Getting the BTU rating right matters more than any other buying decision. An undersized unit runs constantly and never quite cools; an oversized one short-cycles, leaves the air humid, and wears out early.
The quick rule: about 20-25 BTU per square foot for a standard 8-foot ceiling. A 350 sq ft bedroom lands around 9,000 BTU; an open 1,000 sq ft living area needs 24,000. Then adjust: add roughly 10% for very sunny rooms or 9-10 foot ceilings, add 4,000 BTU for kitchens, and add 600 BTU per person beyond two regular occupants.
Poor insulation or hot climates push you up a size; shaded, well-insulated rooms let you round down. If you sit between two capacities, our system finder handles the math and points you at the exact matching systems, it takes under a minute.