How many BTU do I need?
You need the BTU per hour the ENERGY STAR chart lists for your room's area, then 10% less for a heavily shaded room or 10% more for a very sunny one, 600 BTU more for each regular occupant beyond two, and 4,000 BTU more if the unit cools a kitchen.[estar_room] The full chart:
| Area (sq ft) | Capacity (BTU/h) | Tons |
|---|---|---|
| 100 to 150 | 5,000 | 0.4 |
| 150 to 250 | 6,000 | 0.5 |
| 250 to 300 | 7,000 | 0.6 |
| 300 to 350 | 8,000 | 0.7 |
| 350 to 400 | 9,000 | 0.8 |
| 400 to 450 | 10,000 | 0.8 |
| 450 to 550 | 12,000 | 1.0 |
| 550 to 700 | 14,000 | 1.2 |
| 700 to 1,000 | 18,000 | 1.5 |
| 1,000 to 1,200 | 21,000 | 1.8 |
| 1,200 to 1,400 | 23,000 | 1.9 |
| 1,400 to 1,500 | 24,000 | 2.0 |
| 1,500 to 2,000 | 30,000 | 2.5 |
| 2,000 to 2,500 | 34,000 | 2.8 |
The chart assumes an 8-foot ceiling; rooms with higher ceilings may need more capacity.[estar_room] Each row runs from its first area up to (not including) the next row's start, so 250 sq ft uses the 250–300 row. Below 100 sq ft the smallest units (5,000 BTU) are the floor. Above 2,500 sq ft the chart ends, because a space that large needs a whole-house load calculation.
BTU per square foot
The chart works out to roughly 20 to 25 BTU per square foot in the middle of its range (12,000 BTU for 450–550 sq ft is 22–27 BTU per sq ft; 18,000 BTU for 700–1,000 sq ft is 18–26). Small rooms get more BTU per square foot because the smallest unit is 5,000 BTU, and large open areas get fewer.[estar_room] A single "BTU per square foot" rule ignores sun, ceiling height, insulation and windows, which is why the chart adds adjustments and why whole houses need a load calculation such as the HVAC size calculator.
Worked example
A 20 × 20 ft living room is 400 sq ft. The chart's rows run from one area up to the next, so 400 sq ft starts the 400–450 sq ft row: 10,000 BTU. It faces west and gets afternoon sun, so add 10%: 11,000 BTU. Three people usually use it, one more than two, so add 600: 11,600 BTU per hour. Choose a unit rated at or just above that capacity.
Why oversizing is a mistake
Bigger is not better. The Department of Energy lists oversizing among the installation problems that cause efficiency losses and occupant discomfort, and warns against rule-of-thumb sizing.[femp][doe_guide] An oversized unit also short-cycles: it reaches the thermostat setting quickly and shuts off before it has run long enough to pull much moisture from the air. An undersized unit runs constantly and never reaches the set temperature on hot days. Sizing to the chart, with honest adjustments, avoids both.
What a room air conditioner costs to run
Running cost is watts × hours × price. A 10,000 BTU unit with a combined energy efficiency ratio (CEER) of 11 draws about 909 watts; eight hours a day for 30 days uses 218 kWh, about $39.36 a month at the U.S. average of 18.04¢ per kWh.[eia861m] Put your unit's watts in the kWh calculator and your state's rate from electricity rates by state. For cooling a whole house with a heat pump instead, see heat pump cost.
Frequently asked questions
How many BTU do I need for a 12 × 12 room?
144 sq ft is in the 100–150 sq ft row: 5,000 BTU, before sun, people or kitchen adjustments.
How many BTU for 500 square feet?
12,000 BTU (450–550 sq ft row), or 13,200 BTU if the room is very sunny.
How many square feet does a 10,000 BTU air conditioner cool?
400 up to 450 sq ft by the ENERGY STAR chart, before adjustments.
How many BTU is a ton of cooling?
12,000 BTU per hour. A 24,000 BTU unit is 2 tons.
Sources
- Room Air Conditioners: sizing chart (capacity in Btu per hour by area, based on an 8-foot ceiling) and adjustments
- Purchasing Energy-Efficient Residential Air-Source Heat Pumps (cold-climate models; oversizing causes efficiency losses and discomfort)
- Energy Efficient Heating & Cooling Systems guide ("Insist that your contractor use ACCA Manual J"; rules of thumb ignore climate and insulation)
- Monthly Electric Utility Sales and Revenue Data (Form EIA-861M), by state and sector, data through August 2026 (sales_revenue.xlsx)