Air Conditioner BTU Calculator: What Size AC Do I Need?
Room air conditioners need roughly 20 BTU per square foot. A 12 × 15 ft room (180 sq ft) needs about 6,000 BTU by the ENERGY STAR chart, or 6,600 BTU if it's very sunny.
Floor area: 180 sq ft
Sunlight
Recommended size
6,000 BTU
In tons
0.5 tons
Buy the nearest size at or above this. Much bigger isn't better: an oversized unit cools in short bursts and leaves the room clammy.
Show the math
- 1
Base size from the ENERGY STAR chart
180 sq ft → up to 250 sq ft row → 6,000 BTU/h
Roughly 20 BTU per square foot for a typical room with 8-foot ceilings.
= 6,000 BTU
Read the steps as text
- Base size from the ENERGY STAR chart. 180 sq ft → up to 250 sq ft row → 6,000 BTU/h Roughly 20 BTU per square foot for a typical room with 8-foot ceilings.
- In tons of cooling. 6,000 ÷ 12,000 = 0.5 tons One ton of cooling is 12,000 BTU per hour, the heat needed to melt a ton of ice in a day.
Sizing from the ENERGY STAR chart
ENERGY STAR publishes a sizing chart for room air conditioners that matches floor area to cooling capacity in BTU per hour, from 5,000 BTU for up to 150 sq ft to 34,000 BTU for up to 2,500 sq ft. The calculator finds your room's row, then applies ENERGY STAR's adjustments.
Those adjustments are: subtract 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU for each person beyond two who regularly uses the room, and add 4,000 BTU for a kitchen. The chart assumes standard 8-foot ceilings, so rooms with high ceilings or poor insulation may need more.
Why bigger isn't better
An oversized air conditioner cools the room quickly and then switches off before it has removed much humidity, leaving the air cold and clammy. It also cycles on and off more, which wastes energy and wears the unit. An undersized one runs constantly and may never reach the set temperature.
Cooling capacity is also given in tons: one ton is 12,000 BTU per hour. Whole-home central air systems should be sized by a professional using a full load calculation (ACCA Manual J), which accounts for insulation, windows, climate and ductwork.
Frequently asked questions
- What size air conditioner do I need for 400 square feet?
- About 9,000 BTU by the ENERGY STAR chart (the 350–400 sq ft row), plus any adjustments for sun, extra people or a kitchen.
- How many square feet does a 12,000 BTU unit cool?
- Up to about 550 sq ft in typical conditions, per the ENERGY STAR chart's 450–550 sq ft row.
- What does BTU mean?
- British thermal unit, the heat needed to raise one pound of water by 1 °F. An AC's BTU rating is how much heat it can remove per hour.
- Does this work for heating?
- Heating needs depend heavily on climate and insulation, so use this as a rough guide only. A heat pump's heating capacity is usually listed separately from its cooling capacity.