When choosing the right air conditioning unit for your home, one of the most important factors is size. But how do you determine what size air conditioner you need? Too small, and your system will struggle to cool your rooms. Too large, and you’ll waste energy and experience unnecessary wear and tear. In this guide, we’ll explore everything you need to know to find the perfect fit — whether you’re cooling a single room or your entire house in the UK.

Why AC Sizing Matters

Air conditioning sizing isn’t about the physical dimensions of the unit — it’s about cooling capacity, usually measured in British Thermal Units (BTUs). The right-sized air conditioner will efficiently maintain your desired indoor temperature without overworking or underperforming.

What Happens if the AC Is the Wrong Size?

• Undersized units will constantly run, trying to meet the cooling demand. This leads to higher energy bills, insufficient cooling, and early breakdown.
• Oversized units cool the room too quickly without adequately dehumidifying, resulting in cold, clammy air and frequent on-off cycling, which reduces lifespan.

How to Calculate the Right Size (BTUs)

The general rule of thumb is: multiply the room’s square footage by 60 to get a rough BTU estimate.

Example:

• Room Size: 20 square metres
• BTUs Needed: 20 x 60 = 1,200 BTUs

However, this is a basic estimate. The actual requirement depends on several other factors:

Other Key Factors That Influence AC Size

1. Insulation

Homes with good loft, wall, and window insulation need less cooling capacity than draughty, older properties.

2. Ceiling Height

Higher ceilings mean a larger volume of air to cool — this must be factored in.

3. Windows and Sunlight

Rooms with large, south-facing windows will heat up quicker. Consider higher BTUs for sun-exposed areas.

4. Number of Occupants

More people = more body heat. For rooms with more than two people, add roughly 600 BTUs per extra person.

5. Appliances and Electronics

Kitchens and home offices often generate extra heat. Factor this into your cooling requirement.

6. Usage Type

• Bedrooms: quieter units, often smaller capacity
• Living rooms: larger capacity for bigger spaces
• Conservatories: may require a specialised unit due to glass and heat gain

BTU Guide by Room Size (Approximate)

For larger or open-plan areas, consider multi-split systems or ducted systems.

Split vs Portable Units: Sizing Considerations

Split Systems

These are wall-mounted and permanently installed. They’re more efficient and quieter but require professional installation. Perfect for long-term solutions.

Portable Units

Cheaper and movable, but often less powerful. Good for small rooms or occasional use — check energy rating and BTU.

When to Get a Professional Survey

While online BTU calculators offer rough estimates, nothing beats a professional assessment. At TRS Cooling, we conduct detailed surveys considering your home’s layout, insulation levels, sun exposure, and intended use. This ensures you get the most efficient and cost-effective system.

Other Sizing Mistakes to Avoid

• Ignoring insulation levels or property age
• Choosing based solely on square footage
• Forgetting additional heat sources (e.g. kitchens)
• Opting for maximum BTUs “just in case” (often a waste)

Why Choose TRS Cooling?

We’ve helped homeowners across Oxfordshire and the surrounding areas find perfectly sized, professionally installed air conditioning systems for their needs. Whether you’re cooling a cosy cottage or a modern open-plan home, our engineers offer expert guidance and full installation support.

• Free surveys and recommendations
• Trusted local engineers
• Energy-efficient units tailored to your needs

Final Thoughts

Choosing the right-sized air conditioner isn’t just about comfort — it affects efficiency, cost, lifespan, and energy use. Avoid the guesswork and speak to the experts at TRS Cooling for a no-obligation quote and personalised recommendation.

Get in touch today to schedule your free home survey.