What BTUs mean for heating and cooling
A BTU (British thermal unit) is a measure of heat. Air conditioners and heaters are rated in BTUs per hour: how much heat they can remove from or add to a room each hour. Picking the right size matters. Too small, and the unit runs constantly without keeping up. Too big, and an air conditioner cools the air quickly and shuts off before it removes much humidity, leaving the room cold and clammy.
How to measure the room
Measure the room's length and width in feet and multiply to get square feet. If the room is open to another space with no door, include that space too, since the air will mix. Note how much direct sun the room gets, how many people usually use it, and whether it includes a kitchen.
The formula
For cooling, the calculator starts at 20 BTU per sq ft, in line with ENERGY STAR's room air conditioner sizing guidance, then adjusts:
- +10% for a very sunny room, −10% for a heavily shaded one
- +600 BTU for each regular occupant over 2
- +4,000 BTU if the unit cools a kitchen
For heating, it multiplies area by a BTU-per-square-foot figure for your climate, from about 30 in warm regions to about 60 in cold ones. These are rough rules of thumb for a single room.
Worked example
A 400 sq ft sunny living room used by 3 people, no kitchen:
- Base: 400 × 20 = 8,000 BTU
- Sunny: 8,000 × 1.10 = 8,800 BTU
- One extra person: + 600 = 9,400 BTU/hr cooling
- Heating in a moderate climate: 400 × 40 = 16,000 BTU/hr
Room air conditioners are sold in standard sizes, so you would shop for a unit in the 10,000 BTU range, close to the estimate rather than far above it.
Common mistakes
- Oversizing "to be safe." Bigger units short-cycle and dehumidify poorly.
- Ignoring ceiling height. The rules of thumb assume a typical 8 ft ceiling. Rooms with high or vaulted ceilings, lots of glass, or poor insulation need more capacity.
- Sizing a whole house with a room calculator. Central HVAC should be sized with a Manual J load calculation, which accounts for insulation, windows, orientation, air leakage and climate. An HVAC contractor can run one.
- Overloading a circuit. Larger window and portable units can draw a lot of current, and some need a dedicated circuit or a 240 V outlet. Check the unit's plug type and electrical requirements before buying.
Buying tips
- Window air conditioners commonly range from about 5,000 to over 20,000 BTU. Compare the BTU rating to your estimate and the window size to the unit's installation range.
- Portable air conditioners may list two ratings, an older ASHRAE figure and a newer, typically lower, DOE "SACC" figure. Compare units using the same rating.
- Look at efficiency ratings (CEER for room ACs) as well as BTUs. More efficient units cost less to run.
- For electric space heaters, 1 watt ≈ 3.41 BTU/hr, so a 1,500 W heater produces about 5,100 BTU/hr.
Room cooling