Article — AC Tonnage Calculator
AC tonnage calculator: how to size your air conditioner
AC tonnage is your air conditioner's cooling capacity in tons, where 1 ton equals 12,000 BTU per hour. A 1,500 sq ft house in a temperate climate typically needs 3 to 4 tons (36,000 to 48,000 BTU/h). The starting rule is about 30 BTU per square foot, then multiply by climate, insulation, sun exposure, and occupancy adjustments. Florida needs 35% more capacity than the same-sized house in Seattle.
Correctly sizing an AC matters more than picking the highest-efficiency unit. An undersized AC runs constantly without ever cooling the room; an oversized AC cools in five minutes, shuts off, leaves the air humid, and then cycles back on. The right size matches the actual cooling load, which is a function of climate, insulation, and how many people generate heat inside.
What is AC tonnage?
A ton of air conditioning is a unit of cooling power equal to 12,000 BTU per hour. The unit dates from the 19th century, when ice was the primary refrigerant. One ton of ice melting over a 24-hour period absorbs approximately 288,000 BTU, which works out to 12,000 BTU/h. The unit survived because it produces convenient round numbers for home AC sizes: most residential units fall between 1.5 and 5 tons.
Window units are usually labeled in BTU/h (5,000, 8,000, 12,000, 18,000) because they target single rooms. Central air conditioners and heat pumps are labeled in tons (1.5, 2, 2.5, 3, 3.5, 4, 5) because they cool whole houses. Commercial units go higher (10, 15, 20+ tons) for office buildings and warehouses.
Willis Carrier invented modern air conditioning in 1902, but it took until the 1960s for AC to become standard in American homes. Window units (developed in the 1930s) and central AC (1950s) transformed where people could live: cities like Phoenix, Las Vegas, and Houston exploded in population after AC made summer temperatures bearable.
The AC tonnage formula
BTU = sqft × 30 × climate × insul × extras cooling loadTons = BTU ÷ 12,000 convertClimate × 1.35 tropical maxInsulation × 0.70 passive houseThe 30 BTU/h per square foot is a residential rule of thumb that combines roof, wall, window, infiltration, and internal heat loads at typical temperate-climate conditions. For a 1,500 sq ft house: baseline = 1,500 × 30 = 45,000 BTU/h = 3.75 tons. Climate and insulation adjustments then move this number up or down.
AC tonnage by square footage
For a temperate climate (most of the US and UK) with average insulation:
- 500 sq ft 1.25 tons (15,000 BTU/h)
- 800 sq ft 2 tons (24,000 BTU/h)
- 1,000 sq ft 2.5 tons (30,000 BTU/h)
- 1,500 sq ft 3.75 tons (45,000 BTU/h)
- 2,000 sq ft 5 tons (60,000 BTU/h)
- 2,500 sq ft 6 tons (75,000 BTU/h) — often two units
- 3,000 sq ft 7.5 tons (90,000 BTU/h) — usually zoned
- Apartment 800 sq ft single 1.5-ton unit common
AC tonnage and climate
The cooling load depends on how hot it actually gets, not just the building. Tropical climates (Miami, Honolulu, San Juan) need 35% more capacity than temperate climates because summer humidity and overnight temperatures are higher. Hot-dry climates (Phoenix, Vegas, southern California) need 25% more because daytime temperatures hit 110°F regularly, but lower humidity makes nighttime cooling possible.
Cool climates (Pacific Northwest, northern UK, southern Canada) need 15% less. Cold climates (interior Canada, Alaska) often skip dedicated cooling entirely or use a single small heat pump that doubles as a space heater. The climate multiplier is the largest single adjustment to the baseline number.
AC tonnage and insulation
Insulation level affects how much cool air leaks out and how much sun heat leaks in. The four bands:
Poor (×1.25). Old single-pane windows, no wall insulation, leaky doors. Common in pre-1970 housing stock.
Average (×1.00). Modern double-pane windows, code-minimum insulation, weather-stripped doors. Most US housing built 1980 to 2010.
Good (×0.85). Low-E double-pane glass, R-19 wall insulation, R-38 attic insulation. Recent construction in energy-conscious markets.
Excellent (×0.70). Triple-glazed windows, R-40+ walls, R-60+ attic, mechanical ventilation with heat recovery. Passive house standard.
The cost of oversizing AC
A common reflex is to err on the bigger side, but oversizing is worse than undersizing. The problem is short-cycling: the AC reaches setpoint in 5 minutes, shuts off, then the room warms back up in 10 minutes, and the cycle repeats. Each startup uses more electricity than steady-state running.
Short cycles also do not remove humidity. The evaporator coil needs to run continuously for water to condense and drain away. A unit that runs 5 minutes on / 10 minutes off leaves the air clammy at 72°F. A correctly-sized unit runs 20 minutes on / 5 minutes off and produces dry, comfortable air at the same temperature.
An undersized AC reveals itself immediately: the temperature never reaches setpoint and you call someone. An oversized AC works at first but kills the compressor in 8 years instead of 15. The room never feels right because of the humidity. Always err on the smaller side and add zoning if needed.
Common AC tonnage mistakes
The first mistake is ignoring ceiling height. The 30 BTU/sq ft rule assumes 8-foot ceilings. A house with cathedral ceilings (12 ft) needs about 20% more capacity than the floor area alone suggests. A modern open-plan kitchen with 10-foot ceilings adds 10%.
The second mistake is sizing a single zone for the whole house. A 3,000 sq ft house with one 7.5-ton AC will overcool the closest rooms and never reach setpoint at the far end of the ductwork. Two smaller units, or a single large unit with zone dampers, gives much more even cooling for similar total capacity.
Tonnage vs. SEER rating
Tonnage is how much cooling the unit produces. SEER (Seasonal Energy Efficiency Ratio) is how much electricity it uses to produce that cooling. A SEER-20 unit uses about half the power of a SEER-10 unit for the same cooling output. Federal minimum SEER as of 2023 is 14 (northern states) or 15 (southern states). High-efficiency units run SEER 20+.
Higher SEER costs more upfront. The payback depends on local electricity prices and cooling hours. In Phoenix with 2,000+ cooling hours per year and 15 cents/kWh, upgrading from SEER 14 to SEER 18 pays back in 5–7 years. In Seattle with 200 cooling hours per year, the same upgrade may take 20 years.
If you are between two standard tonnages (say, 3.25 calculated), pick the smaller (3 ton) and add a small portable unit for the hottest 5 days of the year. The smaller central unit will dehumidify better the other 360 days and last longer.