proper dimensioning in heating mode:

Heating capacity is higher than cooling (typically by 15–30 %) at the same outside temperature of 7 °C
With a drop in outside temperature, the capacity decreases – at –10 °C, the capacity may be only 60–70 % of the nominal
Air conditioners with "Arctic" function or extended range operate down to –25 °C outside temperature
To use air conditioning as a primary heat source, it is necessary to dimension the capacity for the winter load, not the summer one

For more information on combining air conditioning with a heat pump and traditional heating, see the article Air conditioning in professional heating – combination with a heat pump and heating.

Practical recommendations before purchase

Before ordering an air conditioner, answer these questions and note the answers. They will help you with the selection or when consulting with a seller:

What is the area of the room (m²) and ceiling height?
Which cardinal direction do the windows face? How many square meters of window area are in the room?
Is the room in an attic, on the ground floor, or on a higher floor (what is the space above/below it)?
What is the insulation (new build, panel building, old house without insulation)?
How many people and what appliances are in the room during a typical day?
Do you plan to use the air conditioner only for cooling, or also for heating?
Do you have restrictions for the outdoor unit (balcony, municipal regulation, aesthetic requirements)?

If you are unsure about the calculation, it is always better to round the capacity up to the nearest capacity level. Slightly overdimensioning an inverter unit is a better choice than underdimensioning, which will be noticeable every summer during heatwaves.

Frequently asked questions (FAQ)

Can I use one air conditioner for multiple rooms?

It depends on the layout. If the rooms are connected via open doors or an open space (e.g., open floor plan), one larger unit may be sufficient. For separate rooms behind closed doors, it practically does not work – the cold air will not reach the adjacent room. In such a case, the solution is a multi-split air conditioner with multiple indoor units connected to one outdoor unit. More in the article Split vs. multi-split air conditioning – which is more cost-effective.

The air conditioner has 9 000 BTU – how many square meters will it cover?

9 000 BTU = approx. 2.6 kW of cooling capacity. Under normal conditions (standard insulation, ceiling height 2.5 m, north or east orientation, small number of people), it covers 15–22 m². With a south orientation, large windows, or multiple people, the actual area drops to 12–16 m². Never trust catalog claims of "up to 25 m²" for 9 000 BTU without verifying the conditions.

Why does my air conditioner cool, but the air is still humid and stuffy?

The most likely cause is an overdimensioned unit that runs in short cycles. In a short running period, it does not manage to condense enough moisture from the air. Another possibility is a dirty filter or evaporator – restricted airflow over the evaporator reduces the dehumidification efficiency. Check the filter (procedure in the article Maintenance and servicing of air conditioners – what to do regularly and what to leave to a professional) and if that does not help, verify the capacity dimensioning.

How much does a dirty outdoor unit affect the performance of the air conditioner?

Significantly. A dusty condenser (fins of the outdoor unit) increases the refrigerant condensation temperature, which directly reduces the cooling capacity and energy efficiency. In extreme cases (condenser clogged with pollen, willow fluff, or insects), the capacity may drop by 20–30 % and electricity consumption increases. The outdoor unit should be cleaned at least once a year, ideally before the summer season. Protective accessories such as URLPH0 can help.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.