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AC Size Calculator
Our free AC Size Calculator takes your room dimensions, usage type, insulation quality and local climate to recommend the perfect air conditioning capacity in both kW and BTU/hr. Stop guessing — get an accurate size in seconds.
How to Calculate Air Conditioning Size
The cooling capacity needed for a room depends on its floor area, ceiling height, how the space is used, how well it's insulated, and the local climate. The general rule of thumb for UK residential properties is 100–150 W per square metre of floor area.
Kitchens and sun rooms need more cooling because of heat gains from appliances and solar radiation. Bedrooms need less. Poor insulation or a hot climate pushes the requirement up by 10–15%.
Our calculator multiplies your floor area by a base wattage for the room type, then adjusts for insulation and climate to give you a precise kW and BTU/hr recommendation.
What Size Air Conditioner Do I Need?
For a typical UK living room (4 m × 4 m with 2.4 m ceilings), you'll need approximately 2.0–2.5 kW (7,000–8,500 BTU). A large open-plan kitchen of 30 m² may need 4–5 kW (14,000–17,000 BTU).
Always round up slightly rather than down — an undersized unit will run continuously and never reach setpoint, while a slightly oversized unit will cool quickly and dehumidify effectively.
Frequently Asked Questions
What happens if my AC is oversized?
An oversized air conditioner short-cycles: it cools the room too quickly, shuts off, then restarts. This wastes energy, causes temperature swings, and reduces dehumidification because the unit doesn't run long enough to remove moisture.
How many BTU do I need per square metre?
For UK climates, allow 100–150 W (340–512 BTU) per square metre of floor area. Living rooms typically need 130 W/m², bedrooms 120 W/m², and kitchens 150 W/m².
Does ceiling height affect AC sizing?
Yes. Taller ceilings mean more air volume to cool. Our calculator accounts for ceiling height in the room volume calculation, though the primary driver is floor area.
AC Size Calculator
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