How the size is worked out
Air conditioners are rated in BTUs per hour, a measure of how much heat they can remove from a room. This calculator uses the sizing chart published by ENERGY STAR, the US government's energy efficiency program. It starts from the floor area of the room and then adjusts for sunlight, the number of people and whether the room is a kitchen.
Floor area = length × width
Base capacity = the BTU rating for that area in the chart below
Adjustments: −10% if heavily shaded, +10% if very sunny, +600 BTU for each person above two, +4,000 BTU for a kitchen
In the example above, a 16 × 20 ft room is 320 square feet, which falls in the 300 to 350 square foot band. With normal sun and two people, the chart recommends 8,000 BTU.
| Area to cool | Capacity needed |
|---|---|
| 100 to 150 sq ft | 5,000 BTU |
| 150 to 250 sq ft | 6,000 BTU |
| 250 to 300 sq ft | 7,000 BTU |
| 300 to 350 sq ft | 8,000 BTU |
| 350 to 400 sq ft | 9,000 BTU |
| 400 to 450 sq ft | 10,000 BTU |
| 450 to 550 sq ft | 12,000 BTU |
| 550 to 700 sq ft | 14,000 BTU |
| 700 to 1,000 sq ft | 18,000 BTU |
| 1,000 to 1,200 sq ft | 21,000 BTU |
| 1,200 to 1,400 sq ft | 23,000 BTU |
| 1,400 to 1,500 sq ft | 24,000 BTU |
| 1,500 to 2,000 sq ft | 30,000 BTU |
| 2,000 to 2,500 sq ft | 34,000 BTU |
The chart assumes a standard 8 ft ceiling. Rooms with high or vaulted ceilings hold more air, so consider the next size up.
Why the right size matters
A unit that is too small will run constantly on hot days without getting the room comfortable. A unit that is too big is not better either: it cools the air quickly and switches off before it has removed much moisture, which can leave the room feeling cold and clammy, and the frequent cycling uses more energy. Matching the size to the room gives you the best comfort for the electricity you pay for.
Choosing a unit
- Room air conditioners come in standard sizes such as 5,000, 6,000, 8,000, 10,000 and 12,000 BTU. Pick the size closest to the result, going up if you are between two sizes and the room gets a lot of sun.
- Check that your outlet and circuit can handle the unit. Larger models may need a dedicated circuit or a 230 volt outlet, which is listed on the box.
- Look for the ENERGY STAR label, which means the unit uses less electricity than standard models of the same size.
Frequently asked questions
What size air conditioner do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 square feet, which calls for a 5,000 BTU unit under normal conditions. If the room is very sunny, 5,500 BTU, so a 6,000 BTU model is the closer standard size.
How many BTUs do I need for 500 square feet?
12,000 BTU, according to the ENERGY STAR chart, before any adjustment for sun, people or a kitchen.
Is a bigger air conditioner better?
No. An oversized unit cools the room quickly but does not run long enough to remove humidity, so the room can feel damp, and the constant switching on and off wastes energy.
Can I use this for central air conditioning?
No. This chart is meant for single-room units such as window and portable air conditioners. Sizing a central system for a whole house depends on insulation, windows, climate and ductwork, and should be done by an HVAC contractor using a load calculation.
This calculator gives a starting point based on the ENERGY STAR guide. Insulation, ceiling height and your local climate can change what you need.