BTU Calculator
Estimate the BTU per hour of air conditioning a room needs from its size, ceiling height, sun exposure and occupants. Example: a 400 square foot room with 4 people needs about 9,200 BTU, so a 10,000 BTU unit.
Show steps
How to use the BTU Calculator
- Enter the area of the room in square feet and the ceiling height.
- Choose the sun exposure and number of occupants.
- Mark whether the room is a kitchen, then press Calculate.
Formula and method
BTU = area x 20 x (ceiling height / 8), adjusted +10% for sun or -10% for shade, plus 600 BTU for each person beyond two and 4,000 BTU for a kitchen. 12,000 BTU per hour equals one ton of cooling.
Worked example: 400 sq ft, 4 people
| Step | Result |
|---|---|
| Base (400 x 20) | 8,000 |
| Occupants beyond 2 (2 x 600) | 1,200 |
| Total | 9,200 BTU |
| Standard unit | 10,000 BTU |
BTU by room size (8 ft ceiling)
| Room area | BTU needed |
|---|---|
| 150 sq ft | 3,000 |
| 250 sq ft | 5,000 |
| 300 sq ft | 6,000 |
| 350 sq ft | 7,000 |
| 400 sq ft | 8,000 |
| 550 sq ft | 11,000 |
| 700 sq ft | 14,000 |
| 1000 sq ft | 20,000 |
Sizing matters
An oversized unit cools quickly but cycles on and off, which leaves humidity high. An undersized unit runs constantly and cannot reach the set temperature on hot days. Choose the nearest standard size at or above your estimate.
Frequently asked questions
What is a BTU?
The heat needed to raise one pound of water by one degree Fahrenheit.
How many BTU per square foot?
About 20 for a typical room.
How many BTU do I need for 400 sq ft?
About 8,000 before adjustments, so a 8,000 to 10,000 BTU unit.
What does a ton of cooling mean?
12,000 BTU per hour.
Do ceilings change the size?
Yes, higher ceilings add volume, so more BTU.
Is this for heating too?
It is for cooling; heating needs depend on climate and insulation.
Sources
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