BTU Calculator for Room Air Conditioners

Find the cooling capacity, in BTU per hour, a room air conditioner needs for your room, using ENERGY STAR's sizing table and adjustments.

Cooling capacity needed
7,000 BTU/hr
Look for a unit of about
7,000 BTU/hrThe smallest ENERGY STAR table size at or above your total
In watts of cooling
2,051 W
In tons of cooling
0.58
Room area
280 sq ft
  • Based on ENERGY STAR's room air conditioner sizing table and adjustments. It is a rule of thumb: insulation, windows, climate and the room's layout all matter, and a professional load calculation is the way to size a central system.
  • Bigger is not better. An oversized unit cools the air quickly but shuts off before removing much humidity, leaving the room cold and clammy.

How the number was built

StepBTU/hr
Base for 280 sq ft7,000
Total7,000

How room AC sizing works

Air conditioner capacity is measured in BTU per hour: how much heat the unit can remove from the room each hour. ENERGY STAR publishes a sizing table for room air conditioners that starts at 5,000 BTU for 100 to 150 sq ft and rises to 34,000 BTU for 2,000 to 2,500 sq ft. Its capacities assume an 8-foot ceiling.

The table is then adjusted for the room. ENERGY STAR says to reduce capacity by 10% if the room is heavily shaded, increase it by 10% if it is very sunny, add 600 BTU for each person beyond two who regularly uses the room, and add 4,000 BTU if the unit will cool a kitchen. This calculator applies those steps in that order and shows each one.

Why not just buy the biggest unit?

An air conditioner also dehumidifies, and it needs to run for a while to do that. An oversized unit hits the thermostat setting quickly and cycles off, so the room feels cold but damp. An undersized unit runs constantly and never quite catches up on hot afternoons. Matching the size to the room gives steadier comfort and usually lower running costs.

Check the electrical side too. Larger window units may need a dedicated circuit or a 240 V outlet; the product's manual lists its requirements.

Rooms that need special care

High ceilings, open-plan spaces that connect to other rooms, top-floor rooms under a hot roof and rooms with large west-facing windows all need more cooling than floor area alone suggests. For ceilings above 8 feet this calculator scales the base figure by room volume, which is a rough approximation of our own rather than part of ENERGY STAR's guidance.

For a whole house or a central system, contractors size equipment with a detailed load calculation that accounts for insulation, windows, air leakage and local climate. Use this tool for single rooms and window or portable units.

The formula

Base = ENERGY STAR table value for the room areaAdjusted = Base × (H ÷ 8 when H > 8)Adjusted = Adjusted × 0.9 (shaded) or × 1.1 (very sunny)Total = Adjusted + 600 × max(0, P − 2) + 4,000 × kitchen
  • H = ceiling height in feet (the height step is our approximation)
  • P = number of people who regularly use the room
  • kitchen = 1 if the room is a kitchen, otherwise 0
  • 1 BTU/hr = 0.293 W; 12,000 BTU/hr = 1 ton of cooling

Example: a sunny 14 × 20 ft living room used by three people

  1. Area: 14 × 20 = 280 sq ft. The ENERGY STAR table gives 7,000 BTU for 250 up to 300 sq ft.
  2. Very sunny, so add 10%: 7,000 × 1.10 = 7,700 BTU.
  3. One person more than two: 7,700 + 600 = 8,300 BTU.
  4. The next size up in the table is 9,000 BTU, so look for a unit of about 9,000 BTU (8,300 BTU is about 2,432 W of cooling).

Frequently asked questions

What size air conditioner do I need for a 12x12 room?

A 12 × 12 room is 144 sq ft, which falls in ENERGY STAR's 100 up to 150 sq ft row: 5,000 BTU, before adjustments for sun, people or a kitchen.

How many square feet does a 10,000 BTU air conditioner cool?

In ENERGY STAR's table, 10,000 BTU is listed for 400 up to 450 sq ft with 8-foot ceilings and average conditions.

How many BTU do I need for a kitchen?

ENERGY STAR says to add 4,000 BTU if the unit is used in a kitchen. A 200 sq ft kitchen starts at 6,000 BTU and becomes 10,000 BTU.

How many BTU is one ton of air conditioning?

One ton of cooling is 12,000 BTU per hour, so an 18,000 BTU unit is 1.5 tons.

Does ceiling height matter?

Yes. ENERGY STAR's capacities assume 8-foot ceilings. A taller room holds more air and usually needs more capacity; this calculator scales by volume as an approximation.

Sources

Last reviewed for 2026. How we calculate.