How Many BTU Do I Need? UK Room Size Guide 2026
BTU (British Thermal Units) is the standard measure of cooling capacity for air conditioners. Choosing the right BTU for your room is the single most important decision when buying a portable air conditioner. Too little BTU and the unit will struggle to cool the room. Too much BTU and the unit will cycle on and off inefficiently, wasting energy and potentially causing humidity issues. This guide explains exactly how to calculate the right BTU for any UK room.
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Full Comparison Table
| Product | BTU | Room Size | Noise | Energy | Price | Action |
|---|---|---|---|---|---|---|
De'Longhi Pinguino PAC EX100 Silent Premium Pick | 10,000 | Up to 25m² | 44 dB | A+ | 479 | Check price |
Inventor Chilly 9000 BTU Portable Air Conditioner Best Overall | 9,000 | Up to 20m² | 48 dB | A | 289 | Check price |
Whynter ARC-14S Dual Hose 14000 BTU Heatwave Pick | 14,000 | Up to 40m² | 56 dB | B | 549 | Check price |
Electriq 12000 BTU Portable AC with Heat Pump Best Overall | 12,000 | Up to 32m² | 52 dB | A | 379 | Check price |
MeacoCool MC Series 9000 BTU Best Under £300 | 9,000 | Up to 22m² | 46 dB | A | 249 | Check price |
Woods Milano 7000 BTU Portable Air Conditioner Best Under £300 | 7,000 | Up to 16m² | 42 dB | A+ | 219 | Check price |
Calculating the right BTU for your specific room and conditions
The basic BTU formula
Start with: Room area (m²) × 337 = Base BTU. For example, a 15m² bedroom: 15 × 337 = 5,055 BTU. Round up to the nearest standard BTU size (7,000, 9,000, 12,000, 14,000). So a 15m² bedroom needs a 7,000 BTU model.
Adjustments for UK conditions
Add 10% for high sun exposure (south-facing rooms). Add 10% for poor insulation. Add 600 BTU per person above 2 occupants. Add 500 BTU for home offices with computers. Round up to the nearest standard BTU size after adjustments.
BTU by room type
Small bedroom (8–12m²): 7,000 BTU. Medium bedroom (12–20m²): 9,000 BTU. Large bedroom / small living room (20–25m²): 9,000–12,000 BTU. Medium living room (25–35m²): 12,000 BTU. Large living room / open plan (35m²+): 14,000 BTU.
Why oversizing is a problem
An oversized air conditioner cools the room quickly but cycles on and off frequently. This short-cycling wastes energy, creates temperature fluctuations and can leave the room feeling damp because the unit doesn't run long enough to dehumidify properly. Always choose the right size, not the biggest available.
Full Reviews
De'Longhi
De'Longhi Pinguino PAC EX100 Silent
Best for: Home offices, master bedrooms and open-plan living areas
Pros
- Best-in-class 44 dB noise level
- A+ energy rating reduces running costs
- Real Feel technology adjusts humidity and temperature
Cons
- Premium price point above £400
- Larger footprint than budget models
Inventor
Inventor Chilly 9000 BTU Portable Air Conditioner
Best for: Bedrooms, small living rooms and UK flats up to 20m²
Pros
- Whisper-quiet at 48 dB — ideal for bedrooms
- Compact footprint fits most UK flats
- Energy class A — low running costs
Cons
- Single-hose design is less efficient than dual-hose
- Remote control feels plasticky
Whynter
Whynter ARC-14S Dual Hose 14000 BTU
Best for: Large living rooms, open-plan kitchens and heatwave conditions
Pros
- Dual hose design maximises efficiency
- 14000 BTU handles large open-plan spaces
- Auto-drain means less manual emptying
Cons
- Higher running costs at B energy class
- Noticeably louder at 56 dB
Electriq
Electriq 12000 BTU Portable AC with Heat Pump
Best for: Medium rooms needing both cooling and heating capability
Pros
- Heats and cools — useful year-round in UK climate
- A energy class despite 12000 BTU output
- Wifi-enabled with smartphone app
Cons
- App can be unreliable on first setup
- Exhaust hose is shorter than competitors
379
at Appliances Direct
Affiliate link — price correct at time of writing
Meaco
MeacoCool MC Series 9000 BTU
Best for: Budget-conscious buyers wanting reliable UK brand support
Pros
- Excellent value under £250
- UK brand with strong customer support
- True 9000 BTU output (not inflated)
Cons
- App connectivity not available
- Water tank needs emptying in humid weather
Woods
Woods Milano 7000 BTU Portable Air Conditioner
Best for: Small bedrooms, box rooms and home office nooks
Pros
- Quietest model at 42 dB
- A+ energy rating — very low running cost
- Slim design fits small UK bedrooms
Cons
- Only suitable for rooms up to 16m²
- No app or smart home connectivity
How we choose products
Specification analysis
We compare BTU output, noise levels, energy class, wattage and room size suitability from manufacturer data.
Buyer feedback
We analyse thousands of verified buyer reviews from UK retailers to identify real-world strengths and weaknesses.
Value assessment
We assess price, running costs, warranty and retailer reliability to identify genuine value for UK buyers.
We compare products based on specifications, retailer information, buyer feedback, price, features and suitability. We do not claim hands-on testing unless clearly marked. Read our editorial policy →
Related guides: BTU Calculator · Best 9000 BTU AC · Best 12000 BTU AC · Best Portable AC UK
How Many BTU Do I Need? UK Room Size Guide 2026 — FAQ
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