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Start with a baseline of 20 BTU/hr per square foot for cooling in moderate climates. Heating baselines range from 25–40 BTU/hr per square foot depending on insulation and design temperature difference.
Base BTU = Room area (sq ft) × BTU/sq ftMultiply the base load by factors for ceiling height above 8 feet (+10% per extra foot), poor insulation (+20–30%), sunny exposure (+10%), kitchen heat (+4,000 BTU), and occupancy (+600 BTU per person above two).
Adjusted BTU = Base BTU × Ceiling factor × Insulation factor × Exposure factor + Add-onsApply regional multipliers: Zone 1 (hot/humid) may need 30–35 BTU/sq ft cooling; Zone 5–7 (cold) heating loads increase 30–50% over moderate baselines due to larger indoor-outdoor temperature differentials.
Final BTU = Adjusted BTU × Climate multiplierUpdated: July 2026
A 180 sq ft bedroom with 8-foot ceilings, average insulation, and one sunny window needs roughly 5,400–6,500 BTU cooling. A 6,000 BTU window unit handles the load with modest headroom for afternoon sun.
A 400 sq ft living area with 9-foot ceilings and poor attic insulation above. Base 8,000 BTU plus 10% ceiling adjustment and 25% insulation penalty yields ~11,000 BTU. A 12,000 BTU mini-split is appropriate.
A partially below-grade 150 sq ft office loses less heat than above-grade rooms. Reduced exposure factor and moderate insulation give ~3,500 BTU heating need — a 4,000 BTU electric heater suffices.
A 200 sq ft room in Minnesota with R-13 walls needs far more heating BTU than the same room in San Diego with R-19 walls. Always apply insulation and climate multipliers.
Heating and cooling loads differ. Heat pumps list both capacities separately. A unit rated 36,000 BTU heating may only deliver 30,000 BTU cooling at your design conditions.
Oversized or undersized HVAC equipment wastes energy and fails to maintain comfort. This calculator estimates heating and cooling BTU requirements from room dimensions, insulation quality, sun exposure, and local climate so you can size window units, mini-splits, or central systems correctly.