12,000 BTU per ton
RecommendedThe Rule of Thumb, and What It Is For
The standard starting point for room cooling is about 20 BTU per hour per square foot of floor area. A 300 square foot room therefore needs roughly 6,000 BTU per hour.
Base cooling loadRoom area in square feet x 20 = base BTU per hour
This is a rule of thumb for sizing a single room unit. It is not a design calculation.
This is an estimate, not a designA rule of thumb per square foot ignores insulation levels, window area and orientation, ceiling height, air leakage, duct losses and local climate. For a whole house, a Manual J load calculation is the correct method, and any competent HVAC contractor will perform one.
Adjustments
The base figure assumes an average room: normal ceiling height, moderate window area, two occupants, no unusual heat sources. Adjust it for the conditions that actually apply.
- Sunny room: add 10 percent. This applies to rooms with large south or west facing glazing that takes direct afternoon sun.
- Heavily shaded room: subtract 10 percent. This applies to rooms shaded all day by trees, an overhang or an adjacent building.
- Kitchen: add 4,000 BTU. Cooking appliances, an oven and a refrigerator add a large and concentrated heat load.
- Occupancy: add 600 BTU for each person over two who regularly occupies the room at the same time.
Worked example
A 400 square foot open plan kitchen and dining area, west facing with large windows, regularly used by four people.
- Base: 400 x 20 equals 8,000 BTU per hour.
- Sunny room: add 10 percent, so 8,000 x 1.10 equals 8,800.
- Kitchen: add 4,000, giving 12,800.
- Two extra people: add 1,200, giving 14,000 BTU per hour.
The rule of thumb alone would have suggested 8,000 BTU. The adjusted figure is 75 percent higher, which shows how much the corrections matter in rooms that are not average.
Room Size and Cooling BTU
Room areaBase BTU at 20 per sq ftSunny room, add 10 percentShaded room, less 10 percent
100 sq ft2,0002,2001,800
150 sq ft3,0003,3002,700
250 sq ft5,0005,5004,500
350 sq ft7,0007,7006,300
450 sq ft9,0009,9008,100
550 sq ft11,00012,1009,900
700 sq ft14,00015,40012,600
1,000 sq ft20,00022,00018,000
Kitchen and occupancy additions are applied on top of these figures, not included in them.
Heating Loads Are a Wider Range
Heating does not have a single per square foot number that works nationally, because the load depends on how cold it gets outside and how well the building holds heat. The usual working range is 30 to 60 BTU per square foot.
SituationBTU per square footTypical case
Mild climate, good insulation30 to 35Southern states, modern construction
Moderate climate, average insulation35 to 45Mid latitude states, typical housing stock
Cold climate, average insulation45 to 55Northern states, older construction
Very cold climate or poor insulation55 to 60Far north, or a poorly sealed older building
A 1,500 square foot house therefore falls somewhere between 45,000 BTU in a mild climate with good insulation and 90,000 BTU in a cold climate with poor insulation. That is a factor of two, and it is exactly why a per square foot number cannot be the basis for buying equipment.
What the Rule of Thumb Leaves Out
Ceiling height
Floor area assumes roughly an 8 foot ceiling. A room with a 12 foot or vaulted ceiling holds half again as much air. Scale the result by the ratio of actual ceiling height to 8 feet.
Windows
Glass is the weakest part of the envelope in both directions. A wall of single glazed west facing windows can dominate the cooling load of a room. Modern low emissivity double glazing changes the picture substantially, and neither case is captured by floor area.
Insulation and air leakage
Two identical rooms, one in a 1930s building with no wall insulation and one in a modern airtight build, can differ by a factor of two or more in both heating and cooling load. Air leakage in particular is often the largest single line item in an older building.
Humidity
Cooling equipment does two jobs: lowering temperature and removing moisture. In humid climates a significant part of the capacity goes into latent load, meaning condensing water out of the air, and none of that appears in a floor area estimate.
Ductwork
On a ducted system, undersized or leaky ducts limit delivered capacity regardless of what the equipment is rated at. Ducts in an unconditioned attic can lose a large share of the output before the air reaches a room.
Using the Estimate Sensibly
- For a window or portable unit in one room, the adjusted rule of thumb is reasonable. Pick the nearest standard capacity and lean slightly under rather than over.
- For a single mini split head, use the estimate as a sanity check on the installer's proposal, not as the specification.
- For a whole house system, insist on a Manual J load calculation. Sizing central equipment off square footage is the most common cause of oversized systems.
- Do not round up habitually. Oversized cooling equipment short cycles, fails to dehumidify and wears out faster. Being slightly under capacity on the hottest few hours of the year costs far less than being permanently oversized.
Bigger is not betterThe instinct to add margin is the wrong instinct with cooling equipment. An oversized unit reaches the thermostat setpoint quickly, shuts off before it has removed moisture, and leaves the space cold and clammy. Correct sizing matters more than extra capacity.