Sizing Your Air Conditioner: How to Calculate kW Room by Room (and Why Oversizing Costs You)
L'équipe Proclimo
11 Oct 2026 - 08 min read
There is one installation mistake that stays invisible, triggers no error code and shows up on no maintenance report: incorrect sizing. An oversized air conditioner cools fast — and that is precisely the problem. An undersized one runs flat out without ever reaching the setpoint. In both cases, the owner pays: in electricity bills, in comfort, and in equipment lifespan.
And contrary to what some rushed quotes suggest, the calculation does not boil down to "100 W per square metre". Here is the method serious refrigeration engineers apply — the one you can run yourself before a salesperson walks through your door.
Why the "100 W per m²" rule no longer cuts it
The historical rule of thumb — 100 watts of cooling capacity per square metre — was calibrated on 1980s-1990s housing stock, with standard ceiling heights and no oversized glazing. In 2026, it produces wrong answers in both directions:
- In a RE 2020 or well-renovated home, with external insulation and triple glazing, the real load can drop to 50-70 W/m².
- In a converted south-facing attic under a poorly insulated roof, it can climb to 150, even 180 W/m².
Between the two, the factor is 1 to 3. In other words, applying the same rule everywhere is like buying shoes without knowing your size. The CSTB and ADEME regularly point out, in their guides on summer comfort, that cooling demand depends first and foremost on solar gains and the thermal inertia of the building fabric, long before floor area.
The room-by-room calculation method
The principle: start from volume, not floor area, then apply correction factors. It's a simplified version of a professional heat load calculation, but reliable enough to sanity-check a quote.
Step 1 — Calculate the volume
Volume = length × width × ceiling height.
A living room measuring 5 m × 6 m with a 2.50 m ceiling comes to 75 m³. With a 2.80 m ceiling (an older Haussmann-era flat), it comes to 84 m³ — that's 12% more demand for the same floor area.
Step 2 — Apply a base load
| Insulation level | Base load |
|---|---|
| RE 2020 home / recent whole-house renovation | 25 W/m³ |
| Decent insulation (RT 2005-2012, double glazing) | 32 W/m³ |
| Older, poorly insulated home, single glazing | 40 to 45 W/m³ |
For our 75 m³ living room with decent insulation: 75 × 32 = 2,400 W, or 2.4 kW.
Step 3 — Correct for orientation and glazing
This is the step that gets skipped far too often, and it's the one that shifts the result the most.
- South or west facing with no solar shading: +15 to +25%
- Large picture window (more than 20% of the room's floor area): +10 to +20%
- Top floor directly under the roof: +15 to +20%
- North-facing room, shaded, on an intermediate floor: −10%
Step 4 — Add internal gains
Each occupant gives off roughly 100 W of sensible heat at rest. An oven, a washing machine, a router and two computers in the same room easily add 400 to 600 W. For a kitchen open to the living room, always build in an increase.
For a home office with two workstations, a printer and two people, add 500 W to the calculation without hesitation.
Step 5 — Check the result
Back to our living room: 2,400 W base load, +20% for a west-facing picture window, +300 W of internal gains → roughly 3.2 kW.
A 3.5 kW split unit is therefore a sound match. A 5 kW split would be clear oversizing by more than 50%.
note
Watch the units: manufacturers sometimes quote in BTU/h. 12,000 BTU/h ≈ 3.5 kW. To convert, divide the BTU figure by 3,412.
Oversizing: the most common and most expensive mistake
People often assume a machine "with some headroom" is a good investment. With direct-expansion air conditioning, the opposite is true.
Short cycles that wear out the compressor
An oversized unit reaches the setpoint in a few minutes, shuts down, then restarts. This short cycling multiplies compressor starts, which are precisely the moments when it draws the most power and suffers the most wear. Even on an inverter machine, capable of modulating, there is a minimum output below which it cannot go: typically 25 to 35% of rated capacity. A 5 kW unit installed on a 2 kW load will spend its life switching off.
Cold but humid air
This is the most unpleasant consequence, and the least understood. An air conditioner dehumidifies because air passes over a cold coil long enough for water vapour to condense. If the machine stops after eight minutes, it has lowered the temperature without having had time to remove the moisture. The result: a cold, clammy feel to the air, unpleasant, and a temptation to lower the setpoint even further. Occupants then complain about the "icy draught" while relative humidity stays at 65%.
An indoor hygrometer placed in the room is the best way to observe the phenomenon: if the temperature drops to 23 °C but relative humidity stays above 60%, the sizing needs revisiting.
A higher bill, not a lower one
The published seasonal efficiencies (SEER, SCOP) are measured at part load, in line with the European ecodesign regulation and the EN 14825 standard. But those figures assume the machine works within its optimal range. A unit permanently running below its minimum modulation point drifts away from its rated efficiency. You mechanically lose part of the SEER printed on the energy label — the very same label we broke down in our article on the new energy label for reversible air conditioners.
Undersizing: less common, but not harmless
Conversely, an undersized machine runs continuously at full load during heat peaks, never reaching the setpoint. The symptoms:
- The compressor never stops between 2 pm and 8 pm in August.
- The gap between setpoint and measured temperature stays above 2 °C.
- July and August bills explode without comfort following suit.
Undersizing is especially common when the AC was chosen for one room, then used to cool a larger volume — a split installed in a living room expected to "spread" cooling into the kitchen and hallway. In that case the error isn't capacity but the air distribution strategy: cold air, being heavier, does not travel on its own past doors and partition walls.
Single split, multi-split, ducted: sizing follows different logic
The single split
This is the simple case: one outdoor unit for one indoor unit. The outdoor unit's capacity must match the room's load, full stop.
Multi-split: beware the diversity factor
A multi-split carries two traps.
First, the sum of the indoor capacities often exceeds the outdoor capacity. That's normal and intentional: the assumption is that rooms don't all demand their maximum at the same time. A 6.8 kW condensing unit can feed four 2.5 kW indoor units, i.e. 10 kW combined. But if all four of your rooms face due south and are called upon simultaneously at 4 pm, none of them will reach its setpoint.
Second, the condensing unit's minimum output: a multi-split with a single indoor unit running overnight in a 10 m² bedroom ends up in exactly the oversizing scenario described above.
tip
If most of your usage is at night and limited to one bedroom, two properly sized single splits are often more efficient than a four-way multi-split.
Ducted systems
Here sizing comes with an airflow calculation on top: air volumes per outlet, pressure losses, duct lengths. A ducted system correctly sized on capacity but undersized on airflow delivers little air, noisily. That's one of the reasons we insist on seeing duct sizes detailed in the quote.
Five questions to ask your installer before signing
A professional who sizes properly will answer these five questions without hesitation:
- Have you done a heat load calculation, even a simplified one? A record of areas, heights, glazing and orientations should appear somewhere.
- What is the minimum modulation output of the proposed condensing unit? It's in the manufacturer's technical documentation, rarely on the quote.
- What design outdoor temperature did you use? In France it ranges from 28 °C in Brittany to 36-38 °C in the South-East, depending on the climate data professionals use.
- And what about heating mode? A reversible air-to-air heat pump has to be sized against two different loads. The winter heating demand is generally higher than the cooling demand. Sizing on cooling alone means buying inadequate heating — the classic trap in northern and eastern France.
- What diversity factor did you apply on the multi-split?
These points tie in directly with reading a quote line by line, which we cover in our article on the points to check before signing an air conditioning quote.
Measure before deciding: the habit that changes everything
Before requesting quotes, spend two weeks documenting your home. It costs nothing and defuses 80% of sales conversations.
- Put a connected temperature logger in each room to be cooled and record temperatures over a hot week. You'll know which room actually heats up, and at what time of day.
- Log relative humidity alongside: a room at 29 °C and 45% humidity doesn't call for the same answer as a room at 27 °C and 70%.
- Test solar shading. In many cases, external blackout blinds on a west-facing window remove 1 kW of load — a whole capacity step down, and several hundred euros off the quote.
- Check how airtight your attic and doors are. A smartphone thermal camera costing under €300 reveals in one evening the thermal bridges and air leaks no calculation can guess.
Three worked examples
12 m² bedroom, north facing, intermediate floor, 2015 building
Volume: 12 × 2.50 = 30 m³. Base 32 W/m³ = 960 W. North facing: −10%. One occupant: +100 W. Result: roughly 0.95 kW.
The smallest split on the market is generally 2 to 2.5 kW. We're already in structural oversizing: hence the value of connecting this bedroom to an existing multi-split rather than adding a dedicated single split, or choosing a model whose modulation goes very low.
40 m² open living room and kitchen, due south, 1975 house insulated in 2010
Volume: 40 × 2.50 = 100 m³. Base 32 W/m³ = 3,200 W. Due south with no blinds: +20% = 3,840 W. Open kitchen + 3 occupants: +600 W. Result: roughly 4.4 kW → a 5 kW split is justified.
With external blinds: you fall back to around 3.8 kW, and a 4 kW model is enough.
25 m² converted attic, top floor, roof insulated to 20 cm
Volume: 25 × 2.30 (average height under the sloping ceiling) = 57.5 m³. Base 40 W/m³ = 2,300 W. Directly under the roof: +20% = 2,760 W. Two occupants: +200 W. Result: nearly 3 kW for 25 m², i.e. 120 W/m². The 100 W/m² rule would have underestimated the load by 20%.
Key takeaways
| Mistake | Main consequence |
|---|---|
| Oversizing | Short cycling, cold and humid air, compressor wear, degraded real-world SEER |
| Undersizing | Continuous operation, setpoint never reached, high summer bills |
| Poorly balanced multi-split | Rooms left unsatisfied at peak hours |
| Sizing on cooling alone | Reversible heat pump inadequate for heating in winter |
Good sizing costs nothing extra: half an hour of measurements and a calculation. Yet it is what separates a quiet, economical, comfortable installation from a machine that clunks on and off every ten minutes for eight years.
tip
Always ask for the heat load calculation, even a simplified one, to be attached to the quote. In the event of a dispute over the comfort achieved, it's the document that establishes whether the proposed capacity was appropriate — useful under both the workmanship guarantee and the ten-year structural warranty.
To dig deeper, ADEME's guides on summer comfort and manufacturers' technical datasheets (modulation ranges, capacity curves as a function of outdoor temperature) are public, usable sources. And if an installer refuses to show you their calculation, that in itself tells you something about the quality of their work.
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