How the size is calculated
Base BTU from the ENERGY STAR chart by area
× Ceiling height ÷ 8 ft · ± 10% for shade or sun · + 600 per extra person · + 4,000 for a kitchen
× Ceiling height ÷ 8 ft · ± 10% for shade or sun · + 600 per extra person · + 4,000 for a kitchen
Example: A 20 × 20 ft room (400 sq ft) with 8-ft ceilings, normal sun and 2 people needs 9,000 BTU (0.75 tons). If it faced strong afternoon sun and 3 people used it, the size would rise to about 10,500 BTU.
ENERGY STAR sizing chart
- 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 · 2,000–2,500: 34,000
Heating instead? See what a space heater costs to run with the space heater cost calculator.
Frequently asked questions
How many BTU do I need per square foot?
Roughly 20 BTU per square foot for rooms of average height. The ENERGY STAR chart recommends 9,000 BTU for 350–400 sq ft and 12,000 BTU for 450–550 sq ft.
What adjustments should I make?
Reduce capacity by 10% for a heavily shaded room, increase by 10% for a very sunny room, add 600 BTU for each person beyond two, and add 4,000 BTU for a kitchen.
What happens if my AC is too big?
An oversized unit cools the air quickly and shuts off before removing moisture, leaving the room cold and clammy and wasting energy through short cycling.
How many BTU in a ton?
One ton of cooling equals 12,000 BTU per hour. Central systems are sized in tons; window and portable units in BTU.