Home & Cooking
BTU Calculator
Estimate the BTU/hr capacity to cool or heat a room. Factors in climate, insulation, sun exposure, occupants, windows, and whether it is a kitchen.
Room and climate details
Recommended capacity
3,600 BTU/hr
About 0.3 cooling tons
- Room area
- 180 sq ft
- Ceiling height
- 8 ft
- Baseline BTU
- 3,600 BTU/hr
- After modifiers
- 3,600 BTU/hr
Breakdown
- Baseline (180 sq ft x 20 BTU/sq ft)+3,600 BTU
- Insulation (average, x1)0 BTU
- Sun exposure (average, x1)0 BTU
- Occupants (baseline 2 included)0 BTU
- Windows (2, no extra load)0 BTU
- Kitchen (none)0 BTU
Frequently Asked Questions about the BTU Calculator
How many BTUs do I need to cool a room?
Start with about 20 BTU per square foot for a moderate climate. A 12 x 15 ft room is 180 sq ft, so the baseline is 3,600 BTU/hr. Add roughly 600 BTU per regular occupant beyond the first two, 1,000 BTU for each window above two, and 4,000 BTU if the room is a kitchen. Adjust by climate: hot zones add 10%, very hot zones add 25%, cool zones drop 15%.
What is a BTU and how does it relate to tons of AC?
A BTU is a British Thermal Unit, the energy needed to raise one pound of water by 1 degree Fahrenheit. AC capacity is rated in BTU per hour. One ton of cooling equals 12,000 BTU/hr, so a 6,000 BTU/hr window unit is half a ton and a 24,000 BTU/hr central system is two tons.
Does insulation really change the BTU calculation?
Yes. Poor insulation (older home, single-pane windows) multiplies the baseline load by about 1.2, while a well-sealed home with double-pane windows drops it to about 0.85. That alone can shift a 6,000 BTU unit recommendation up to 7,200 or down to 5,100, which is the difference between a comfortable room and an overworked compressor.
Why does sun exposure work differently for heating versus cooling?
For cooling, a sunny room takes on more heat through windows and walls, so the load goes up about 10%. A shaded room drops about 10%. Heating is the reverse: sun acts as free passive heating, so a sunny room loses less, while a shaded north-facing room loses more and needs about 10% more heating capacity.
Should I round up or down when picking an AC unit?
Round up only slightly. An oversized AC cools the air quickly but does not run long enough to remove humidity, leaving the room cold and clammy. An undersized unit runs constantly and never reaches the setpoint. Aim for an AC rated within 10-15% of the calculated load and bias upward if the room gets strong afternoon sun.