How to Calculate Indicated Airspeed

Indicated airspeed is not actual airspeed but a calculation based on several factors.
Indicated airspeed is not actual airspeed but a calculation based on several factors. (Image: Thinkstock Images/Comstock/Getty Images)

Indicated air speed is the speed at which a plane is traveling as indicated by the pilot's instruments. The instrument which measures flight speed is called the air speed indicator. Variants in indicated airspeed versus calculated or true airspeed are typically caused by imperfections in the manufacturing and/or installation processes as well as changes in altitude or temperature. Most pilots use on-board flight computers or airspeed calculation charts to learn their true speed, but if you wish to find your indicated airspeed you may.

Locate your airspeed indicator on the instrument panel of the airplane cockpit. The airspeed indicator is a round dial with pin markings and numbers which increase in a clockwise direction. The airspeed is indicated in knots per hour.

Read your airspeed indicator display to learn your indicated airspeed. If you wish to learn your indicated airspeed without the use of your airspeed indicator, you can perform a mathematical equation to do so.

Calculate your True Airspeed by entering your Calculated Airspeed, outside temperature and pressure altitude into the on-board flight computer. If you do not have an on-board flight computer, you may also use an online true airspeed calculator.

Subtract 2 percent from your true airspeed to obtain your calculated airspeed. Calculated airspeed is regarded as equivalent to indicated airspeed.

Tips & Warnings

  • Calculate your Calibrated Airspeed. Many modern airplanes will provide a calculated airspeed as determined by the on-board flight computer. The calculated airspeed takes into account mechanical errors and any inconsistencies caused by the installation or manufacturing of the speed measuring equipment.
  • Indicated airspeed is based on air pressure readings and may not be completely accurate.

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