How powerful an air conditioner do you need?

This worksheet, adapted from a printed version published by the Association of Home Appliance Manufacturers for the continental United States, can help you estimate how much cooling capacity you need.

You'll need:

  • a tape measure
  • a scratch paper
  • a pocket calculator

The worksheet guides you through the measurements needed to calculate the size of the air conditioner, and then it automatically calculates the final answer for you. We suggest that you print out the worksheet first, fill it out, then enter the information online.

Make the measurements listed.

For each dimension or area, round it to the nearest whole number before entering it in the appropriate box.


This worksheet is to be used only for the area to be cooled by a single air conditioner.

If the room is connected to another by a permanently open door or archway more than 5 feet wide, consider the two rooms as one area and make all the necessary measurements in both rooms.

1. Geographical location
  Select the city nearest to your location with a similar climate (continental US only).
2. Walls
  Measure the length of all walls, in feet. Add up three separate lengths:


Outside walls not exposed to direct sunlight
(facing north and/or shaded by adjacent buildings)


Outside walls exposed to direct sunlight
(facing other directions and unshaded)


Interior walls

Enter the lengths in the three boxes below, and select the appropriate wall frame type.

Outside walls facing north, and/or shaded

Total wall length
  feet sq.

Outside walls facing other directions, unshaded

  feet sq.

Interior walls

  feet sq.
  Wall frame construction


(Note: For wall frame construction there are two choices: uninsulated or masonry up to 8 inches thick, insulated or masonry more than 8 inches thick.)
3. Ceiling
  Determine the ceiling height, in feet. Enter that figure in the box.
(Note: For a sloped ceiling, determine the average height
between the highest and lowest points.)

Ceiling height
  feet sq.

  Determine the ceiling area (length x width), in square feet. Enter that figure in the box, and select the appropriate category for insulation above the ceiling.  

Ceiling area
  sq. feet

  Insulation above ceiling


(Note: For insulation above ceiling, the choices are: uninsulated, no space above; uninsulated, attic above; insulated, no space above; insulated, attic above; occupied space above.)
4. Floor
  If the floor is on ground or over a permanent basement, skip this step and move on to step 5. Otherwise, determine the floor area (length x width) in feet, enter it in the box. The floor area is usually the same as the ceiling area.  

Floor area
  sq. feet

5. Doors and arches
  If the room is connected to another by a permanently open door or archway more than 5 feet wide you should consider it as one room; skip this step and go on to step 6. Otherwise, enter the width of the door or archway into the box.  

Total width
  feet sq.

6. Windows
  Calculate the area (height x width) of each window. Take the measurements in inches, then divide by 144 to determine the square footage. Jot down the area of each window for use below. Add up the total window area for each wall, enter it in the appropriate box and select the shade and glass type that best describes the windows on each wall.  
(Note: For shades the choices are: none, inside, and awnings. For overhead eaves, use shade type "awnings." Shades are not applicable for North walls. For glass, the choices are: single, double, and triple. For storm windows, use glass type "double." For low-e or block windows, use triple.)

Wall orientation

Adjustment factors




Window area
  sq. feet


      sq. feet


      sq. feet


      sq. feet


      sq. feet


      sq. feet


      sq. feet


      sq. feet
7. Heat from people
  Enter the number of people who normally use the room (use a minimum of 2).

People in room
  (min. 2)

8. Kitchen
  If the room includes your kitchen appliances, check the checkbox below and skip step 9.  

Room includes kitchen

9. Heat from lights and appliances
  If this area includes the kitchen, do not enter this number and check the checkbox in step 8 instead. Otherwise, add up the wattage of all lights and appliances in the room, not including the air conditioner itself, and enter the total here.  

Total wattage of non-kitchen appliances

Total wattage

(See a chart of typical wattage of household products.)

10. Hours of operation
  Select how you would typically use the air conditioner in the given area. 

For use at any time of day


For use at night only

The number calculated tells how much heat builds up in the room each hour. The air conditioner's cooling capacity (BTU per hour) should nearly match the resulting number. A difference of about 5 percent between the number calculated and the air conditioner capacity shouldn't be significant.