How air conditioner size is chosen
Room air conditioners are rated by their cooling capacity in BTU per hour (British thermal units). Too small a unit runs constantly without cooling the room; too large a unit cools quickly but switches off before it removes humidity, leaving the room cold and clammy. The US ENERGY STAR program publishes a simple sizing chart based on floor area, with a few adjustments, and this calculator follows it.
ENERGY STAR sizing chart
| Area to be cooled | Capacity needed |
|---|---|
| 100 up to 150 sq ft | 5,000 BTU/h |
| 150 up to 250 sq ft | 6,000 BTU/h |
| 250 up to 300 sq ft | 7,000 BTU/h |
| 300 up to 350 sq ft | 8,000 BTU/h |
| 350 up to 400 sq ft | 9,000 BTU/h |
| 400 up to 450 sq ft | 10,000 BTU/h |
| 450 up to 550 sq ft | 12,000 BTU/h |
| 550 up to 700 sq ft | 14,000 BTU/h |
| 700 up to 1,000 sq ft | 18,000 BTU/h |
| 1,000 up to 1,200 sq ft | 21,000 BTU/h |
| 1,200 up to 1,400 sq ft | 23,000 BTU/h |
| 1,400 up to 1,500 sq ft | 24,000 BTU/h |
| 1,500 up to 2,000 sq ft | 30,000 BTU/h |
| 2,000 up to 2,500 sq ft | 34,000 BTU/h |
Adjustments: reduce capacity by 10% if the room is heavily shaded; increase it by 10% if the room is very sunny; add 600 BTU/h for each person beyond two who regularly uses the room; add 4,000 BTU/h if the unit is used in a kitchen.
Worked example: an 18 × 14 ft sunny living room
- Area: 18 × 14 = 252 sq ft, which falls in the 250–300 sq ft band: 7,000 BTU/h.
- Very sunny: 7,000 + 10% = 7,700 BTU/h.
- Three people usually in the room: one more than two, so + 600 = 8,300 BTU/h.
Units are sold in steps, so choose the nearest size at or above the result, here an 8,000 to 9,000 BTU/h model. In metric terms 8,300 BTU/h is about 2.4 kW of cooling.
BTU, kW and tons
| BTU/h | kW (cooling) | Tons of refrigeration |
|---|---|---|
| 5,000 | 1.47 | 0.42 |
| 8,000 | 2.34 | 0.67 |
| 12,000 | 3.52 | 1 |
| 18,000 | 5.28 | 1.5 |
| 24,000 | 7.03 | 2 |
One BTU/h is 0.293 W, and one "ton" of cooling is 12,000 BTU/h. Mini-split and central systems are often described in tons.
When a simple chart is not enough
The chart assumes a typical room with standard 8 ft ceilings and average insulation. High or vaulted ceilings, poor insulation, large west-facing windows, hot climates and rooms open to other spaces all raise the load. For whole-house systems, heat pumps and mini-splits, HVAC contractors use a full load calculation such as ACCA Manual J, which considers insulation, windows, orientation and local climate.
Tips for choosing a unit
- Check the electrical supply. Larger window units may need a dedicated circuit or a 230 V outlet.
- Compare efficiency: look at the CEER rating in the US; higher means lower running costs.
- Place the unit where air can circulate freely, away from heat sources such as lamps and TVs that confuse the thermostat.
Step by step: sizing a room air conditioner
- Measure the room's length and width and multiply for the floor area.
- Include any space that is open to the room without a door.
- Find the base capacity in the chart.
- Adjust for shade or sun, people and kitchen use.
- Choose the nearest unit size at or above the result, and check the outlet it needs.
Common mistakes
- Buying the biggest unit "to be safe". Oversized units dehumidify poorly and cycle on and off.
- Forgetting connected spaces. If a room opens onto a hallway or kitchen without a door, include that area.
- Using the house size for a single-room unit.
Frequently asked questions
How many BTU do I need for a 12×12 room?
A 12 × 12 ft room is 144 sq ft, so the ENERGY STAR chart gives 5,000 BTU/h. Add 10% if it is very sunny or 600 BTU/h for each extra person beyond two.
What size AC for 500 square feet?
500 sq ft is in the 450–550 sq ft band, so about 12,000 BTU/h (1 ton), before adjustments for sun, people or a kitchen.
How many square feet does 12,000 BTU cool?
Roughly 450 to 550 sq ft according to the ENERGY STAR chart, under typical conditions.
How do I convert BTU to kW?
Multiply BTU/h by 0.000293. 12,000 BTU/h is about 3.5 kW of cooling capacity.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly but switches off before it removes enough humidity, so the room feels damp and the unit cycles on and off. Choose the size that matches the room.
Sources
- ENERGY STAR (US EPA / DOE), Room Air Conditioners: "How to choose the right size" sizing chart and adjustments.
- ACCA Manual J, Residential Load Calculation (Air Conditioning Contractors of America).
- NIST Special Publication 811: 1 BTU (IT)/h = 0.2930711 W.
Last reviewed 20 October 2026. Results are estimates; see our disclaimer.