When summer hits and your home feels like a sauna, an air conditioner can be a lifesaver. But picking the right size isn’t just about grabbing the flashiest model off the shelf - it’s about nailing the balance between comfort, efficiency, and keeping your energy bills from giving you a heart attack.
Get it wrong, and you’re either stuck with a unit that’s puffing like an old steam train or one that’s overkill, costing you a fortune.
This guide, tailored for UK homeowners, will walk you through choosing the perfect air conditioner size.
💨 Ready to chill without the guesswork? Find out exactly what size air conditioner you need - and get a fixed price in seconds here.
Key Takeaways:
Air conditioner size is measured in BTUs (British Thermal Units).
You’ll need 60–125 BTUs per square metre, depending on room use, insulation, and sun exposure.
Oversized or undersized units waste energy, hike up costs, and leave you uncomfortable.
A simple formula or calculator, paired with real-world tips, ensures you get it spot-on.
Why Getting the Right AC Size Matters
Choosing the correct air conditioner size is like picking the right jacket, it needs to fit your home’s needs perfectly. Too small, and it’ll struggle to cool your space, leaving you sweaty and annoyed.
Too big, and it’ll blast cold air too quickly, switching on and off like a fidgety teenager, wasting energy and leaving rooms damp. A properly sized unit keeps your home comfortable, saves on running costs, and lasts longer.
“Picking the right AC size is like choosing a pair of shoes - too small and you’ll struggle, too big and you’re just wasting money. A properly sized unit keeps your home cool and your bills sane.” - Patrick Garner, Gas Safe Engineer.
BTUs Explained: The Nuts and Bolts of AC Sizing
BTUs, or British Thermal Units, measure how much cooling power an air conditioner has. One BTU is the energy needed to cool 0.45 kg of water by 0.55°C. In plain English, it’s how much “oomph” your unit has to battle heat.
The higher the BTUs, the more cooling capacity - but don’t go thinking bigger is always better.
An oversized unit cools too fast, skipping dehumidification, which leaves your room feeling clammy. An undersized unit runs nonstop, burning energy and wearing itself out.
To get it right, you need to match the BTUs to your room’s size, layout, and quirks. Think of it as tailoring a suit, you need to account for every detail.
Let’s break it down step by step…
Room Size vs BTU Guide
The starting point for sizing an air conditioner is your room’s floor area, measured in square metres.
The table below gives a baseline for typical UK homes, assuming standard ceiling heights (2.4–3 metres), average insulation, and typical summer conditions (we’re not talking Sahara-level heat here).
Room Size (Square Metres) | Recommended BTUs | Typical Room Type |
10-15 | 4,000–5,500 | Small bedroom, home office |
15-25 | 5,500–7,500 | Medium bedroom, study |
25-35 | 7,500–10,000 | Large bedroom, small lounge |
35-45 | 10,000–14,000 | Living room, dining area |
45-60 | 14,000–18,000 | Open-plan kitchen-diner, large lounge |
60-80 | 18,000–24,000 | Spacious living area, multi-room setup |
Pro Tip: For whole-house cooling or multi-room setups, add up the BTUs for each room. Check out our air source heat pump guide for an alternative cooling option.
Factors That Tweak Your BTU Needs 🤔
Room size is just the beginning. Several factors can nudge your BTU requirements up or down. Here’s what to consider:
Ceiling Height: Standard ceilings (2.4–3 metres) are covered in the table above. For lofty spaces (over 3 metres), add 10% more BTUs per extra metre of height to account for the extra air volume.
Insulation Quality: Older UK homes with draughty windows or thin walls lose cool air fast. Bump up your BTUs by 15–20% for poor insulation. Modern, well-insulated homes (think double glazing and cavity wall insulation) can stick to the lower end of the BTU range.
Sun Exposure: South-facing rooms with large windows act like greenhouses. Add 10–15% more BTUs for sunny spots. North-facing or shaded rooms? Reduce BTUs by 10%.
Occupancy: Each person in the room adds about 400 BTUs per hour due to body heat. A busy living room with four people needs an extra 1,600 BTUs.
Appliances and Electronics: Heat-generating kit like ovens, fridges, or gaming PCs can add 1,000–2,000 BTUs. Kitchens often need 20% more cooling power than bedrooms.
Room Purpose: Quiet bedrooms or studies need less cooling (60–80 BTUs per square metre), while active spaces like kitchens or living rooms need more (100–125 BTUs per square metre).
For more on making your home energy-efficient, see our guide on energy-efficient home upgrades.
Quick Sizing Formula with a Worked Example
For a quick-and-dirty estimate, use this formula:
Room area (square metres) × BTUs per square metre (60–125, based on factors above) = Required BTUs.
Real-Life Example: Let’s say you’ve got a 30-square-metre living room in a south-facing, older home with standard ceilings, two occupants, and a big TV that’s always on.
Base Calculation: Start with 30 square metres × 100 BTUs (mid-range for a living room) = 3,000 BTUs.
Adjustments:
South-facing windows: Add 15% (3,000 × 0.15 = 450 BTUs).
Poor insulation: Add 15% (3,000 × 0.15 = 450 BTUs).
Two people: Add 800 BTUs (2 × 400).
TV and electronics: Add 1,000 BTUs.
Total: 3,000 + 450 + 450 + 800 + 1,000 = 5,700 BTUs.
Round up to a standard unit size, like 6,000 or 7,500 BTUs, to avoid undercooling.
For pinpoint accuracy, use an online BTU calculator or, better yet, use our online tool. Multi-room or whole-home systems? Our air conditioning installation team can tailor a solution.
Common Sizing Mistakes to Dodge
Don’t trip over these classic blunders:
Going Overboard “Just in Case”: A massive unit might seem like a safe bet, but it’ll short-cycle, leaving your room cool but muggy, and your wallet lighter.
Ignoring Room Specifics: Forgetting about sun exposure, insulation, or that toasty gaming PC throws off your BTU estimate.
Guesstimating Room Size: Whip out a tape measure - guessing can lead to a unit that’s way off. Multiply length by width for accurate square metres.
Skipping Professional Advice: Portable units are fine for DIY, but wall-mounted or multi-split systems need expert sizing. Heatable’s installers can save you from costly mistakes.
Forgetting Maintenance Costs: Undersized units wear out faster, and oversized ones guzzle energy. A right-sized unit keeps running costs low.
Curious about cooling alternatives? Check out our guide on heat pumps here.
Stay Cool with Heatable 😎
Sizing an air conditioner doesn’t have to be a faff. Measure your room, factor in sunlight, insulation, and occupancy, and use our BTU guide or formula to get it right.
For a seamless setup, trust Heatable’s air conditioning installation services to keep your home cool and your bills under control. Want to explore other cooling options?
Check out our guides on air source heat pumps, energy-efficient home upgrades, or boiler vs AC cooling.
Beat the heat, save money, and enjoy your summer - Heatable’s got you covered.
Next Steps For Your AC Journey:
When planning to install air conditioning for your home, there are several important factors to consider. Make sure to refer to the following guides to help you make informed decisions:
To dive deeper into these topics, head over to our advice section, check out our YouTube channel for informative videos.
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