How to use the AC size calculator
- Measure the length and width of the room you want to cool.
- Choose the sun exposure. Heavily shaded rooms need less cooling and very sunny rooms need more.
- Enter how many people regularly use the room.
- Tick the kitchen box if the unit will cool a kitchen, which produces extra heat.
ENERGY STAR sizing chart
| Area to cool | Capacity |
|---|---|
| 100–150 sq ft | 5,000 BTU/h |
| 150–250 sq ft | 6,000 BTU/h |
| 250–300 sq ft | 7,000 BTU/h |
| 300–350 sq ft | 8,000 BTU/h |
| 350–400 sq ft | 9,000 BTU/h |
| 400–450 sq ft | 10,000 BTU/h |
| 450–550 sq ft | 12,000 BTU/h |
| 550–700 sq ft | 14,000 BTU/h |
| 700–1,000 sq ft | 18,000 BTU/h |
Adjustments from the same ENERGY STAR guidance:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people: add 600 BTU/h for each additional person.
- Kitchen: add 4,000 BTU/h.
Source: ENERGY STAR room air conditioners.
Worked example
A 15 × 20 ft bedroom (300 sq ft) used by two people:
- The chart gives 7,000 BTU/h.
- If the room is very sunny: 7,000 × 1.1 = 7,700 BTU/h, so pick the next common size up.
Why size matters
- An undersized unit runs constantly and still cannot keep the room cool.
- An oversized unit cools quickly but switches off before removing enough humidity, leaving the room cold and clammy.
- 1 ton of cooling equals 12,000 BTU/h, and 1 kW is about 3,412 BTU/h.
Frequently asked questions
How many BTU do I need for a 12 × 12 room?
A 12 × 12 room is 144 sq ft, which falls in the 100–150 sq ft range, so about 5,000 BTU/h under normal conditions.
How many BTU do I need for 500 square feet?
500 sq ft falls in the 450–550 sq ft range, so about 12,000 BTU/h, before adjustments for sun, people or a kitchen.
What is 12,000 BTU in tons and kW?
12,000 BTU/h is 1 ton of cooling, or about 3.5 kW.
Can I use this for central air conditioning?
No. This chart is for room air conditioners. Central systems should be sized with a full load calculation, such as ACCA Manual J, done by an HVAC professional.