Cooling capacity is calculated in kW but sold in BTU/h: 1 kW is 3,412 BTU/h and 860 frigories/h. That unit mismatch is why a unit advertised as "9,000 BTU" is catalogued at 2.5 kW when it actually delivers 2.64. And it is the first sizing trap, before internal gains even come into it: the figure you are held to on the quotation is the rated cooling capacity in kW, not the round number on the marketing plate.
Preliminary sizing works on volume rather than floor area: count 30 to 35 W/m³ in an insulated dwelling facing south or west, 25 to 30 W/m³ facing north, and up to 50 to 60 W/m³ under a roof or behind a large unshaded window. Those ratios give you a quotation envelope, not a calculation note: the capacity an inspection will look at remains the rated capacity in the sense of Regulation (EU) No 206/2012, measured under standard test conditions.
Understanding air conditioning cooling capacity: the basics before you calculate
Cooling capacity, kW, BTU: speaking the same language on site
For air conditioning, cooling capacity indicates the system's ability to remove heat. It is expressed in kW (kWf) or BTU/h. A simple benchmark: 1 kW is roughly 3,412 BTU/h. The trap sits in the supplier documentation: always convert to check that the figure quoted is the "cooling" capacity and not the electrical power drawn, or you will price a unit three times too small. Put the kWf figure on your quotation, that is the one you are held to.
What "capacity" really covers: gains, heat loss and summer comfort
"Capacity" isn't just about floor area. It has to offset gains (sun, occupants, appliances), as well as heat loss and air infiltration. The goal is summer comfort: holding a realistic setpoint (often 26°C), limiting temperature swings, and managing humidity, especially in heavily glazed rooms or under the roof. To go further on summer comfort strategies without oversizing, passive cooling (free cooling, night purge ventilation) can usefully complement the sizing.
Single-split, multi-split, ducted: how the system affects sizing
The system changes the sizing approach. A single-split is calibrated room by room. A multi-split shares one outdoor unit, so you need to look at the simultaneity of demand. With a ducted system, add in duct losses (lengths, bends, airtightness) and airflow balancing. The right capacity is the one that delivers cooling without short cycling. That room-by-room logic is the one the tool follows: Argile works out the load of each room and the coverage of the indoor units placed on the plan.
Quick answer
How many indoor units for this home?
The rules of thumb from the section above, applied to your case. The number moves with the partitioning and the number of floors, not with floor area alone.
Indoor units advised
2 units
Why that number
The baseline for the layout: one unit for the living space, and one on the sleeping side from two bedrooms up.
Refine it room by room with the configurator further down the article.
A first estimate, to be confirmed by a room-by-room heat-loss calculation.
An Argile tool
The conversion table, and why commercial sizes land just below
| Marketing plate (BTU/h) | Actual capacity (kW) | Equivalent (frigories/h) | Catalogue size (kW) |
|---|---|---|---|
| 9,000 | 2.64 | 2,268 | 2.5 |
| 12,000 | 3.52 | 3,024 | 3.5 |
| 18,000 | 5.28 | 4,537 | 5.0 |
| 24,000 | 7.03 | 6,049 | 7.0 |
Regulation (EU) No 206/2012 sets ecodesign requirements for air conditioners up to 12 kW rated cooling capacity and defines the SEER and SCOP you will be compared on. What counts is the rated capacity, measured under standard test conditions, not the peak figure printed in bold on the brochure. On a hot site the machine will deliver less than its rated output, and that margin belongs in the calculation rather than in a last-minute jump to the next size up.
Data to collect on site for reliable sizing
Floor area, volume, ceiling height: don't confuse m² and m³
For accurate air conditioning sizing, record the floor area of each room, then the ceiling height. A mezzanine, an open stairwell, or a cathedral ceiling can quickly change the picture. What matters is the actual volume to be cooled, in m³, not just the floor area. Those measurements are taken on a phone during the visit, where Argile extracts the floor areas, ceiling heights and dimensions of each room.
Insulation, glazing, orientation: what really matters for air conditioning
Note the insulation of the loft, walls and floors, as well as any weak spots. For windows, specify single or double glazing, and whether shutters or blinds are present. Orientation (south and west) and solar protection weigh more heavily than you'd think on cooling needs and solar gains.
Occupancy, appliances, solar gains: the "small details" that skew the calculation
Count the occupants, their usage schedule, and heat-generating appliances (cooking, IT equipment, lighting). Identify heavily glazed rooms, conservatories, and late-afternoon gains. These internal gains help avoid oversizing, which often means short cycling and discomfort.
Air conditioning capacity calculation methods: from quick to precise
Quick per-m² estimate: when to use it (and when to avoid it)
For a first estimate, you can start from a rule of thumb per m². This is a quick baseline for comparing quotes, not for sizing. Avoid it if you have large bay windows, a loft, a very sunny room, or significant internal gains, because oversizing makes the air conditioner short-cycle and undermines comfort.
Calculating by volume and insulation level: a more robust approach
A simple but more reliable method is to work in m³ and correct for insulation, airtightness and orientation. This gives a consistent capacity for most dwellings, especially if you distinguish living areas from bedrooms and factor in existing solar protection.
| Configuration recorded on survey | Volume ratio (W/m³) | Equivalent at 2.50 m (W/m²) |
|---|---|---|
| Dwelling insulated to a modern standard, north or east facing | 25 to 30 | 63 to 75 |
| Dwelling insulated to a modern standard, south or west facing | 30 to 35 | 75 to 88 |
| Dwelling insulated to 1990s or early 2000s standards | 35 to 40 | 88 to 100 |
| Uninsulated dwelling, or room in a converted loft | 40 to 50 | 100 to 125 |
| Unshaded glazing over more than a quarter of the facade, or top floor under a flat roof | 50 to 60 | 125 to 150 |
Two corrections sit on top of the raw ratio: around 100 W per permanent occupant beyond two, and the actual dissipated power of the equipment in the room, read off the rating plate rather than guessed. An open-plan kitchen with an induction hob and an oven moves the result by a full commercial size, and it is the survey record that lets you defend it.
Full heat balance: the right choice for complex cases in 2026
As soon as the project moves beyond standard, run a full cooling load calculation. This is the safest route to validate sizing, zone by zone, taking into account walls, glazing, solar shading, ventilation, occupancy scenarios, and local design temperatures, using software and up-to-date weather data. To also frame the effects of oversizing, you can refer to the most common sizing mistakes.
Choosing the right air conditioning capacity: avoiding under- and oversizing
Signs of insufficient capacity: discomfort, long cycles and excess consumption
Insufficient capacity is easy to spot. The air conditioner struggles to reach the setpoint, especially late in the afternoon. It runs almost continuously, with long cycles. The result: the air stays lukewarm in certain rooms and the bill climbs, because the unit is often running flat out.
Risks of excessive capacity: short cycling, humidity and breakdowns
Conversely, excessive capacity causes frequent starts and stops. These short cycles degrade comfort, limit dehumidification, and leave a sense of humid air. Over time, the compressor and components wear out faster, increasing the risk of breakdowns.
Safety margin and settings: aiming for the right balance without "inflating" the capacity
The right approach is a load calculation (floor area, insulation, glazing, orientation, internal gains), followed by a small, reasonable margin. To keep a good balance, set the setpoint around 25-26°C, reduce solar gains (shutters, blinds), and maintain the filters and heat exchangers.
Field checks and points of attention before quoting and installing
Checking the layout: units, line lengths and drainage constraints
Before any air conditioning job, visit the site. Check the mounting supports, load transfer, maintenance access, and noise impact on neighbours. Measure the actual lengths of refrigerant lines and any height differences. Plan ahead for wall penetrations, cable trunking, and above all condensate drainage, ideally by gravity, otherwise with a lift pump.
Air quality and humidity: settings and useful options by room
Air conditioning does not replace ventilation. Check the mechanical ventilation, air inlets, and any rooms with odour or moisture issues. In bedrooms, favour quiet control and simple programming. In kitchens and bathrooms, secure dehumidification, condensate drainage, and, if needed, finer filtration depending on use.
Documenting your calculation assumptions: securing your quote and the client's needs in 2026
Record your sizing assumptions: floor area, insulation, solar gains, setpoints, occupancy, and rooms to be treated. Archive photos, measurements, and equipment choices to justify the capacity, the settings, and the commissioning. In 2026, keeping a clear record limits disputes and speeds up after-sales service. To also frame the performance angle, you can refer to measuring the SEER of an air conditioner.
