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.
For a 300 sq ft room with moderate sun exposure, 2 typical occupants, and not a kitchen: the DOE sizing chart gives a base rating of 7,000 BTU for that square footage. Since sun exposure is moderate (no adjustment), occupants are exactly 2 (no extra), and it's not a kitchen, no further adjustments apply, so the recommended rating stays at 7,000 BTU, equivalent to about 0.58 tons (7,000 ÷ 12,000).
Assuming a bigger BTU rating is always better, an oversized unit cycles on and off too quickly, which wastes energy and leaves a room feeling clammy since it doesn't run long enough to properly dehumidify, correct sizing (not maximum sizing) is the goal.
Forgetting the occupancy adjustment only applies beyond 2 people, this calculator only adds extra BTU for occupants past the first two, a room with exactly 1 or 2 typical occupants gets no occupancy adjustment.
Treating this as a substitute for a professional load calculation in edge cases, this estimate doesn't account for insulation quality, ceiling height or extreme climates, homes with those factors may need sizing beyond what this general estimate suggests.
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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