Air Conditioner Size Calculator
Air conditioners are rated in BTU per hour, which is a heat removal rate rather than a weight or a size. Getting the number right stops you buying a unit that cannot keep up, or one that is so oversized it never runs long enough to dehumidify.
Cooling load
6,000
- Tons of cooling
- 0.5
- Cooling capacity in watts
- 1,758.4262
- Current draw at 240 volts
- 7.3268
- BTU per square foot
- 20
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
The formula this calculator uses
BTU = area x 20 x (ceiling height / 8) x climate factor x kitchen factor
- area
- The floor area of the room, in square feet
- 20
- The baseline BTU per square foot at 8 foot ceilings
- climate
- A factor above or below 1 for hot or mild regions
- tons
- BTU per hour divided by 12,000
How to check the result by hand
- 1
Measure the floor area
Length times width for a rectangular room, and split an L-shape into two rectangles. A 15 by 20 foot room is 300 square feet.
- 2
Take twenty BTU per square foot
The baseline for a temperate climate and an 8 foot ceiling. Three hundred square feet gives 6,000 BTU per hour.
- 3
Adjust for ceiling height
Multiply by the ceiling height divided by 8. A 10 foot ceiling multiplies the load by 1.25, taking 6,000 BTU up to 7,500.
- 4
Apply the climate and kitchen factors
Typically 1.1 for a hot climate and another 1.1 for a kitchen, applied on top of the height factor.
- 5
Round to a whole unit size
Window units come in 5,000 BTU steps, so round up rather than down. Then check the electrical draw against the circuit.
For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Air Conditioner Size page.