What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Room floor area: 400 sq ft. The ENERGY STAR chart covers 100–2,500 sq ft.
- Sun exposure: Normal.
- People who regularly occupy the room: 2.
- Is the unit for a kitchen?: No.
What happens, step by step
- Start with the ENERGY STAR chart: 10,000 BTU/h for 400 sq ft.
- Normal sun, so the base stays as it is.
- Two people or fewer, so no extra for occupants.
- Not a kitchen, so no cooking heat is added.
- The window unit needs about 10,000 BTU/h, or 0.83 tons.
The answer: recommended cooling capacity is 10,000 BTU/h.
How it’s calculated
- Base BTU/h from the ENERGY STAR room AC chart by floor area
- Shaded −10%, very sunny +10%
- +600 BTU/h for each regular occupant beyond two
- +4,000 BTU/h if the room is a kitchen
- Tons = BTU/h ÷ 12,000
Worked example
A 400 sq ft sunny room used by three people.
Inputs
- Room floor area
- 400 sq ft
- Sun exposure
- Very sunny (+10%)
- People who regularly occupy the room
- 3
- Is the unit for a kitchen?
- No
Results
- Recommended cooling capacity
- 11,600 BTU/h
- Tons of cooling
- 0.97 tons
- Base capacity for area
- 10,000 BTU/h
Assumptions and limits
- Standard 8 ft ceilings and typical insulation.
- Percentage adjustment is applied to the base value before occupant and kitchen additions.
Sources: ENERGY STAR, Room Air Conditioner sizing guidance.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
What size AC for a 400 square foot room?
The ENERGY STAR chart gives 10,000 BTU/h for 400–450 sq ft. Add 10% for a very sunny room and 600 BTU/h per extra regular occupant.
Is a bigger air conditioner better?
No. An oversized unit short-cycles, removes less humidity and can leave the room cool but clammy.