What BTU Air Conditioner Do I Need? Room Size Calculator
Choosing the right BTU rating for your portable air conditioner is the single most important buying decision you will make. Get it right and your room will be comfortable through the hottest UK heatwave. Get it wrong and you end up with a unit that either short-cycles and leaves the room clammy, or runs full power 24/7 without ever catching up.
BTU stands for British Thermal Unit and is the standard measure of cooling power. The higher the BTU, the more heat the unit can remove per hour. But bigger is not always better — an oversized unit cools so fast it does not run long enough to dehumidify, leaving the room feeling cold and damp.
Here is exactly how to calculate the right BTU for your room, with a simple formula, a size chart, and our product recommendations at each level.
BTU Room Size Calculator
Enter your room dimensions and conditions to find the minimum BTU rating you need.
The Simple BTU Formula
The 337 multiplier is a simplified version of the standard HVAC calculation adapted for UK rooms with typical 2.4 metre ceilings and average insulation. It gives you a baseline. You then adjust for the specific conditions of your room.
This formula works for portable air conditioners in typical UK homes. For fixed split-system installations, a professional heat load calculation (often called a Manual J) is more accurate, but for portable units this rule of thumb will get you to the right size bracket.
Room Size to BTU Chart
| Room Size | Area (m²) | Min BTU | Recommended Unit |
|---|---|---|---|
| Small bedroom / home office | 10-15 m² | 5,000-7,000 BTU | 7,000 BTU |
| Medium bedroom / living room | 15-25 m² | 7,000-10,000 BTU | 9,000 BTU |
| Large living room / conservatory | 25-35 m² | 10,000-14,000 BTU | 12,000 BTU |
| Open-plan / large conservatory | 35+ m² | 14,000+ BTU | 14,000+ BTU |
The 7,000-10,000 BTU range is the most common for UK homes. A standard 20 m² bedroom or living room falls squarely in this bracket. The 9,000 BTU units (like the MeacoCool MC9000 or AEG Comfort 9000) are the sweet spot for most buyers.
Factors That Affect the BTU You Need
The formula gives you a baseline, but every room is different. Here is how to adjust the calculation for your specific situation.
High Ceilings
If your ceiling is over 2.4 m (common in Victorian conversions and new-build open-plan homes), you have more air volume to cool.
South-Facing / Sunny Room
South-facing rooms and rooms with large windows that get direct afternoon sun absorb significantly more heat.
Kitchen
Ovens, kettles, and hobs generate substantial heat. A kitchen needs more cooling power than a bedroom of the same size.
Extra People
Each person adds roughly 600 BTU of body heat. A home office with two people needs more cooling than a single-occupancy bedroom.
North-Facing / Shaded
North-facing rooms or rooms shaded by trees or neighbouring buildings stay cooler naturally. You can size down slightly.
Poor Insulation
Single-glazed windows, draughty rooms, loft conversions, and conservatories lose cool air fast. Add 20% for these spaces.
Our Product Picks by Room Size
Small Room (10-15 m²) — 7,000 BTU
ElectriQ Eco 7000
Medium Room (15-25 m²) — 9,000 BTU (Most Popular)
MeacoCool MC9000
AEG Comfort 9000
Large Room (25-35 m²) — 12,000 BTU
DeLonghi Pinguino PACEL 12000
Open-Plan / Large Spaces (35+ m²) — 14,000+ BTU
Meaco Cirro+ 14000
Frequently Asked Questions
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Meaco vs ElectriQ vs AEG — Which Portable Air Conditioner? — Our full head-to-head comparison of the top 2026 models.
Best Portable Air Conditioners UK 2026 — Our ranking of the best units this year.
