Find the right air conditioner size in BTU per hour for a room, based on its square footage, sun exposure, number of people and whether it’s a kitchen.
Buy a window unit that’s too small and it runs nonstop without ever getting the room cool. Go too big and it cools so fast it shuts off before pulling moisture out of the air, which leaves the room cold and clammy. This BTU calculator starts from the ENERGY STAR sizing chart for room air conditioners and then adjusts for sun, extra people and kitchen heat, so you can shop for the right capacity.
How many BTU do I need for my room?
Find your room’s square footage on the ENERGY STAR chart, then adjust. A 12 × 20 ft bedroom is 240 sq ft, which falls in the 150 to 250 sq ft range at 6,000 BTU. If the room gets strong afternoon sun, add 10% for 6,600 BTU. Add 600 BTU for each regular occupant beyond two, and 4,000 BTU if the unit will cool a kitchen.
ENERGY STAR room AC sizing chart
| Area to be cooled (sq ft) | Capacity needed (BTU per hour) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
Adjustments
Heavily shaded room: base × 0.90
More than 2 people: + 600 BTU per extra person
Kitchen: + 4,000 BTU
Tons = BTU per hour ÷ 12,000
When a room chart isn’t enough
The chart is meant for single rooms and window or portable units. For central air in a whole house, or a room with high ceilings, lots of glass or poor insulation, ask an HVAC contractor for a Manual J load calculation. It accounts for insulation, windows, climate and air leaks in a way no square-foot table can.
How to use the BTU Calculator
Enter the room size
Type the length and width in feet, or put the total square footage straight into the area box.
Adjust for sun, people and kitchen
Choose the sun exposure, how many people regularly use the room and whether it is a kitchen.
Read the BTU size
See the recommended capacity in BTU per hour, the base value from the chart and the size in tons.
What does a BTU actually measure?
A British thermal unit (BTU) is the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It equals about 1,055 joules. When an air conditioner is rated at 10,000 BTU, the full unit is really BTU per hour: the rate at which it removes heat from the room.
Useful equivalents:
| Unit | Equals |
|---|---|
| 1 watt of heat | 3.412 BTU/h |
| 1 kilowatt | 3,412 BTU/h |
| 1 ton of refrigeration | 12,000 BTU/h |
The ton comes from the ice era. Melting one short ton (2,000 pounds) of ice absorbs about 288,000 BTU, and spreading that over 24 hours gives 12,000 BTU per hour. That is why a central system described as a 2-ton unit has a capacity of 24,000 BTU/h.
The watt figure helps with heat from inside the room: a 1,000-watt gaming PC or space heater adds roughly 3,400 BTU/h that the air conditioner must remove.
Room size drives most of the load, so start with an accurate area from the square footage calculator.
What do EER, CEER and SEER2 ratings mean?
Capacity in BTU tells you how much heat a unit removes. Efficiency ratings tell you how much electricity it uses to do it.
- Energy Efficiency Ratio (EER) is cooling output in BTU/h divided by power input in watts at one fixed test condition. An 8,000 BTU/h unit with an EER of 10 draws about 800 watts while cooling.
- Combined Energy Efficiency Ratio (CEER) is the rating the U.S. Department of Energy uses for room air conditioners. It works like EER but also counts the power used in standby and off modes, so it reflects a whole day of real use.
- SEER2 applies to central air conditioners and heat pumps. It estimates efficiency across a typical cooling season, and it replaced the older SEER test on January 1, 2023 with conditions closer to real ductwork.
Higher numbers are better in every case. Two units of the same BTU size can differ a lot in running cost, so compare ratings, not just price.
To turn watts into dollars, enter the wattage and your hours of use in the electricity cost calculator.
Frequently asked questions
How many BTU do I need for a 12 × 12 room?
A 12 × 12 room is 144 sq ft, which ENERGY STAR puts at 5,000 BTU. Go to 5,500 if it gets heavy sun.
Is a bigger air conditioner better?
No. An oversized unit cycles on and off too fast, so it doesn’t remove humidity well and the room can feel damp. Match the size to the room.
What does BTU mean?
BTU stands for British thermal unit, the heat needed to raise one pound of water by 1°F. AC capacity is listed in BTU per hour of heat removed.
How many BTU is a ton of air conditioning?
One ton of cooling equals 12,000 BTU per hour. A 24,000 BTU unit is a 2-ton unit.
Does this work for portable air conditioners?
It gives a starting point. Portable units are often rated two ways, so compare the SACC (seasonally adjusted) rating with the number here.
Sources and further reading
Related tools
Last reviewed: October 2026