Airspeed Tool
True Airspeed Calculator (TAS)
Convert between indicated airspeed, true airspeed, and Mach number at any altitude and temperature using ICAO standard atmosphere formulas.
Input mode
kts
ft
°C
ISA: -54.3°C at this altitude
Results
IAS280kts
TAS503kts
Mach0.872
Speed of sound576kts
IAS to TAS Rule of Thumb
For a quick mental estimate, add 2% to your indicated airspeed for every 1,000 ft of altitude. At 10,000 ft that turns 150 kts IAS into about 180 kts TAS, against an exact 175 kts. The rule runs a few knots high below FL200 and falls behind above FL300: at FL350 it is about 25 kts short at 250 kts IAS. Use it to sanity-check a number, and the calculator above when the number matters.
IAS to TAS at Standard Altitudes
| Altitude | 100 kts IAS | 150 kts IAS | 250 kts IAS |
|---|---|---|---|
| 5,000 ft | 108 (110) | 162 (165) | 269 (275) |
| 10,000 ft | 116 (120) | 175 (180) | 291 (300) |
| 20,000 ft | 137 (140) | 205 (210) | 342 (350) |
| 30,000 ft | 163 (160) | 245 (240) | 409 (400) |
| FL350 | 180 (170) | 269 (255) | 449 (425) |
True airspeed in knots at ISA temperature, from the same standard atmosphere the calculator uses. Figures in brackets are the 2% per 1,000 ft estimate.