What you're actually doing through the air.
Your airspeed indicator measures dynamic pressure, not speed — in thin air it under-reads. True airspeed corrects indicated (calibrated) airspeed for the air density at your altitude and temperature: TAS = CAS ÷ √σ. Enter your numbers to get TAS, the density altitude doing the work, and your Mach number.
Uses pressure altitude (29.92 set — or compute it with the pressure altitude calculator) and outside air temperature. IAS is assumed equal to CAS — close for most cruise regimes.
Educational estimates only — not for flight planning or dispatch. Fly your POH numbers.
TAS = CAS ÷ √σ, where σ is the density ratio — the air density at your density altitude divided by sea-level density. The airspeed indicator is a pressure gauge calibrated for sea-level air; at altitude the same true speed produces less dynamic pressure, so the needle reads low. Dividing by √σ undoes that.
The rule of thumb: add roughly 2% per 1,000 ft of altitude. At 8,000 ft, 120 KIAS ≈ 120 × 1.16 ≈ 139 KTAS. The exact math usually lands a few knots lower than the rule — both are shown above so you can see the spread.
Why it matters: flight planning runs on TAS (groundspeed = TAS ± wind), cruise tables quote TAS, and the gap between IAS and TAS is exactly why "high density altitude approaches feel fast" — your groundspeed is higher at the same indicated speed. That's also the link to density altitude's effect on takeoff and landing.
| Pressure altitude (ISA) | 120 KIAS ≈ TAS | σ (density ratio) |
|---|---|---|
| Sea level | 120 kt | 1.000 |
| 4,000 ft | 127 kt | 0.888 |
| 8,000 ft | 135 kt | 0.786 |
| 12,000 ft | 144 kt | 0.693 |
Divide calibrated airspeed by the square root of the density ratio at your density altitude: TAS = CAS ÷ √σ. Or use the rule of thumb — add about 2% per 1,000 ft of altitude. 120 KIAS at 8,000 ft is roughly 135–139 KTAS.
The airspeed indicator measures dynamic pressure, which depends on both speed and air density. Thinner air produces less pressure at the same true speed, so the instrument under-reads — and the gap grows with altitude and temperature.
No. TAS is your speed through the air mass; groundspeed is TAS corrected for wind — a 20-kt headwind turns 140 KTAS into 120 kt over the ground. GPS shows groundspeed, never TAS.
Yes. Warmer than ISA means thinner air (higher density altitude), so the same indicated airspeed converts to a higher TAS. That's why this calculator asks for OAT rather than assuming a standard day.
DensityAlt is a set of free, no-signup aviation calculators. Explore the rest: