BTU Calculator
Use this free online BTU calculator to find the right air conditioner or heater size for any room. Enter your room dimensions, insulation level, and occupancy to get an accurate BTU recommendation instantly.
Enter Values
Choose feet or meters for your room dimensions
The longer side of your room
The shorter side of your room
Standard is 8 ft (2.4 m). Higher ceilings need more BTU
Rooms with large windows or direct sunlight need more cooling
Each person above 2 adds roughly 600 BTU
Kitchens generate extra heat from cooking appliances
Result
Enter values above and click Calculate to see your result.
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Formula
Multiply the room area in square feet by 20 to get the base BTU. Then adjust upward for high ceilings, poor insulation, heavy sun exposure, extra occupants, or kitchen use.
Worked Example
What Is a BTU and Why Does It Matter?
- An undersized AC will run constantly without cooling the room, wasting energy and money
- An oversized AC cools too quickly without removing humidity, leaving the room damp and uncomfortable
- Most residential rooms need between 5,000 and 12,000 BTU depending on size and conditions
- The standard rule of thumb is 20 BTU per square foot of living space
- Kitchens, server rooms, and rooms with many occupants need additional capacity
Getting the right BTU rating saves energy, extends equipment life, and keeps your space comfortable year-round. This calculator accounts for all the major factors HVAC professionals consider when sizing a unit.
Frequently Asked Questions
What is a BTU?
BTU stands for British Thermal Unit. It measures the amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit. For air conditioners, the BTU rating tells you how much heat the unit can remove from a room per hour.
How many BTU do I need per square foot?
The general rule is 20 BTU per square foot for a standard room with 8-foot ceilings. A 150 sq ft bedroom needs about 3,000 BTU, while a 300 sq ft living room needs about 6,000 BTU. Adjust upward for kitchens, sunny rooms, or high ceilings.
What size AC do I need for a 12x12 room?
A 12x12 room is 144 square feet. At 20 BTU per square foot, you need about 2,880 BTU. A 5,000 BTU window unit is the smallest commonly available size and would work well for this room.
Does ceiling height affect BTU requirements?
Yes. Standard calculations assume 8-foot ceilings. Rooms with 9, 10, or 12-foot ceilings have more air volume to condition, so they need proportionally more BTU. This calculator adjusts automatically for non-standard ceiling heights.
Why do kitchens need more BTU?
Kitchens generate significant heat from ovens, stoves, dishwashers, and refrigerators. The industry standard adds approximately 4,000 BTU to the base requirement for kitchen spaces.
What happens if my AC has too many BTU?
An oversized AC cools the air too quickly and shuts off before properly dehumidifying the room. This leads to a cold but clammy environment, frequent on-off cycling that wastes energy, and shorter equipment lifespan.
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