BTU Calculator — What Size AC Do I Need?
Buy the right size air conditioner. Enter the room size and conditions to get the recommended cooling capacity in BTU per hour, based on ENERGY STAR sizing guidance.
Recommended capacity
How to size an air conditioner
- Measure the room's length and width. For open-plan spaces, include every area the unit needs to cool.
- Choose the sun exposure and enter how many people normally use the room.
- Tick the kitchen box if appliances add heat. The calculator shows the recommended BTU per hour; choose a unit at or just above it.
Why AC size matters
An undersized air conditioner runs constantly and never reaches the set temperature. An oversized one is a less obvious problem: it cools the room quickly and shuts off before removing much humidity, leaving the room cold and clammy, and its frequent on-off cycling wastes energy and wears the unit out.
The base capacity comes from the ENERGY STAR room air conditioner sizing chart, which assigns a BTU rating to each range of floor area (for example 6,000 BTU for 150–250 sq ft). It is then adjusted: reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny one, add 600 BTU for each regular occupant beyond two, and add 4,000 BTU for a kitchen.
These guidelines assume standard 8-foot ceilings and average insulation. Rooms with high ceilings, poor insulation or lots of west-facing glass may need more. For central air or heat pumps covering a whole house, a contractor should perform a Manual J load calculation.
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Frequently asked questions
How many BTU do I need per square foot?
Roughly 20 BTU per square foot for a typical room, though the ENERGY STAR chart varies by size range.
What size AC for a 300 sq ft room?
About 7,000 BTU under normal conditions; 8,000 if it is very sunny or used by more than two people.
What is a ton of cooling?
12,000 BTU per hour. A 24,000 BTU unit is a 2-ton unit.
Is bigger better?
No. An oversized unit short-cycles and leaves humidity behind. Choose the size that matches the room.