BTU Calculator
Find out exactly what size air conditioning unit you need for your room. Enter your details below for an instant recommendation.
Your Results
Room Volume
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m³
Recommended BTU
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BTU/hr
Equivalent kW
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kW
Our Recommendation
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This calculator provides an estimate. For an accurate assessment, we recommend a free on-site survey where we measure heat loads, check insulation, and factor in all variables specific to your property.
Or call us on 01273 455584
How BTU Sizing Works
Building Fabric
Heat coming through the walls, roof and floor — based on your floor area, ceiling height and how well the property is insulated.
Sun Through the Glass
The biggest factor in most rooms — how much glazing there is, which way it faces, whether it is shaded, and any rooflights (which take the most sun of all).
Heat Inside the Room
People, lighting and appliances — a kitchen with the oven on needs far more than a bedroom. Added together, these give the kW and BTU rating to look for.
Quick Reference: Room Size Guide
| Room Size | Area (m²) | BTU Needed | kW Rating |
|---|---|---|---|
| Small Bedroom | 10-12 m² | 3,500-4,300 | 2.0 kW |
| Double Bedroom | 12-16 m² | 4,300-5,400 | 2.0-2.5 kW |
| Living Room | 20-30 m² | 7,200-10,700 | 2.5-3.5 kW |
| Large Open Plan | 30-45 m² | 17,500-23,000 | 5.0-7.1 kW |
| Garden Office | 8-15 m² | 4,600-6,500 | 2.0-2.5 kW |
| Conservatory | 12-20 m² | 12,000-19,000 | 3.5-6.0 kW |
Assumes average insulation, a 2.4m ceiling, mixed-facing glazing and typical use — the same method as the calculator above, which will give you a figure for your actual room. An open-plan kitchen is dominated by cooking heat and glass, which is why it needs far more than its floor area suggests.
Frequently Asked Questions
How do I calculate what size air conditioning I need?
A proper sizing adds up three things: the heat coming through the walls, windows and roof (which depends on how well the property is insulated), the sun coming through the glass (the biggest single factor in most rooms — how much glazing, which way it faces, whether it is shaded), and the heat given off inside the room by people, lighting and cooking. Added together that is the cooling load in kilowatts — multiply by 3,412 for BTU. Our calculator above does this automatically—just enter your room details.
What is a BTU and why does it matter?
BTU stands for British Thermal Unit—a measure of cooling (or heating) capacity. A higher BTU rating means more cooling power. Choosing the correct BTU ensures your room cools efficiently without wasting energy or struggling to reach temperature.
What happens if I choose the wrong size unit?
An undersized unit will run constantly without reaching the desired temperature, increasing energy bills and wear. An oversized unit will short-cycle—turning on and off frequently—wasting energy and causing uneven temperatures. Correct sizing is essential for comfort and efficiency.
Is this calculator accurate enough to buy a system?
This gives a good estimate for planning purposes. However, a professional on-site survey considers additional factors like wall construction, glazing type, nearby heat sources, and airflow patterns. We offer free on-site surveys across Sussex to ensure you get the perfect system.
Need Help Choosing the Right System?
Our engineers will carry out a free on-site survey to calculate the exact cooling requirement for your space—no obligation.