Your room
Your result
| Room area | |
|---|---|
| Baseline (20 BTU/sq ft) | |
| Ceiling height adjustment | |
| Sun exposure | |
| Insulation | |
| Kitchen | |
| Extra occupants | |
| Tonnage |
How this estimate works
What BTU means
BTU stands for British Thermal Unit. For an air conditioner, it measures cooling capacity: how much heat the unit can remove from the air in one hour. A bigger room holds more air and more heat, so it needs a bigger BTU number.
The starting point
This calculator begins with the widely used rule of thumb of 20 BTU per square foot of floor area, the same baseline behind the ENERGY STAR sizing chart. A 12 × 15 ft room is 180 square feet, so the baseline is 3,600 BTU — which points to a 5,000 BTU unit, matching the chart's smallest size.
Why oversizing hurts
A unit that is too large cools the air so quickly that it shuts off before it has time to pull moisture out of the air. The room feels cold but damp, and the unit switches on and off constantly (short-cycling), which wastes energy and wears out components. Undersized is uncomfortable; oversized is uncomfortable and clammy. That is why the calculator always snaps up to the nearest standard size but never inflates beyond what your inputs justify.
The adjustment factors
- Ceiling height: cooling works on air volume, so the estimate scales with height relative to a standard 8 ft ceiling.
- Sun exposure: a sunny room gains extra heat (+10%); a shaded room needs less (−10%). These match the adjustments published alongside the ENERGY STAR chart.
- Kitchen: cooking appliances add roughly 4,000 BTU of heat load, again per the chart's published adjustments.
- Occupants: each extra person beyond two adds about 600 BTU, per the chart's published adjustments.
- Insulation: ±10% is our own heuristic, not an official figure. Well-insulated rooms hold cool air longer, so we reduce the estimate for good insulation and raise it for poor insulation.
Frequently asked questions
What is a BTU on an air conditioner?
BTU stands for British Thermal Unit. On an air conditioner it measures cooling capacity: how much heat the unit can remove from the air each hour. A 10,000 BTU air conditioner removes 10,000 BTUs of heat per hour, so a bigger room needs a bigger BTU number.
How do BTU and tons relate for air conditioners?
One ton of cooling equals 12,000 BTU per hour. The term comes from the cooling power of a ton of ice. A 24,000 BTU unit is a 2-ton unit. This calculator shows both so you can compare window units (usually sold by BTU) and larger systems (often sold by tons).
Is it bad to buy an air conditioner that's too big?
Yes. An oversized air conditioner cools the room so fast that it shuts off before it has time to remove humidity from the air. The result is a room that feels cold but clammy, plus short-cycling, which wastes energy and wears the unit out faster. Aim for the right size rather than the biggest size.
Do portable and window air conditioners use the same BTU numbers?
Window air conditioners are generally more efficient than portable units, so a portable AC often needs a higher BTU rating to cool the same room. If you are buying a portable unit, consider going one size up from what this calculator recommends for a window unit.
What if my room's BTU falls between two unit sizes?
Buy the larger size, never the smaller one. An undersized unit runs constantly without ever quite cooling the room, which drives up energy bills and shortens its life. When you are close to the boundary between two sizes, the bigger unit gives you headroom for hot days.