True airspeed (TAS) As a general rule of thumb, the TAS is about 2% higher than IAS for every 1,000 feet (300 m) above sea level. At a typical cruising altitude of 30,000 ft (9,000 m), the TAS is roughly 30% greater than what the cockpit's airspeed indicator displays.

The "rule of thumb" for calculating True Airspeed (TAS) from Indicated Airspeed (IAS) is a quick mental math shortcut used by pilots to estimate their actual speed through the air as they climb to higher altitudes where the air is thinner. The general rule is to add 2% to your Indicated Airspeed for every 1,000 feet of altitude above sea level. For example, if you are flying at 10,000 feet with an Indicated Airspeed of 100 knots, you would add 20% (10×2%) to that speed, giving you an estimated True Airspeed of 120 knots. This adjustment is necessary because an airspeed indicator works by measuring pressure (pitot-static), and as air density decreases with altitude, the instrument "under-reads" the actual speed the aircraft is moving. While this rule is remarkably accurate for standard atmospheric conditions at lower to mid-altitudes, it becomes less precise at very high altitudes or in extreme temperatures. Pilots use TAS for navigation and flight planning because it represents the actual speed of the aircraft relative to the air mass, which, when combined with wind speed and direction, allows them to calculate "Ground Speed," the most critical metric for determining arrival times and fuel consumption during a cross-country flight.