True Airspeed Calculator
Convert indicated airspeed to true airspeed using pressure altitude — the thinner the air, the faster you are really flying.
How it works
TAS ≈ IAS × (1 + 0.02 × altitude ÷ 1000)
An airspeed indicator measures dynamic pressure, not actual speed through the air. As you climb the air thins, so the same dynamic pressure corresponds to a higher true speed — the gap widens by roughly 2% of indicated airspeed for every 1,000 ft. This rule of thumb is accurate to about 10,000 ft; above that, or in unusual temperatures, use a flight computer that accounts for air density directly.
Worked example
Cruising at 8,000 ft pressure altitude showing 120 kt indicated: TAS ≈ 120 × (1 + 0.02 × 8) = 120 × 1.16 = 139 kt. The aircraft is moving through the air about 19 kt faster than the indicator shows.
Sources: True airspeed — Wikipedia
Frequently asked questions
Why is true airspeed higher than indicated?
The airspeed indicator is calibrated for sea-level air density. As you climb, the air is thinner, so the same measured pressure means you are actually moving faster — indicated airspeed under-reads true airspeed by about 2% per 1,000 ft.
Is true airspeed the same as groundspeed?
No. True airspeed is your speed relative to the surrounding air mass; groundspeed is TAS combined with the wind. A headwind lowers groundspeed below TAS, a tailwind raises it above.
How accurate is the 2% rule?
It is a good approximation up to roughly 10,000 ft in near-standard temperatures. For high altitudes or hot/cold days, compute density altitude first and use it in place of pressure altitude for a more exact figure.
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This calculator is for educational purposes. Double-check important results before acting on them.