How to size a room for air conditioning
Measure the room's length and width and multiply. If it opens into another space through a wide opening with no door, count that space too, because the cold air will not stop at the arch. A door you plan to keep closed is where a room ends.
Then look at how the room behaves. Does sun pour in through a large west window on summer afternoons, or is it on the shaded side under a big tree? ENERGY STAR moves the size by 10 percent either way. Count the people who regularly use the room, because each one past the second gives off enough heat to matter.
The chart is built on an 8 foot ceiling. Above that, you may want more capacity, but ENERGY STAR gives no figure and we will not make one up. Other sites use a flat percentage per foot with no source behind it.
A worked example
Here is a 400 square foot room, very sunny, with three people in it most evenings. These lines come from the same formula the calculator above runs.
400 sq ft room, very sunny, three people
- From the chart: 10,000 BTU/h
- Very sunny, plus 10%: 1,000 BTU/h
- One person past the first two: 600 BTU/h
- Total: 11,600 BTU/h
- Next size on the chart: 12,000 BTU/h
Next, price it. The cooling cost calculator estimates a season of running it, and the electricity cost calculator works from a wattage on the unit's own label. If the room is hard to cool, it is worth looking at the home energy savings calculator before buying a bigger unit.
Air conditioner size by room
The sizes for a room with two people and an 8 foot ceiling. Each cell is the next unit size on ENERGY STAR's chart at or above the capacity that room needs.
| Room | Area | Normal | Very sunny | Kitchen |
|---|---|---|---|---|
| Small bedroom, 10 × 12 | 120 sq ft | 5,000 | 6,000 | 9,000 |
| Bedroom, 12 × 12 | 144 sq ft | 5,000 | 6,000 | 9,000 |
| Main bedroom, 14 × 16 | 224 sq ft | 6,000 | 7,000 | 10,000 |
| Living room, 16 × 20 | 320 sq ft | 8,000 | 9,000 | 12,000 |
| Large living room, 20 × 20 | 400 sq ft | 10,000 | 12,000 | 14,000 |
| Open plan area, 20 × 30 | 600 sq ft | 14,000 | 18,000 | 18,000 |
Mistakes that lead to the wrong unit
- Buying big to be safe. ENERGY STAR is direct on this: an oversized unit costs more, wastes energy and does not cool better.
- Using this for central air. A whole house needs ACCA Manual J, which accounts for insulation, windows and climate room by room.
- Counting everyone. The 600 BTU is for each person past the first two, and only those who are regularly in the room.
- Forgetting the kitchen. Cooking adds real heat, which is why a kitchen gets a flat 4,000 BTU added.
- Extrapolating off the chart. Past 2,500 square feet a single room unit is the wrong tool anyway. The result tells you if you are off the chart.
Questions
How many BTU do I need for a 12x12 room?
A 12 by 12 room is 144 square feet, which calls for 5,000 BTU per hour on ENERGY STAR's chart. Add 10 percent if the room is very sunny, or take off 10 percent if it is heavily shaded.
How many BTU do I need for 400 square feet?
A 400 square foot room needs about 10,000 BTU per hour on ENERGY STAR's chart. We read its 400 up to 450 row as starting at 400. A room just under 400 needs 9,000.
Does a kitchen need a bigger AC?
Yes, add 4,000 BTU per hour, which is ENERGY STAR's figure for a unit used in a kitchen. A 300 square foot kitchen goes from 8,000 to 12,000, and the next unit size up covers that exactly.
How big of a room will an 8000 BTU unit cool?
An 8,000 BTU unit suits a room from 300 up to 350 square feet with normal sun, an 8 foot ceiling and up to two people. Sun and extra people reduce that, and heavy shade raises it a little.
Is a bigger air conditioner better?
No, an oversized unit costs more to buy, wastes energy and does not cool better, as ENERGY STAR says. Size to the room and resist the urge to move up.
Can I use this to size central air or a mini split for a house?
No. This chart is for a room air conditioner in one room. A whole house needs a proper load calculation, known as ACCA Manual J, and a mini split needs its maker's own sizing.
This BTU figure is a rule of thumb for one room and a room air conditioner with an 8 foot ceiling. It is not a load calculation, and it does not size central air, which needs ACCA Manual J. Insulation, windows and climate all matter, so check the manufacturer's sizing before you buy, and remember an oversized unit cools worse.
Sources and review
- Room air conditioners, ENERGY STAR, specification effective 2023.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Capacity
Capacity = Chart × (1 ± 10%) + 600 × (People − 2) + 4,000 if a kitchen
- Chart
- BTU per hour from ENERGY STAR's table, by room area
- People − 2
- Regular occupants past the first two
Worked through: 10000 × 1.1 + 600 × 1 = 11600 BTU/h
The constants, and where they come from
CHART
Published figureCooling capacity by room area: up to these square feet, this many BTU per hour
- 0
- 150, 5000
- 1
- 250, 6000
- 2
- 300, 7000
- 3
- 350, 8000
- 4
- 400, 9000
- 5
- 450, 10000
- 6
- 550, 12000
- 7
- 700, 14000
- 8
- 1000, 18000
- 9
- 1200, 21000
- 10
- 1400, 23000
- 11
- 1500, 24000
- 12
- 2000, 30000
- 13
- 2500, 34000
Applies to: One room with a room air conditioner and an 8 foot ceiling. It is not for central air
Source: Room air conditioners, ENERGY STAR, published 2023-10, read 2026-11. This source is about 3 years old, and it is the one the figure rests on.
ENERGY STAR's chart. Its rows read 'X up to Y', and reading that as including X and not Y is our interpretation. The page gives no factor for higher ceilings, so none is applied.
CHART_STARTS_AT_SQ_FT
Published figureSmallest room on the chart, in square feet
Applies to: One room with a room air conditioner and an 8 foot ceiling. It is not for central air
Source: Room air conditioners, ENERGY STAR, published 2023-10, read 2026-11. This source is about 3 years old, and it is the one the figure rests on.
The chart runs from 100 to 2,500 square feet, and outside that the result says it is off the chart rather than extrapolating.
SUN_ADJUSTMENT
Published figureShare of capacity added for a very sunny room, or removed for a heavily shaded one
Applies to: One room with a room air conditioner and an 8 foot ceiling. It is not for central air
Source: Room air conditioners, ENERGY STAR, published 2023-10, read 2026-11. This source is about 3 years old, and it is the one the figure rests on.
Taken off the chart value before the flat additions. The source gives no order, and this order is our choice.
BTU_PER_EXTRA_PERSON
Published figureBTU per hour added for each regular occupant past the first two
Applies to: One room with a room air conditioner and an 8 foot ceiling. It is not for central air
Source: Room air conditioners, ENERGY STAR, published 2023-10, read 2026-11. This source is about 3 years old, and it is the one the figure rests on.
Stated by ENERGY STAR as 600 BTUs for each additional person, where more than two regularly occupy the room.
PEOPLE_INCLUDED
Published figureOccupants already allowed for in the chart
Applies to: One room with a room air conditioner and an 8 foot ceiling. It is not for central air
Source: Room air conditioners, ENERGY STAR, published 2023-10, read 2026-11. This source is about 3 years old, and it is the one the figure rests on.
The extra is only for people past the first two, so a couple gets nothing added.
KITCHEN_BTU
Published figureBTU per hour added for a unit used in a kitchen
Applies to: One room with a room air conditioner and an 8 foot ceiling. It is not for central air
Source: Room air conditioners, ENERGY STAR, published 2023-10, read 2026-11. This source is about 3 years old, and it is the one the figure rests on.
Stated by ENERGY STAR as 4,000 BTUs when the unit is used in a kitchen.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the BTU calculator for a room air conditioner
- 1
Measure the room's length and width
- 2
Say whether it is very sunny or heavily shaded
- 3
Count the people who regularly use it
- 4
Say if the unit is going in a kitchen
- 5
Read the capacity, and the unit size to look for
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- One room, a room air conditioner, and an 8 foot ceiling, which the chart is built on
- The chart runs from 100 to 2,500 square feet, and we do not extrapolate past it
- The sun adjustment comes off the chart value first, then people and kitchen are added. The source gives no order
- The unit size is the next capacity on the chart at or above what you need
Pro tips
- Measure the room, not the floor plan, since an open doorway adds the space beyond
- Check the unit's real capacity on the EnergyGuide label, not the box's headline
- Look at how well the room is insulated before moving up a size