How the sizing estimate works
The finder uses a transparent rule of thumb, not a black box. Here is every number it applies, so you can check the result rather than trust it.
What this is, and what it is not
This is a cooling-load rule of thumb built from published installer and retailer guidance. It is meant to put you in the right class of unit and to err slightly generous, because portables lose some of their rated output in real rooms. It is not a full EN 12831 engineering survey; for a permanent installation, a heating and cooling engineer should run a proper load calculation.
The steps
- Start from the floor area in m² and a base of 65 W per m².
- Adjust for ceiling height, sun, insulation and whether the room sits under the roof.
- Add body heat for people over two, and any heat-making equipment.
- Add a 12% margin for how portables behave in practice.
- Recommend the smallest standard size that covers the result.
Every factor
| Factor | Applied | Why |
|---|---|---|
| Room envelope | 65 W per m² | From the 20 BTU/h per sq ft rule, about 215 BTU/h (63 W) per m² at a 2.44 m ceiling, rounded for a 2.5 m room. |
| High ceiling (over 3 m) | ×1.2 | More air volume to cool. |
| Shaded room | ×0.9 | North-facing or shaded by trees and buildings. |
| Very sunny room | ×1.15 | Large south or west glazing, hot afternoons. |
| Good insulation | ×0.85 | New build or recent renovation. |
| Older / draughty | ×1.25 | Single glazing or thin walls push toward the 150 to 175 W/m² installers quote for hard rooms. |
| Top floor under roof | ×1.12 | Heat builds under the roof. |
| Each person over two | +175 W | About 600 BTU/h of body heat per extra person. |
| Computers and TV | +250 W | A desktop PC and several screens. |
| Kitchen | +1200 W | Oven, hob and fridge, about 4,000 BTU/h. |
| Portable margin | ×1.12 | Portables dump some heat near the room and single-hose units draw warm air back in, so we add 12%. |
Size mapping
The conversion is fixed: 1 kW is 3412 BTU per hour. We pick the smallest of these standard sizes that covers your adjusted requirement.
| Capacity | BTU/h | Typical room, standard ceiling |
|---|---|---|
| 2.0 kW | 7,000 | up to ~25 m² |
| 2.6 kW | 9,000 | ~25 to 34 m² |
| 2.9 kW | 10,000 | ~30 to 40 m² |
| 3.5 kW | 12,000 | ~35 to 46 m² |
| 4.1 kW | 14,000 | ~45 to 55 m² |
When the adjusted figure passes 4.1 kW, one portable will struggle. The tool says so and points you toward two units, a mobile split with shading, or a fixed installation.
Why a margin, and why not more
Published "up to X m²" ratings assume light load and good conditions; the same 3.5 kW unit is rated to 46 m² by one brand and 60 m² by another. We size to the conservative end and add 12%, which covers single-hose inefficiency without pushing you into an oversized unit that short-cycles and wastes energy.
Sources
The factors come from common installer and retailer sizing guidance: the 20 BTU/h per sq ft envelope rule, the roughly 100 W/m² installer shorthand for average rooms, the ±10% sun adjustment, about 600 BTU/h per extra person, about 4,000 BTU/h for a kitchen, and a 10 to 15% portable allowance. The EU energy-label and refrigerant points are covered in the energy label guide.
Try it on your room
Answer a few questions and see the number, with this working shown.