BTU / AC Size Calculator
Tell us about your room and we'll estimate the cooling power (BTUs) and the air conditioner size that fits it best.
📖 How it works & FAQWhat a BTU actually measures
A BTU (British Thermal Unit) is the amount of energy needed to change 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 pull out of a room per hour. More square footage and more heat sources mean you need more BTUs. This calculator starts from the widely used rule of roughly 20 BTUs per square foot, then adjusts for the things that make a room harder to cool.
Why sun, people, and kitchens change the number
Two rooms of the same size can need very different cooling. A room that bakes in afternoon sun gets a 10% bump, while a heavily shaded room drops 10%. Every person past the first two adds about 600 BTU because bodies give off heat. Kitchens add a flat 4,000 BTU to cover the stove, oven, and fridge. Getting the size right matters: an undersized unit runs non-stop and never cools, while an oversized one cools too fast, short-cycles, and leaves the air clammy.
How to use it
- Enter your room's floor area in square feet (length times width).
- Pick the sun exposure that best matches the room.
- Enter how many people are usually in the room at once.
- Choose whether the room is a kitchen.
- Read the BTUs needed, then match it to the suggested standard AC size.
These figures are practical estimates for typical homes, not a professional HVAC load calculation.
FAQ
- How do I measure a room's square footage?
- Multiply the room's length by its width in feet. For an L-shaped space, split it into rectangles, size each, and add them together.
- What if my number lands between two AC sizes?
- We round up to the next standard unit so you never fall short. A slightly larger unit is fine; going too big causes short-cycling and humidity problems.
- Is bigger always better for an air conditioner?
- No. An oversized AC cools the air quickly but shuts off before removing moisture, leaving the room cold and damp. Matching the size to the load is the goal.
- Does ceiling height matter?
- This estimate assumes standard 8-foot ceilings. For very tall or vaulted rooms, add roughly 10-20% more BTUs or ask an HVAC pro for a full load calculation.