Air Conditioner Size Guide – How to Find the Right Capacity
The most common mistake when buying an air conditioner: choosing the wrong size for the room. Too weak and the room never gets cool. Too powerful and the unit short-cycles — wasting energy, drying out the air and wearing out the compressor faster. The fix: size correctly before you buy.
The Basic Formula
25 watts per m² of floor area — the accepted benchmark for standard living spaces in Europe.
Example: 30 m² living room × 25 W = 750 W ≈ 2,560 BTU → right unit: 9,000 BTU (2.5 kW)
Quick-Reference Table: Room Size → Recommended Capacity
| Room Size | Capacity (kW) | BTU | Typical Use |
|---|---|---|---|
| Up to 15 m² | 1.5 – 2.0 kW | 5,000 – 7,000 | Small bedroom, small office |
| 15 – 25 m² | 2.0 – 2.5 kW | 7,000 – 9,000 | Bedroom, home office |
| 25 – 35 m² | 2.5 – 3.5 kW | 9,000 – 12,000 | Living room, open kitchen |
| 35 – 50 m² | 3.5 – 5.0 kW | 12,000 – 18,000 | Large living room, loft |
| 50 – 70 m² | 5.0 – 7.0 kW | 18,000 – 24,000 | Open-plan living/dining |
| 70 – 100 m² | 7.0 – 10.0 kW | 24,000 – 34,000 | Large open space |
BTU vs. kW – What's the Difference?
Both describe the same thing: how much heat an air conditioner can remove per hour. 1 kW = 3,412 BTU/h. European product specifications use kW; American and some older specifications use BTU. To convert: divide BTU by 3,412 to get kW. Common AC sizes: 9,000 BTU = 2.6 kW / 12,000 BTU = 3.5 kW / 18,000 BTU = 5.3 kW.
Factors That Increase Your Requirements
The 25 W/m² formula applies to standard conditions. Go one size up if:
- 🌞 Strong direct sunlight (south-facing, top floor, glass wall): +15–25%
- 📐 Ceiling height above 2.6 m: add ~10% per additional 30 cm
- 👥 More than 2 people regularly in the room: +600 W per additional person
- 🖥️ Heavy heat loads (servers, kitchen equipment): calculate individually
- 🏚️ Poor insulation (old building, attic room): +20%
Factors That Reduce Your Requirements
- 🌲 North-facing or permanently shaded: −10–15%
- 🏠 Well-insulated building (new build, KfW 55 or better): −10–20%
What Happens With the Wrong Size?
Too small: The unit runs flat-out continuously, never reaches the target temperature, consumes more power and wears out faster. Room feels stuffy on hot days.
Too large: Frequent short-cycling (on/off every few minutes), poor dehumidification, uncomfortable temperature swings and unnecessarily high purchase cost.
Multiple Rooms: Multi-Split Instead of Multiple Singles
If you want to cool two or more rooms, a multi-split system is almost always the more economical solution: one outdoor unit, multiple indoor units — fewer wall penetrations, lower installation cost, central control.
👉 Browse all split air conditioners at Jacob Warm
Frequently Asked Questions
My room is 28 m² but has a big south-facing window. What size?
Start with 28 × 25 W = 700 W (2.0 kW), then add 20% for the south-facing glass: 700 × 1.20 = 840 W. Round up to the next standard size: 2.5 kW (9,000 BTU).
Can I use one air conditioner for an open-plan kitchen/living area of 55 m²?
Yes — a single 7.0 kW (24,000 BTU) floor console or ceiling cassette would handle this. Alternatively, a multi-split with two 3.5 kW heads gives more flexible temperature zones.
Does AC capacity affect heating performance too?
Yes. Most inverter split units provide both cooling and heating. The heating capacity of a unit is typically 10–20% higher than its cooling capacity, so a 2.5 kW (cooling) unit usually delivers around 3.0 kW of heating. Always check the heating capacity in the spec sheet.
