What size air conditioner do you need?

What size air conditioner do I need for my room?

Multiply the room's length by its width and look the area up in ENERGY STAR's table: 150 to 250 square feet needs about 6,000 BTU per hour, 350 to 400 needs 9,000, 450 to 550 needs 12,000. A 12 by 15 ft bedroom is 180 square feet — a 6,000 BTU unit.

How do sun, people and a kitchen change the size?

ENERGY STAR adjusts the table: take off 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU for every person beyond two who regularly uses the room, and add 4,000 BTU if it is a kitchen. The same 180 square foot room, sunny and used by four people, needs 6,000 × 1.1 + 1,200 = 7,800 BTU.

Is a bigger air conditioner better?

No. An oversized unit cools the air so fast that it shuts off before it has pulled much moisture out, leaving the room cold and clammy, and it wastes energy by cycling on and off. Pick the nearest size at or above the result.

How many BTUs is a ton of air conditioning?

12,000 BTU per hour — the cooling from melting a ton of ice in a day. A 24,000 BTU unit is 2 tons. In watts, 12,000 BTU per hour is about 3.5 kW of cooling.

What about a whole house?

Room units top out around 2,500 square feet in the ENERGY STAR table. For central air or a heat pump for a whole home, a contractor should size it with a Manual J load calculation, which accounts for insulation, windows and your climate.

Does the calculator use ceiling height?

No. The ENERGY STAR table is based on the floor area, with corrections for sun, people and a kitchen. A room with an unusually high ceiling, a lot of glass or poor insulation can need more cooling than the table gives, and a Manual J load calculation will account for those.

How do I convert BTU per hour to watts?

One BTU per hour is 0.29307 watts. A 12,000 BTU/h air conditioner removes heat at about 3.52 kW. That is cooling power, not electric power: the unit draws less electricity than that because it moves heat rather than making it.

What if the room is open to another room?

Add the floor areas of the connected spaces, because the air conditioner has to cool both. If you only cool one room with the doorway closed, use that room's area.

How the calculation works

The calculator multiplies the room's length by its width to get the floor area in square feet and finds the row of the ENERGY STAR sizing table that includes that area. The table gives the cooling capacity in BTU per hour. Then it applies the ENERGY STAR corrections: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU more for each person beyond two who regularly uses the room, and 4,000 BTU more for a kitchen. The result is the capacity to look for; choose the unit whose rating is the nearest at or above it.

Worked example

A living room is 320 square feet, gets a lot of sun and is used by 4 people. The table gives 8,000 BTU per hour for 320 square feet. The sun correction is 8,000 × 1.1 = 8,800. Two people beyond the first two add 2 × 600 = 1,200, for 10,000 BTU per hour in all. A 200 square foot kitchen with the normal two people needs 6,000 + 4,000 = 10,000 BTU per hour. A shaded 180 square foot bedroom needs 6,000 × 0.9 = 5,400.

Common mistakes

• Sizing for the largest room in the house. The unit should match the room it will cool. A bigger unit than needed cycles on and off too quickly.
• Forgetting the kitchen allowance. Cooking adds heat, which is why the table adds 4,000 BTU for a kitchen.
• Mixing up BTU and tons. One ton of cooling is 12,000 BTU per hour. A 1.5-ton unit is 18,000 BTU per hour, not 1,500.

Sources and related calculators

Sizing table and corrections: ENERGY STAR guidance on sizing a room air conditioner. Unit conversion: 1 BTU/h = 0.29307107 W (International Table BTU). Related: CFM calculator, attic ventilation calculator and ceiling fan size calculator.