Room AC Size Calculator
Find the right size window, portable or mini-split air conditioner for a room from its area, sun exposure, occupants and use, following the ENERGY STAR sizing chart.
Calculator
12,000 BTU per hour
- Room area
- 500 sq ft
- Chart starting point
- 12,000 BTU
- Cooling power
- 3.5 kW
- Tons
- 1.0
Look for a unit rated at or just above 12,000 BTU. A much larger unit will short-cycle and leave the room clammy.
How this calculator sizes a room
It follows the ENERGY STAR method for room air conditioners in four steps:
- Find the starting capacity from floor area.
Length times width gives the area, which maps to a capacity on the ENERGY STAR chart below.
- Adjust for sun.
Subtract 10 percent for a heavily shaded room or add 10 percent for a very sunny one.
- Add for people.
Add 600 BTU for each person beyond two who regularly uses the room.
- Add for a kitchen.
Add 4,000 BTU if the unit will cool a kitchen, where cooking adds heat.
The result is the minimum capacity to look for. Units are sold in steps (5,000, 6,000, 8,000, 10,000, 12,000 BTU and so on), so choose the nearest size at or slightly above the result.
ENERGY STAR sizing chart
Capacity for a room with average sun, two occupants and no kitchen.
| Area to cool | Area in m² | Capacity |
|---|---|---|
| 100 to 150 sq ft | 9 to 14 m² | 5,000 BTU/h |
| 150 to 250 sq ft | 14 to 23 m² | 6,000 BTU/h |
| 250 to 300 sq ft | 23 to 28 m² | 7,000 BTU/h |
| 300 to 350 sq ft | 28 to 33 m² | 8,000 BTU/h |
| 350 to 400 sq ft | 33 to 37 m² | 9,000 BTU/h |
| 400 to 450 sq ft | 37 to 42 m² | 10,000 BTU/h |
| 450 to 550 sq ft | 42 to 51 m² | 12,000 BTU/h |
| 550 to 700 sq ft | 51 to 65 m² | 14,000 BTU/h |
| 700 to 1,000 sq ft | 65 to 93 m² | 18,000 BTU/h |
| 1,000 to 1,200 sq ft | 93 to 111 m² | 21,000 BTU/h |
| 1,200 to 1,400 sq ft | 111 to 130 m² | 23,000 BTU/h |
| 1,400 to 1,500 sq ft | 130 to 139 m² | 24,000 BTU/h |
| 1,500 to 2,000 sq ft | 139 to 186 m² | 30,000 BTU/h |
| 2,000 to 2,500 sq ft | 186 to 232 m² | 34,000 BTU/h |
Frequently asked questions
Is a bigger air conditioner better?
No. An oversized unit cools the room quickly and shuts off before it has removed much moisture, so the room feels cold and clammy, and frequent starts waste energy. ENERGY STAR specifically warns against buying a unit that is too large for the space.
How many square feet does a 12,000 BTU air conditioner cool?
On the ENERGY STAR chart, 12,000 BTU per hour covers 450 to 550 square feet in a room with average sun, two occupants and no kitchen. Sunny rooms, more people or kitchen use push that number down.
What about rooms with high ceilings?
The ENERGY STAR chart assumes ordinary 8-foot ceilings. For taller ceilings the room holds more air, so consider the next size up or have a professional run a load calculation.
Can I use this to size central air or a heat pump for my whole house?
Use it only as a rough check. Whole-house systems should be sized with a Manual J load calculation, which accounts for insulation, windows, air leakage and your climate. Ask contractors to show you the calculation before you accept a quote.
Sources and method
- Room Air Conditioners: sizing chart and adjustments ENERGY STAR (U.S. EPA)
- Room Air Conditioners U.S. Department of Energy, Energy Saver (archived copy, 2025)
- Residential Load Calculation (Manual J) Air Conditioning Contractors of America
Last reviewed . Results are estimates for planning; they do not replace an on-site assessment by a qualified professional.