What is a BTU calculator?
A BTU calculator helps you estimate how much cooling power an air conditioner needs to keep a room comfortable. You enter the floor area of the room, and the calculator returns the required cooling capacity expressed in BTU per hour (BTU/h). It is a fast way to get into the right ballpark before you shop for a window unit, a portable air conditioner, or a mini-split.
What does BTU mean?
BTU stands for British Thermal Unit, a measure of heat energy. One BTU is roughly the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. When the rating is written as BTU/h (often shortened to just “BTU” on product labels), it describes a rate of heat the air conditioner can remove from a room in one hour. A higher BTU/h figure means a more powerful unit.
BTU formula
This calculator uses the common rule of thumb of 20 BTU per square foot of floor area:
If you enter the room area in square meters, the calculator first converts it to square feet before applying the formula. The conversion is:
How to size an air conditioner
Sizing matters more than most people expect. An air conditioner that is too small runs constantly and still never reaches the target temperature. One that is too large cools the air quickly but cycles on and off so often that it fails to remove humidity, leaving the room cold and clammy. The 20 BTU per square foot figure aims for a balanced middle ground for a typical room.
To use the result, round up to the nearest standard size offered by manufacturers. For example, if the calculator suggests 4,300 BTU/h, a 5,000 BTU/h unit is a sensible choice.
This is a rule of thumb
The 20 BTU per square foot guideline assumes an average room with standard ceiling height, ordinary insulation, and a moderate climate. Real rooms vary, so treat the result as a starting point and adjust it for your conditions:
- Sunlight: add roughly 10% for a very sunny room, and you can subtract about 10% for a heavily shaded one.
- Occupancy: add about 600 BTU/h for each regular occupant beyond the first two.
- Kitchens: add around 4,000 BTU/h, since cooking appliances generate extra heat.
- Ceiling height: the rule assumes about 8 ft; taller ceilings mean more air volume and a higher requirement.
- Insulation and climate: poorly insulated rooms or hot climates need more capacity, while well-insulated rooms in mild climates need less.
For a more detailed estimate that weighs ceiling height, occupants, insulation, climate zone, and heat from devices, use our air conditioner calculator.
Units of measurement
Cooling capacity is most often quoted in BTU/h, especially in North America. Elsewhere you may see it in watts or kilowatts. The conversion is:
So a 5,000 BTU/h unit is about 1,465 W, or roughly 1.47 kW.
Worked examples
Example 1: a 200 ft² bedroom
A 200 ft² room needs about 4,000 BTU/h of cooling. A 5,000 BTU/h window unit would comfortably cover it.
Example 2: a 500 ft² studio
A 500 ft² space needs about 10,000 BTU/h. Larger portable units and many mini-splits land in this range.
Example 3: an area given in square meters
Suppose a room is 30 m². First convert to square feet:
Then apply the rule:
A unit of around 7,000 BTU/h would be a safe choice once you round up.
BTU requirements by room size
The table below shows approximate cooling capacity for common room sizes. These figures use the 20 BTU per square foot rule and may vary with your specific conditions.
| Room area (ft²) | Room area (m²) | Cooling capacity (BTU/h) |
|---|---|---|
| 150 | 13.9 | 3,000 |
| 200 | 18.6 | 4,000 |
| 300 | 27.9 | 6,000 |
| 400 | 37.2 | 8,000 |
| 500 | 46.5 | 10,000 |
| 700 | 65.0 | 14,000 |
Frequently asked questions
How many BTU do I need per square foot?
A common rule of thumb is 20 BTU per hour for each square foot of floor area. This works well for an average room, but you should adjust the figure for sunlight, occupancy, kitchen heat, ceiling height, insulation, and climate.
Is it better to have too many or too few BTU?
Neither extreme is ideal. An undersized unit runs nonstop and never cools the room, while an oversized unit cycles off too quickly to remove humidity, leaving the air cold and damp. Aim for the right size and round up only modestly to the nearest standard rating.
How do I convert BTU/h to watts?
Multiply BTU/h by about 0.293 to get watts. For example, 5,000 BTU/h is roughly 1,465 W, or about 1.47 kW.
Can I enter the room area in square meters?
Yes. The calculator converts square meters to square feet automatically using 1 m² ≈ 10.7639 ft² before applying the 20 BTU per square foot rule.
Does ceiling height affect the BTU requirement?
Yes. The rule of thumb assumes a standard ceiling of about 8 ft. Higher ceilings hold more air volume, so a room with tall ceilings needs more cooling capacity than the floor-area calculation alone suggests.