Step 1: measure the room
ENERGY STAR asks you to multiply the full length of the room by its width. Measure in feet, converting inches to fractions of a foot, so 11 feet 6 inches is 11.5 feet.
For a triangular area, ENERGY STAR says to multiply the length by the width and divide by two. For a complex shape, it says to subdivide it into squares, rectangles and triangles, work out each one and add them up. The calculator above takes a length and a width, so for an odd-shaped room, work out the area yourself and enter a length and a width that multiply to it.
An 11 by 13 foot bedroom is 143 square feet.
Step 2: read the ENERGY STAR capacity chart
The chart gives the capacity for each band of floor area. Each band includes its lower number and stops just before its upper one, so a 149 square foot room needs 5,000 BTU/hr and a 150 square foot room needs 6,000 BTU/hr. The capacities are calculated for an 8-foot ceiling.
| Area to be cooled (square feet) | 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 |
Step 3: adjust for sun, people and a kitchen
ENERGY STAR lists four adjustments. A heavily shaded room takes 10 percent off. A very sunny room adds 10 percent. If more than 2 people regularly use the room, add 600 BTU per hour for each additional person. In a kitchen, add 4,000 BTU per hour.
Example: a sunny 12 by 20 foot room (240 square feet) is 6,000 BTU/hr on the chart. Very sunny adds 600 BTU/hr, and three people beyond the first two add 1,800 BTU/hr. That totals 8,400 BTU/hr.
ENERGY STAR does not say whether the sun and shade percentages apply to the chart value or to the running total. The calculator applies them to the chart value, then adds the people and kitchen amounts.
Step 4: round down to a size you can buy
In its article on choosing a window air conditioner, ENERGY STAR says to round the total down to the nearest thousand and look for that capacity. The 8,400 BTU/hr above would be a 8,000 BTU/hr unit. The calculator shows the rounded number as the headline and the exact total under it.
When this chart does not apply
- Mini splits and central air. ENERGY STAR says the contractor should verify the right size for a ductless system with a calculation called Manual J, which works out the heating and cooling your home needs. The room air conditioner chart is not that calculation.
- Oversizing and undersizing. ENERGY STAR says that a system that is either too large or too small for its space will struggle to meet your comfort needs.
- Open floor plans. The ENERGY STAR ductless guidance notes that open spaces are generally easier to cool and that it can be challenging for heat to move through an open doorway. The room air conditioner chart does not say how it treats a room that opens onto another space.
- Tall ceilings. The chart is calculated for an 8-foot ceiling and ENERGY STAR gives no ceiling adjustment. For a much taller room, talk to an HVAC professional.
After you pick a size
BTU per hour measures how much heat the unit can remove, not how much electricity it uses. To see what the unit costs to run once you know its power draw, use the appliance electricity cost calculator.