How to size an air conditioner
- Measure the room and calculate its floor area (length × width).
- Find the base capacity from the ENERGY STAR chart below.
- Adjust for conditions: reduce by 10% for a heavily shaded room, increase by 10% for a very sunny one, add 600 BTU for each regular occupant beyond two, and add 4,000 BTU if the unit cools a kitchen.
- Choose a unit at or just above the result.
The calculator also scales the result for ceilings higher than 8 ft, since a taller room holds more air.
ENERGY STAR BTU sizing chart
| Area to be cooled | Capacity needed |
|---|---|
| 100–150 sq ft | 5,000 BTU/hr |
| 150–250 sq ft | 6,000 BTU/hr |
| 250–300 sq ft | 7,000 BTU/hr |
| 300–350 sq ft | 8,000 BTU/hr |
| 350–400 sq ft | 9,000 BTU/hr |
| 400–450 sq ft | 10,000 BTU/hr |
| 450–550 sq ft | 12,000 BTU/hr |
| 550–700 sq ft | 14,000 BTU/hr |
| 700–1,000 sq ft | 18,000 BTU/hr |
| 1,000–1,200 sq ft | 21,000 BTU/hr |
| 1,200–1,400 sq ft | 23,000 BTU/hr |
| 1,400–1,500 sq ft | 24,000 BTU/hr |
| 1,500–2,000 sq ft | 30,000 BTU/hr |
| 2,000–2,500 sq ft | 34,000 BTU/hr |
Worked example
A 15 × 20 ft living room is 300 sq ft, which the chart puts at 7,000 BTU. If it faces south with big windows (+10%) and four people often use it (+1,200 BTU for the two extra people), you need about 8,900 BTU — so a 10,000 BTU unit is the right choice.
Why bigger isn't better
An oversized air conditioner cools the room quickly and shuts off before it has removed enough moisture, leaving the space cold and clammy. It also cycles on and off more often, which wastes energy and wears the compressor. An undersized unit runs constantly and may never reach the set temperature on hot days. Matching capacity to the room gives the best comfort and efficiency.
BTU, tons and kilowatts
- BTU per hour (British thermal units) measures how much heat an air conditioner removes in an hour.
- 1 ton of cooling = 12,000 BTU/hr — the unit used for central AC and heat pumps.
- 1 kW of cooling ≈ 3,412 BTU/hr — common on metric labels.
For whole-house central air, room-by-room rules of thumb are not enough — an HVAC contractor should run a Manual J load calculation that accounts for insulation, windows, orientation and climate.
Frequently asked questions
How many BTU do I need for my room?
Use the ENERGY STAR chart: about 5,000 BTU for 100–150 sq ft, 8,000 BTU for 300–350 sq ft, 12,000 BTU for 450–550 sq ft and 18,000 BTU for 700–1,000 sq ft, then adjust for sun, occupants and kitchens.
How many square feet does a 12,000 BTU air conditioner cool?
About 450–550 sq ft under average conditions, according to the ENERGY STAR sizing chart.
What size AC do I need for a 12×12 room?
A 12 × 12 room is 144 sq ft, so a 5,000 BTU unit is usually enough — go up to 6,000 BTU if the room is very sunny or used as a home office with equipment.
How many BTU is 1 ton?
One ton of cooling equals 12,000 BTU per hour. A 2-ton central AC unit is 24,000 BTU/hr.
Does ceiling height affect BTU?
Yes. Standard charts assume 8 ft ceilings. For higher ceilings, scale the BTU up in proportion — a 10 ft ceiling needs roughly 25% more capacity.