HowMuchIt / Room AC
What size air conditioner do I need?
Room air conditioners are sized in BTU per hour. Start from the room's floor area, then adjust for how sunny it is, how many people use it and whether it is a kitchen. The calculator does the chart and the adjustments.
Work out your room air conditioner size
| Room up to | BTU per hour |
|---|---|
| 150 sq ft | 5,000 |
| 250 sq ft | 6,000 |
| 300 sq ft | 7,000 |
| 350 sq ft | 8,000 |
| 400 sq ft | 9,000 |
| 450 sq ft | 10,000 |
| 550 sq ft | 12,000 |
| 700 sq ft | 14,000 |
| 1,000 sq ft | 18,000 |
| 1,200 sq ft | 21,000 |
| 1,400 sq ft | 23,000 |
| 1,500 sq ft | 24,000 |
| 2,000 sq ft | 30,000 |
Then: heavily shaded, 10 percent less; very sunny, 10 percent more; 600 BTU for each person over two; 4,000 BTU more in a kitchen.
What a BTU is
A BTU, or British thermal unit, is the heat needed to warm one pound of water by one degree Fahrenheit. An air conditioner's cooling power is given in BTU per hour: how much heat it can pull out of the room in an hour. A small bedroom needs a few thousand; a big living room needs several times more. The floor area is the main thing, which is why the chart starts from square feet: length times width.
Why bigger is not better
An air conditioner that is too big for the room cools the air quickly and then stops, before it has had time to pull the moisture out. The room feels cold and clammy, and the unit keeps turning on and off, which is called short cycling. A unit sized to the room runs longer, steadier cycles and dries the air as it cools. So pick the size closest to the result, not two sizes up for safety.
Before you buy
- Measure the window or wall opening: the unit has to fit it.
- Read the unit's manual for the plug and circuit it needs before you buy.
- For a whole house or rooms over 2,000 sq ft, a contractor does a full load calculation.
Good to know
- Room air conditioner sizing by floor area (BTU per hour): 100-150 sq ft 5,000; 150-250 6,000; 250-300 7,000; 300-350 8,000; 350-400 9,000; 400-450 10,000; 450-550 12,000; 550-700 14,000; 700-1,000 18,000; 1,000-1,200 21,000; 1,200-1,400 23,000; 1,400-1,500 24,000; 1,500-2,000 30,000.1,2
- If the room is heavily shaded, reduce the capacity by 10 percent.1,3
- If the room is very sunny, increase the capacity by 10 percent.1,3
- If more than two people regularly use the room, add 600 BTU for each extra person.1,3
- If the unit cools a kitchen, add 4,000 BTU.1,3
- Bigger is not better: an oversized unit cools the air before it has removed enough humidity, so the room feels cold and clammy.1,2,3
- An oversized unit also short cycles: it turns on and off too often instead of running full cooling cycles.2,4
- A BTU (British thermal unit) is the heat needed to raise one pound of water by 1 degree Fahrenheit; air conditioner cooling power is given in BTU per hour.2,5,6
How we checked this page
Checked on October 26, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- Room Air Conditioners, ENERGY STAR (U.S. EPA / DOE)
- What Size Air Conditioner Do I Need, TCL
- How to Size a Window Air Conditioner, Consumer Reports
- What Is AC Short Cycling: Causes, Effects, and Troubleshooting, Carrier
- British thermal units (Btu), U.S. Energy Information Administration
- British thermal unit, Merriam-Webster