True Airspeed Calculator
Enter your indicated airspeed and pressure altitude to estimate true airspeed using the common 2 percent per 1000 feet rule of thumb. True airspeed grows with altitude because the air is thinner, so the aircraft moves faster through it than the airspeed indicator shows.
How to estimate true airspeed
- Enter your indicated airspeed in knots.
- Enter your pressure altitude in feet.
- Read the estimated true airspeed and the percent added.
Examples
Light aircraft at 5000 ft
IAS 100 kt, 5000 ft
TAS 110 kt (+10%)
Cruise at 8000 ft
IAS 120 kt, 8000 ft
TAS 139.2 kt (+16%)
Frequently asked questions
What is the difference between indicated and true airspeed?
Indicated airspeed is what the airspeed indicator reads from dynamic pressure. True airspeed is your actual speed through the air mass, which is higher at altitude because the air is less dense.
What formula does this calculator use?
It uses the rule of thumb true airspeed = indicated airspeed times (1 + 0.02 times altitude in thousands of feet), which adds about 2 percent per 1000 feet.
How accurate is the 2 percent per 1000 feet rule?
It is a handy cockpit approximation. Real true airspeed depends on air density, so temperature and pressure both matter. For precise numbers use a flight computer or an E6B that takes temperature.
Does temperature change true airspeed?
Yes. Warmer than standard air is less dense and raises true airspeed, while colder air lowers it. This simple rule assumes a standard atmosphere and ignores temperature.
What altitude should I enter?
Use pressure altitude, the value shown when the altimeter is set to 29.92 inHg. It removes local pressure variation so the estimate stays consistent.
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