BTU stands for British thermal unit, a unit of heat energy. An air conditioner's rating in BTU per hour is how much heat it can remove from a room every hour, so a higher rating means more cooling. Manufacturers publish that rating for each model. The chart this calculator uses works from floor area rather than volume, so the ceiling height needs a separate check.
How to use this calculator
- Measure the room's length and width in feet, from wall to wall at floor level. For an odd-shaped room, use the square footage calculator to get the total floor area, then enter a length and width that multiply to that area.
- Pick the sun setting that describes the room over a typical day. Choose Heavily shaded only when trees or buildings block most of the direct sun, Very sunny for a room with large windows in direct sun for much of the day, and Average for everything in between.
- Enter the people who regularly use the room, and leave out guests. Set the room type to Kitchen only when the unit will cool a kitchen; for any other room, keep Living or bedroom.
- Check the ceiling. The chart assumes a ceiling of 8 ft, and a higher ceiling may need more capacity than the result shows.
- Look for a unit rated at or near the result in BTU per hour, and do not buy one rated far above it.
Worked examples
Bedroom
Room: 12 × 14 ft
- Air conditioner size
- 6,000 BTU per hour
- Room area
- 168 sq ft
- Chart starting point
- 6,000 BTU/h
Crowded sunny living room
Room: 18 × 20 ft · Sun: Very sunny · People who regularly use the room: 4
- Air conditioner size
- 11,000 BTU per hour
- Room area
- 360 sq ft
- Chart starting point
- 9,000 BTU/h
- After adjustments
- 11,100 BTU/h
Kitchen
Room: 12 × 16 ft · Room type: Kitchen
- Air conditioner size
- 10,000 BTU per hour
- Room area
- 192 sq ft
- Chart starting point
- 6,000 BTU/h
- After adjustments
- 10,000 BTU/h
Shaded home office
Room: 10 × 12 ft · Sun: Heavily shaded
- Air conditioner size
- 5,000 BTU per hour
- Room area
- 120 sq ft
- Chart starting point
- 5,000 BTU/h
- After adjustments
- 4,500 BTU/h
Air conditioner size by room size and sun, for a living room or bedroom
| Room size | Sun: Heavily shaded | Sun: Average | Sun: Very sunny |
|---|---|---|---|
| 10 × 12 ft | 5,000 | 5,000 | 5,000 |
| 12 × 15 ft | 5,000 | 6,000 | 6,000 |
| 15 × 20 ft | 6,000 | 7,000 | 7,000 |
| 20 × 25 ft | 10,000 | 12,000 | 13,000 |
Each cell: Air conditioner size (BTU per hour). Inputs not in the table use the calculator's starting values.
How the chart sizes a room
The calculator multiplies the length by the width to get the floor area, then finds the matching band in the ENERGY STAR sizing chart. A room smaller than 100 sq ft uses the smallest band. A room larger than the chart covers gets a message instead of a size, which points to either several units or a load calculation for central air.
Next, the chart value is adjusted. A heavily shaded room takes off 10 percent, and a very sunny room adds 10 percent. The chart already allows for 2 people, so each regular occupant beyond that adds 600 BTU per hour. A kitchen adds 4,000 BTU per hour to the total.
The sun change applies to the chart value alone, while the occupant and kitchen amounts are added afterward. ENERGY STAR then rounds the total down to the nearest 1,000 BTU per hour, which is the figure the calculator shows.
Why bigger is not better
An oversized unit costs more to buy and wastes energy, because it has more capacity than the room needs. It can also cool the air before it has removed much humidity, so the room feels cold and clammy instead of comfortable.
Choose the size the calculator gives, not a bigger one as a safety margin. If the room still runs warm, check the inputs first: the sun setting, the people count and the ceiling height. To estimate running cost, use the electricity cost calculator with the power rating from the unit's label.
Whole houses need a different calculation
This page sizes a room, which is the job of a window or portable unit. Central air for a whole house is sized by a contractor with a load calculation, often called Manual J, which looks at the walls, windows, insulation, air leaks and climate. Rules of thumb for whole houses are unreliable, so use that calculation instead.
Mistakes to avoid
- Measuring to the outside of the walls. The chart works from the floor area of the room, so measure between the walls.
- Skipping the ceiling check. The chart assumes a ceiling of 8 ft, and a higher ceiling may need more capacity.
- Leaving the room type on Living or bedroom for a kitchen. The Kitchen setting adds capacity for the heat a kitchen gives off, which the other setting does not.
What size room does each air conditioner size cool?
Start from the unit's rating and find the room it fits. For a living room or bedroom with average sun and two people, 10,000 BTU per hour fits rooms from 400 sq ft up to 450 sq ft. 12,000 BTU per hour fits rooms from 450 sq ft up to 550 sq ft, and 18,000 BTU per hour fits rooms from 700 sq ft up to 1,000 sq ft. A very sunny room, extra people or a kitchen push you toward a bigger unit.
Common questions
Is a small air conditioner enough for a bedroom?
It depends on the bedroom's floor area, its sun setting and how many people use it. Enter those values and compare the result with the unit's rating in BTU per hour. A unit rated well below the result may run for long stretches and struggle on the hottest days, while a much larger unit cools too quickly to remove humidity.
How many BTUs per square foot do I need?
This calculator does not use a per-square-foot rule, because those rules ignore insulation, window area and sun. Enter the room's length and width here instead. For a whole house, use a contractor's load calculation.
What size air conditioner does a kitchen need?
Set the room type to Kitchen and enter the room's size as usual. The calculator adds 4,000 BTU per hour to the total to cover the heat a kitchen gives off, then rounds down as it does for any room. Compare the result with the unit's BTU per hour rating before you buy.
How do I convert BTUs to tons of cooling?
This page works only in BTU per hour and does not convert to tons. Compare the result with the BTU per hour rating in the unit's specifications, since both use the same measure.
How many BTUs do I need for a small square room?
Multiply the room's length by its width to get the floor area. A room from 100 sq ft up to 150 sq ft falls in the smallest chart band, which lists 5,000 BTU per hour. Then adjust for sun, extra people and a kitchen, as the calculator does.
How many BTUs are in a ton of air conditioning?
Air conditioner capacity is also given in tons of cooling. One ton equals 12,000 BTU per hour. Divide your result by that number to compare it with a unit listed in tons.