What is a ton of cooling?
A ton of cooling is 12,000 BTU per hour. The name dates from the days of ice: melting one ton of ice absorbs about 288,000 BTU, and spreading that over 24 hours gives 12,000 BTU per hour. In metric terms, one ton is about 3.5 kW of cooling.
| Tons | BTU/hr | kW |
|---|---|---|
| 1.5 | 18,000 | 5.3 |
| 2 | 24,000 | 7.0 |
| 2.5 | 30,000 | 8.8 |
| 3 | 36,000 | 10.6 |
| 4 | 48,000 | 14.1 |
| 5 | 60,000 | 17.6 |
Residential central systems typically range from 1.5 to 5 tons in half-ton steps. Larger homes use two systems or a commercial unit.
Square feet per ton: the rule of thumb
You will hear that one ton cools 400 to 600 sq ft, or more. The real figure depends on climate, insulation, windows, ceiling height and how many people live there. The ENERGY STAR room sizing chart, designed for single rooms, shows how coverage per ton grows as the space gets bigger:
| Floor area | Chart capacity | Tons | Sq ft per ton |
|---|---|---|---|
| 400 sq ft | 9,000 BTU/hr | 0.75 | 533 |
| 1,000 sq ft | 18,000 BTU/hr | 1.5 | 667 |
| 1,500 sq ft | 24,000 BTU/hr | 2 | 750 |
| 2,000 sq ft | 30,000 BTU/hr | 2.5 | 800 |
Hot, sunny climates and older homes land at the low end of the square feet per ton range. Mild climates and tight, well-insulated homes cover more area per ton. That spread is the reason the rule of thumb cannot replace a load calculation for a whole house.
What changes the load
- Climate. Hotter design temperatures and more humid air mean more heat to remove.
- Windows. Large west- and south-facing windows let in a lot of heat on summer afternoons. Shading, awnings and low-e glass cut that gain.
- Insulation and air leaks. A well-insulated, tight attic and walls keep heat out; a hot, poorly insulated attic adds a big load to the rooms below.
- Ducts. Leaky ducts running through a hot attic can waste a noticeable share of the cooling before it reaches the rooms.
- People and appliances. Every occupant, computer and cooking appliance adds heat.
Worked example: a bonus room over the garage
For a single space, the chart and its adjustments work well. Take a 650 sq ft bonus room above a garage, with a south-facing dormer that bakes in the afternoon, used by three people. The chart row for up to 700 sq ft gives 14,000 BTU/hr. Very sunny adds 10 percent: 15,400. One person over two adds 600. The total is 16,000 BTU/hr, or 1.33 tons, which works out to about 488 sq ft per ton. A 1.5-ton (18,000 BTU) mini-split is the nearest common size at or above it. Run your own rooms through the AC BTU calculator.
Sizing a whole-house system
For central air or a whole-house heat pump, US contractors should use ACCA Manual J. It totals heat gain room by room from walls, roof, windows by direction, air leakage, ducts, people and appliances. The result usually comes in lower than the old rule of 1 ton per 400 or 500 sq ft, especially in newer homes. Oversized AC cools in short bursts and leaves humidity behind, so a right-sized system feels more comfortable.
If a contractor quotes a size without measuring windows, insulation and ducts, or simply matches the old unit, ask how they arrived at it. The old system may never have been the right size.
Manual S then picks equipment that matches that load at your local design conditions, and Manual D sizes the ducts. Ask your contractor which methods they used.
Heat pumps: size for heating too
A heat pump is rated in tons of cooling, but in a cold climate the heating load is usually the larger one. A 3-ton heat pump might cover the summer cooling load but lose output as outdoor temperatures drop. Cold-climate models publish heating capacity at low temperatures, such as 5°F (−15°C). Compare that figure with your heating load, which the heating BTU calculator can estimate. Many installers size for the larger of the two loads and add backup heat for the coldest days.
Signs your system is the wrong size
- Runs for only a few minutes and the house feels damp: likely oversized.
- Runs constantly on hot afternoons without reaching the setpoint: possibly undersized, or a duct, refrigerant or airflow problem.
- Big temperature swings between rooms: often duct design or air sealing, not tonnage.
Safety and installation
Central systems and heat pumps need dedicated 240 V circuits, refrigerant handling by certified technicians and permits in most places. Outdoor units need clearance for airflow and should be secured on a level pad. Check local code and the manufacturer's installation manual.
Frequently asked questions
How many BTU is a 3-ton AC?
36,000 BTU per hour, which is about 10.6 kW of cooling.
How big a house does a 2-ton AC cool?
Roughly 800 to 1,500 sq ft, depending on climate, insulation and windows. The ENERGY STAR room chart puts 24,000 BTU/hr at up to 1,500 sq ft, but a whole house should be sized with a Manual J calculation.
Why is it called a ton?
A ton of cooling is the heat needed to melt one ton of ice over 24 hours, about 12,000 BTU per hour.
Do the math for your own project.
Open the ac btu calculator