BTU Calculator
This uses the basic square-footage method: roughly 20 BTU of cooling per square foot, adjusted for ceiling height, sun exposure, occupancy and kitchens. It's a rule of thumb for ballparking a window or portable AC unit — not an engineering calculation.
The square-footage method ignores insulation, windows, climate, and air leakage — the things that actually drive heating and cooling loads. It's fine for picking a window AC for one room. For sizing a furnace, central AC, heat pump or mini-split, get a Manual J load calculation from an HVAC contractor. Oversized equipment short-cycles and dehumidifies poorly; this rule of thumb can't catch that.
How this is calculated
Base estimate = room area × 20 BTU per square foot, scaled by (ceiling height ÷ 8) for taller rooms.
Then: +10% for very sunny rooms, −10% for heavily shaded rooms, +600 BTU for each person beyond two, and +4,000 BTU for kitchens. The result is rounded to the nearest 100 BTU because the method isn't precise enough to justify finer numbers.
Common questions
What is a BTU?
A British Thermal Unit — the energy needed to raise one pound of water by 1°F. Air conditioners and heaters are rated in BTU per hour, which describes how much heat they can move or produce.
Is bigger always better?
No. An oversized air conditioner cools the air quickly but shuts off before removing much humidity, leaving the room cold and clammy. Matching the load matters more than adding headroom.
Does this work for heating?
Heating loads depend heavily on your climate and insulation, so a single per-square-foot number is even rougher for heat. Use this for ballparking only, and get a proper load calculation for any installed heating equipment.
Why does my HVAC quote differ from this number?
A contractor doing a Manual J accounts for windows, orientation, insulation levels, ductwork and local climate. This calculator doesn't, so real quotes can reasonably land above or below it.