How we worked it out
Updated as you change the numbers.
- Area = length × width = 20 ft × 12 ft = 240 sq ft.
- ENERGY STAR chart for 240 sq ft: 6,000 BTU per hour.
- Total = 6,000 BTU per hour. Pick the nearest size at or just above this.
Method and assumptions
The calculator uses the ENERGY STAR room air conditioner sizing chart, which gives a capacity for bands of floor area, from 5,000 BTU per hour for 100 to 150 sq ft up to 34,000 for 2,000 to 2,500 sq ft. It then applies the chart’s adjustments: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU per hour for each regular occupant over two, and +4,000 for a kitchen.
A unit that is too big cools the room quickly but switches off before it removes humidity, leaving it cold and clammy. Too small and it runs constantly without keeping up.
Tips
- Rooms with high ceilings, poor insulation or big west-facing windows may need the next size up.
- Check the outlet: larger units can need a 230 V circuit.
Questions
What size air conditioner do I need for a 12 × 20 room?
240 sq ft falls in the 150 to 250 sq ft band of the ENERGY STAR chart: 6,000 BTU per hour. In a very sunny room, 6,600, so a 7,000 BTU unit.
Sources
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.