How it works
Room air conditioners, window units, portable units and ductless mini-splits are sized by cooling capacity in BTU per hour. This calculator starts from the ENERGY STAR room sizing chart, then applies the chart's standard adjustments.
- Look up the floor area in the chart to get the base capacity.
- Very sunny room: add 10 percent. Heavily shaded room: subtract 10 percent.
- Add 600 BTU/hr for each person over two who regularly uses the room.
- If the unit cools a kitchen, add 4,000 BTU/hr.
BTU/hr = chart value × sun factor + 600 × (people − 2) + 4,000 if kitchen
tons = BTU/hr ÷ 12,000 · kW = BTU/hr ÷ 3,412
Worked example
A 400 sq ft (37.2 m²) living room with big west-facing windows, used by four people. The chart gives 9,000 BTU/hr. Very sunny adds 10 percent: 9,900. Two extra people add 1,200. Total: 11,100 BTU/hr, or 0.93 tons and 3.25 kW. The nearest common size at or above that is a 12,000 BTU unit.
A 300 sq ft kitchen with normal sun and two people: 7,000 + 4,000 = 11,000 BTU/hr, which also points to a 12,000 BTU unit.
ENERGY STAR room sizing chart
| Area up to | Metric | BTU/hr |
|---|---|---|
| 150 sq ft | 13.9 m² | 5,000 |
| 250 sq ft | 23.2 m² | 6,000 |
| 300 sq ft | 27.9 m² | 7,000 |
| 350 sq ft | 32.5 m² | 8,000 |
| 400 sq ft | 37.2 m² | 9,000 |
| 450 sq ft | 41.8 m² | 10,000 |
| 550 sq ft | 51.1 m² | 12,000 |
| 700 sq ft | 65.0 m² | 14,000 |
| 1,000 sq ft | 92.9 m² | 18,000 |
| 1,200 sq ft | 111.5 m² | 21,000 |
| 1,400 sq ft | 130.1 m² | 23,000 |
| 1,500 sq ft | 139.4 m² | 24,000 |
| 2,000 sq ft | 185.8 m² | 30,000 |
| 2,500 sq ft | 232.3 m² | 34,000 |
The chart assumes 8 ft ceilings. Rooms smaller than 100 sq ft still get the smallest common unit, 5,000 BTU. Above 2,500 sq ft you are sizing a whole house, which needs a proper load calculation.
Why not just buy bigger?
An oversized unit cools the air quickly and shuts off before it has pulled much moisture out of it. The room ends up cold and clammy, and the unit cycles on and off more, which wastes energy and wears it out. An undersized unit runs constantly on hot afternoons and may never reach the set temperature. Aim for the nearest size at or slightly above the estimate.
Practical tips
- Australian units are labeled in kW. Use the metric result: a 3.5 kW split system covers the same job as a 12,000 BTU unit.
- Rooms with high or vaulted ceilings hold more air. Consider the next size up.
- Portable units often list two ratings. Compare the lower, newer test figure (SACC in the US) with this estimate.
- A heat pump mini-split can also heat the room in winter. Check its heating output against the heating BTU calculator, because the heating need in a cold climate can be larger than the cooling need.
Window, portable or mini-split?
All three are sized with the same chart, but they perform differently. Window units are the cheapest and usually the most efficient of the plug-in types, and they need a suitable window. Portable units are easy to move but less efficient, because the hose and the unit itself sit inside the room and leak some heat back in. Ductless mini-splits cost the most to install but run quietly, cool efficiently and, as heat pumps, can also heat the room in winter.
Electrical and installation notes
Large window units and most mini-splits need a dedicated circuit, and some run on 240 volts. Do not run an air conditioner on an extension cord unless the manufacturer allows it. Window units must be secured so they cannot fall. Refrigerant work on split systems must be done by a licensed technician, and local code applies to wiring and outdoor units.
Frequently asked questions
What size air conditioner for a 12 × 12 bedroom?
A 12 × 12 room is 144 sq ft, which falls in the smallest row of the chart: 5,000 BTU/hr. Add 10 percent if it gets strong afternoon sun.
How many square feet does a 12,000 BTU air conditioner cool?
On the ENERGY STAR chart, 12,000 BTU/hr covers rooms from 450 up to 550 sq ft with normal sun and two people.
How many BTU is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour, which is about 3.5 kW.
Why does a kitchen need 4,000 BTU more?
Stoves, ovens and other appliances add a lot of heat while you cook, so the chart adds 4,000 BTU/hr for kitchens.