How to use this btu calculator
- Measure the room; the base size comes from the ENERGY STAR chart.
- Adjust for sun exposure.
- Add people who regularly use the room (600 BTU for each person beyond two).
- Add 4,000 BTU for kitchens.
Example
A sunny 20 × 18 ft living room used by three people needs about 10,500 BTU/h of cooling (0.88 tons).
What to know before you buy
What is a BTU?
A British Thermal Unit is the heat needed to raise one pound of water by one degree Fahrenheit. For air conditioners, BTU per hour measures cooling capacity: 12,000 BTU/h equals one ton of cooling or about 3.5 kW.
Window, portable or mini-split
Window units are the most efficient low-cost option for single rooms. Portable units are easier to install but less efficient. Ductless mini-splits are quiet, efficient and can also heat, making them a good long-term choice for rooms and additions.
Air conditioner size by room area (normal sun, 2 people)
| Room size | Cooling capacity |
|---|---|
| 10 × 15 ft (150 sq ft) | 5,000 BTU/h |
| 15 × 17 ft (255 sq ft) | 7,000 BTU/h |
| 15 × 20 ft (300 sq ft) | 7,000 BTU/h |
| 20 × 20 ft (400 sq ft) | 9,000 BTU/h |
| 20 × 25 ft (500 sq ft) | 12,000 BTU/h |
| 25 × 28 ft (700 sq ft) | 14,000 BTU/h |
Formula
Base capacity from the ENERGY STAR chart by room area (e.g. 150–250 sq ft: 6,000 BTU; 350–400: 9,000; 450–550: 12,000; 700–1,000: 18,000), then ± 10% for sun or shade, + 600 BTU per person over 2 and + 4,000 BTU for a kitchen.
Tips from the pros
- Add capacity for high ceilings: about 10% for each extra foot above 8 ft.
- Place window units away from direct sun if possible.
- For whole-house systems, ask for a Manual J load calculation.
Common mistakes to avoid
- Buying the biggest unit you can afford: oversized units short-cycle and leave the room humid.
- Ignoring insulation and windows, which change the load a lot.
Frequently asked questions
How many BTU per square foot?
Roughly 20–25 BTU per square foot for typical rooms with 8 ft ceilings, a little more for small rooms.
Is a bigger air conditioner better?
No. An oversized unit cycles on and off too quickly and removes less humidity, leaving the room clammy.
What size AC for 500 square feet?
About 12,000 BTU/h (1 ton) for a typical room with normal sun, according to the ENERGY STAR sizing chart.
How many square feet does a 12,000 BTU AC cool?
Roughly 450–550 sq ft under normal conditions.