How to Measure the Running Time of an Air Conditioner

The constant cycling on and off of an air conditioner compressor can become alarming if you think it’s happening too often, and if you think it's running for too long or too short a time.
An air conditioner that's too powerful for the home won't run efficiently.An air conditioner that's too powerful for the home won't run efficiently.
Air conditioning that runs for very short amounts of time is likely too powerful for the space it’s in, cooling everything very quickly. This sounds like an advantage, but it doesn’t give the dehumidifying aspect of the air conditioning a chance to work, making the room feel damp. Measuring how long your air conditioner is running is almost like timing anything else; you just have to repeat it a few times.

Step 1

Set the air conditioner’s thermostat to more than 20 degrees Fahrenheit below the forecast outdoor temperature.

Step 2

Place the thermometer outside if it is not already there. If you have a thermometer indoors that’s wired to show indoor and outdoor temperatures, ensure it is on.

Step 3

Set the stopwatch to zero and confirm you know how to start it. In some cases, you have to press down on a top button, but other models may be different. Wait for the air conditioning to turn on.

Step 4

Start the stopwatch right as the air conditioning turns on. If you are using an analog stopwatch for which a full circle of the hand equals one minute, watch carefully and count how many minutes pass with the air conditioning turned on. Prepare to stop the watch.

Step 5

Stop the watch immediately when the air conditioning turns off and write down the time.

Step 6

Check the outside temperature reading, and write that down along with the date and time.

Step 7

Repeat this a few times throughout the day. Repeat the next day if the temperature is supposed to change. Air conditioners typically cool a room by 20 degrees Fahrenheit when compared to the outside temperature unless you set the thermostat for a warmer temperature. Recording the time and temperature at different points in the day shows you if the air conditioner is actually cooling things down as much as it should.

Things You Will Need

  • Stopwatch
  • Thermometer


  • Tim Carter of Ask the Builder writes that a properly working air conditioner should run for 10 to 15 minutes, so if yours is running a for a different length of time, get the air conditioning serviced.
  • If the air conditioning is running for a longer time, the unit could be too small for the space and would have trouble cooling it effectively.
  • The EPA notes air conditioners don’t reach their greatest level of efficiency if they run for only a few minutes. They must run for at least nine minutes to reach their most efficient state.

About the Author

Suzanne S. Wiley is an editor and writer in Southern California. She has been editing since 1989 and began writing in 2009. Wiley received her master's degree from the University of Texas and her work appears on various websites.