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True Airspeed Calculator

Estimate true airspeed from IAS and altitude

Result

139.20

How it is calculated

The tool applies the 2-percent-per-1,000-feet rule: TAS = IAS × (1 + 0.02 × altitude / 1000). IAS is the indicated airspeed read from the instrument and altitude is the altitude in feet. The result has the same unit as the IAS you entered, so knots in gives knots out and the tool does no unit conversion. It shows two decimals.

The rule is linear and only uses altitude. It does not know about outside temperature, pressure setting, or instrument and position error, which are the things a flight computer would also use. The IAS must be above zero and the altitude cannot be negative; at 0 ft the answer equals the IAS. Treat the output as a planning estimate, not a figure for navigation that must be exact.

Worked example

An aircraft indicates 120 knots at 8,000 ft. The correction is 0.02 × 8,000 / 1,000 = 0.16, which is +16 percent. Multiply: 120 × (1 + 0.16) = 120 × 1.16 = 139.2. The tool shows 139.20, so the aircraft is actually moving through the air about 19 knots faster than the instrument shows. At sea level the same 120 would stay 120.00.

Try this example →

Common mistakes

  • Entering the altitude in thousands of feet (8 instead of 8000). The formula divides by 1,000 itself, so the field needs the full figure in feet.
  • Entering the altitude in metres. The 2 percent rule is defined per 1,000 ft, so convert metres to feet first.
  • Treating the result as groundspeed. TAS is speed through the air; wind still has to be added or subtracted.
  • Trusting it for high or hot-day work. The rule ignores temperature and grows less accurate as altitude increases, so use a flight computer when it matters.
  • Entering a calibrated or ground speed as the IAS. The input must be the reading from the airspeed indicator.

About this calculator

This calculator estimates true airspeed (TAS) from indicated airspeed (IAS) using the rule-of-thumb correction of roughly +2% per 1,000 ft of pressure altitude, since air density drops with altitude and an airspeed indicator — which measures dynamic pressure, not true speed — reads increasingly low relative to the aircraft's actual speed through the air as it climbs.

Pilots use this quick approximation for flight planning and cross-country navigation when precise density-altitude data isn't at hand, since TAS (not IAS) is what determines groundspeed, fuel burn over distance, and time en route; for exact work they switch to a flight computer or E6B that also accounts for temperature deviation from standard.

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Comments (1)

  • Editorial team

    The answer is speed through the air, not speed over the ground. For a flight plan you still have to combine it with the wind to get groundspeed, which this tool does not do. A quick sanity check: at 10,000 ft the correction is plus 20 percent, so 100 knots indicated becomes 120. Tell us whether you enter altitude in feet or would want metres supported.

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