How this calculator works
The calculator looks up the base capacity for your room size, then applies the standard ENERGY STAR adjustments: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU for each regular occupant beyond two, and +4,000 BTU for a kitchen.
Area (sq ft) → base BTU/hr:
100–150: 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
Adjust: shade −10% · sun +10% · +600 per person over 2 · kitchen +4,000
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Room area | 300 sq ft |
| Sun exposure | Normal |
| People who regularly use the room | 2 |
| Is it a kitchen? | No |
| Result | Value |
|---|---|
| Recommended capacity | 7,000 BTU/hr |
| Base size from room area | 7,000 BTU/hr |
| Equivalent tons of cooling | 0.58 tons |
Step by step:
- Base for 300 sq ft = 7,000 BTU
- Sun adjustment × 1 = 7,000 BTU
- + 0 BTU for extra people = 7,000 BTU
Room AC size chart
| Room area | Recommended capacity |
|---|---|
| up to 150 sq ft | 5,000 BTU/hr |
| up to 250 sq ft | 6,000 BTU/hr |
| up to 300 sq ft | 7,000 BTU/hr |
| up to 350 sq ft | 8,000 BTU/hr |
| up to 400 sq ft | 9,000 BTU/hr |
| up to 450 sq ft | 10,000 BTU/hr |
| up to 550 sq ft | 12,000 BTU/hr |
| up to 700 sq ft | 14,000 BTU/hr |
| up to 1000 sq ft | 18,000 BTU/hr |
Common mistakes to avoid
- Buying the biggest unit you can afford — it will leave the room damp.
Frequently asked questions
What size AC for a 300 sq ft room?
About 7,000 BTU in normal conditions; 7,700 BTU if the room is very sunny.
Is a bigger air conditioner better?
No. An oversized unit short-cycles and removes less humidity, so the room feels damp.