Estimate the BTU (British Thermal Unit) rating needed for an air conditioner or heater based on room size, sun exposure, occupancy and use.
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This calculator starts from the standard room-size-to-BTU sizing chart published by the U.S. Department of Energy, which maps room square footage to a baseline BTU rating (for example, 300-350 sq ft calls for about 8,000 BTU). It then adjusts that baseline: +10% for a sunny or south-facing room, -10% for a heavily shaded room, +600 BTU for each occupant beyond two, and +4,000 BTU if the room is a kitchen, since cooking appliances add significant heat.
An undersized unit will run constantly and struggle to cool or heat the space, while an oversized unit cycles on and off too quickly, which wastes energy and can leave a room feeling clammy since it doesn't run long enough to properly dehumidify. Getting the BTU rating right balances comfort and efficiency.
BTU stands for British Thermal Unit, a measure of heat energy. In HVAC sizing, a higher BTU rating means a unit can heat or cool a larger space, or a smaller space more quickly.
For central air conditioning systems, capacity is often described in tons rather than BTU, where 1 ton equals 12,000 BTU/hr. This calculator shows both figures.
No, this calculator uses room square footage as the primary factor, along with sun exposure, occupancy and kitchen use. Poor insulation, very high ceilings, or extreme climates may require sizing beyond this general estimate, consult an HVAC professional for a precise load calculation.
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