Room BTU/hr Calculator for Cooling & Heating

Cooling capacity is rated in BTU per hour, and the quickest honest way to size a single room is the ENERGY STAR rule of thumb: start at about 20 BTU/hr for every square foot of floor. A 340 square-foot bedroom therefore lands near 6,800 BTU/hr, which is why room air conditioners cluster around the 6,000–8,000 BTU marks.

Four adjustments from the same ENERGY STAR chart tune that base number. A heavily shaded room drops 10%; a very sunny room adds 10%. Once more than two people use the room regularly, add 600 BTU/hr per extra person. And because a kitchen dumps stove and oven heat into the air, add a flat 4,000 BTU/hr when you are sizing one. The calculator applies whichever boxes you check and shows the running total.

Heating is sized separately because it depends on your winter climate, not just floor area. We multiply the room area by a climate factor — roughly 30 BTU/hr per square foot in the Deep South, 40 in the middle of the country, and 50 up north — to give a companion heating estimate. Both numbers are planning figures: a Manual J load calculation by an HVAC professional accounts for insulation, windows, and air leakage that a per-square-foot rule cannot.