How to use the AC size calculator
- Measure the length and width of the room, or enter the floor area if you know it.
- Enter the ceiling height. The chart assumes 8 ft (2.4 m); taller rooms need more capacity.
- Choose whether the room is shaded, normal or very sunny.
- Enter how many people usually use the room and tick kitchen if the unit will cool one.
How the size is worked out
1. Floor area → ENERGY STAR chart capacity (8 ft ceiling) 2. × (ceiling height ÷ 8 ft) if the ceiling is higher 3. × 0.9 for heavy shade, × 1.1 for a very sunny room 4. + 600 BTU for each regular occupant beyond two 5. + 4,000 BTU for a kitchen Tons = BTU ÷ 12,000 kW = BTU × 0.000293
Worked example
A 20 × 20 ft family room (400 ft²) that gets a lot of afternoon sun and usually has four people in it:
- Chart: 350–400 ft² → 9,000 BTU
- Very sunny: 9,000 × 1.1 = 9,900 BTU
- Two extra people: + 1,200 = 11,100 BTU → look for a 12,000 BTU unit
ENERGY STAR room AC sizing chart
| Area to cool | Capacity | Area to cool | Capacity |
|---|---|---|---|
| 100–150 ft² | 5,000 BTU | 450–550 ft² | 12,000 BTU |
| 150–250 ft² | 6,000 BTU | 550–700 ft² | 14,000 BTU |
| 250–300 ft² | 7,000 BTU | 700–1,000 ft² | 18,000 BTU |
| 300–350 ft² | 8,000 BTU | 1,000–1,200 ft² | 21,000 BTU |
| 350–400 ft² | 9,000 BTU | 1,200–1,400 ft² | 23,000 BTU |
| 400–450 ft² | 10,000 BTU | 1,500–2,000 ft² | 30,000 BTU |
Choosing the right type of air conditioner
- Window units are the cheapest per BTU and work well for single rooms.
- Portable units are easy to move but less efficient; single-hose models pull warm air into the room, so choose a slightly larger or dual-hose unit.
- Ductless mini-splits are quiet and very efficient, can heat as well as cool, and suit rooms without ducts — they need professional installation.
- Central air is sized for the whole house by a contractor using a Manual J calculation.
Save energy while staying cool
- Choose an ENERGY STAR certified model — certified room units use less energy than standard models.
- Close blinds on sunny windows during the day and seal gaps around window units.
- Set the thermostat a few degrees higher and use a ceiling fan — moving air feels about 4 °F cooler.
- Clean or replace the filter every few weeks in summer.
- Use the electricity cost calculator with the unit’s watts to see what it costs to run.
Why correct sizing matters
An air conditioner removes heat and humidity. A unit that is too small runs constantly and never reaches the set temperature on hot days. One that is too big cools the air so fast that it shuts off before it has pulled moisture out, so the room feels damp, and frequent starts waste energy and shorten the compressor’s life. Matching the BTU rating to the room gives the most comfortable result at the lowest running cost.