How to use it
Measure your room
Enter the length and width in feet or metres — the calculator converts metres to square feet for you.
Describe the conditions
Pick the sun exposure, set how many people normally use the room, and mark it as a kitchen if it is one.
Read the recommendation
The result shows the computed BTU load, the full breakdown, and the next standard unit size up.
BTU sizing by room area
The rule of thumb is about 20 BTU per square foot of floor area, then adjusted for sunlight, occupancy and heat-producing appliances. This table shows typical window and portable unit sizes by room area.
| Room area (sq ft) | Recommended capacity (BTU) |
|---|---|
| 100 – 150 | 5,000 |
| 150 – 250 | 6,000 |
| 250 – 300 | 7,000 – 8,000 |
| 300 – 350 | 8,000 |
| 350 – 400 | 9,000 – 10,000 |
| 400 – 450 | 10,000 |
| 450 – 550 | 12,000 |
| 550 – 700 | 14,000 |
| 700 – 1,000 | 18,000 |
| 1,000 – 1,200 | 21,000 – 24,000 |
About the BTU calculator
The calculator starts from 20 BTU per square foot, then applies the standard adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU for each person beyond two, and 4,000 BTU for kitchens. The breakdown stays visible under the result so you can see exactly how the number was built. Everything runs in your browser — nothing is uploaded.
Questions
What is a BTU?
A BTU (British Thermal Unit) is the energy needed to raise one pound of water by one degree Fahrenheit. Air conditioner capacity is rated in BTU per hour — the amount of heat the unit can remove from a room each hour. A higher BTU rating means more cooling power.
Is a bigger air conditioner always better?
No. An oversized unit cools the air quickly and shuts off before completing a full cycle — called short-cycling. That wastes energy, wears the compressor, and leaves the room clammy because the unit never runs long enough to dehumidify properly. Aim for the size that matches your load, not far above it.
What about ceiling height?
The formula assumes a standard 8 ft ceiling. If your ceiling is higher, add roughly 10% to the result for every extra 2 ft of height, since there is more air volume to cool.
Is anything uploaded?
No. The calculation runs entirely in your browser — your room details never leave your device, and the tool is free with no sign-up.