BTU Calculator

Find the right size window or room air conditioner, in BTU per hour, for a room of 100 to 1,000 square feet. The calculator uses the ENERGY STAR sizing chart and adjusts for sun, shade, the number of people and kitchens. Your inputs stay in your browser.

Your room

The room

Room size

100 to 1,000 square feet.

Sun

10,000 BTU per hour for 320 sq ft. See full results

Example: a 20 × 16 ft room that is very sunny and used by 4 people. Enter your room to update.

Your air conditioner size

Cooling capacity
10,000 BTU per hour
Choose a unit rated at or close to this
Room area
320 sq ft
20 × 16 ft
In tons of cooling
0.83 tons
1 ton = 12,000 BTU per hour

How the size was worked out

StepBTU per hour
Chart capacity for 300–350 sq ft 8,000 BTU
Very sunny: +10% +800 BTU
2 people beyond two, 600 BTU each +1,200 BTU
Not a kitchen 0 BTU
Recommended capacity 10,000 BTU

Based on the ENERGY STAR room air conditioner sizing chart for a room with ordinary ceilings and insulation. Unusual rooms, such as high ceilings, large windows or poor insulation, can need more.

How to size a room air conditioner

  1. Measure the room and multiply length by width for its area in square feet. The example is 20 × 16 = 320 square feet.
  2. Find the capacity for that area in the chart below: 8,000 BTU per hour for the example.
  3. Adjust for the room (ENERGY STAR: choosing the right size): 10% less if it is heavily shaded, 10% more if it is very sunny, 600 BTU for each person beyond two who regularly uses it, and 4,000 BTU if it is a kitchen.

The example is very sunny (+800 BTU) and used by 4 people (+1,200 BTU), so it needs about 10,000 BTU per hour. A shaded 500 sq ft room needs 10,800 BTU, and a 200 sq ft kitchen 10,000 BTU.

Air conditioner size by room area

Cooling capacity for an average room, before adjustments

Area to be cooledCapacity needed
100 to 150 sq ft 5,000 BTU per hour
150 to 250 sq ft 6,000 BTU per hour
250 to 300 sq ft 7,000 BTU per hour
300 to 350 sq ft 8,000 BTU per hour
350 to 400 sq ft 9,000 BTU per hour
400 to 450 sq ft 10,000 BTU per hour
450 to 550 sq ft 12,000 BTU per hour
550 to 700 sq ft 14,000 BTU per hour
700 to 1,000 sq ft 18,000 BTU per hour

Air conditioners come in set sizes. If your result falls between two models, the closer one is usually the better choice; the adjustments above matter more than a few hundred BTU either way.

Why the right size matters

Bigger is not better. A unit that is much too large cools the air quickly and shuts off before it has removed much humidity, so the room can feel cold and clammy, and it costs more to buy. One that is too small runs almost constantly and may not keep up on the hottest days.

BTU, tons and watts

  • A BTU (British thermal unit) is a unit of heat. Air conditioner capacity is the heat it removes per hour, in BTU per hour, often written just “BTU”.
  • A “ton” of cooling is 12,000 BTU per hour, the unit used for central air conditioners and heat pumps. The example is 0.83 tons.
  • 1 BTU per hour is about 0.293 watts of cooling, so 10,000 BTU per hour is about 2,931 watts. That is cooling output, not the electricity the unit uses, which is much less.

Assumptions and limits

  • The chart assumes a room with ordinary ceilings (about 8 feet), insulation and windows. High ceilings, large or west-facing windows, poor insulation or an attic room above can need more capacity.
  • It covers single rooms of 100 to 1,000 square feet. Central air conditioning for a whole house should be sized with a room-by-room load calculation (ACCA Manual J) by a heating and cooling contractor.
  • Portable air conditioners are rated under a different test and often list a lower capacity for the same unit; compare like with like.
  • Heating is not included.

Methodology and testing

Capacity = chart capacity for the area × (1 − 10% if heavily shaded, or + 10% if very sunny) + 600 BTU per person beyond two + 4,000 BTU for a kitchen. Automated tests check every chart row and boundary, each adjustment on its own and together, and that areas outside the chart are rejected. How we build and test calculators.

Sources

  • How to choose the right sized window AC, ENERGY STAR (U.S. Environmental Protection Agency and Department of Energy). Room air conditioner capacity by area to be cooled (100 to 1,000 square feet), and the adjustments: 10% less for a heavily shaded room, 10% more for a very sunny room, 600 BTU per person beyond two, and 4,000 BTU for a kitchen.
  • Square Footage Calculator

    Square feet of a rectangle, circle, triangle or trapezoid in any unit, with square yards, square meters, acres and cost.

  • Home Equity Loan Calculator

    Monthly payment, borrowing limit at common CLTV caps, cash received after closing costs, term comparison and amortization.

  • Roofing Calculator

    Roof area from footprint and pitch, roofing squares, waste, and shingle bundles with cost.