Why sizing matters
The rule of thumb is about 20 BTU per square foot, adjusted for sun, occupants and heat sources like kitchens. Don't just buy bigger: an oversized AC cools the air fast but shuts off before removing humidity, leaving the room cold and clammy — and it wastes energy. Too small, and it runs non-stop and never catches up. Right-sizing is the sweet spot.
A quick estimate for a single room; whole-home systems should be sized by an HVAC professional using a Manual J load calculation.