What this calculator does
Indicated airspeed is what the airspeed indicator shows, based on the pressure of the air hitting the pitot tube. As altitude increases, the air becomes less dense, so the aircraft is actually moving faster through the air than the gauge suggests, even though the gauge reading stays the same for a given amount of lift. True airspeed is the actual speed of the aircraft through the surrounding air mass.
A widely used shortcut among pilots is that true airspeed runs roughly 2% higher than indicated airspeed for every 1,000 feet of altitude, on a standard-temperature day. It is quick enough to do in your head during flight planning or in the cockpit, which is exactly why it has stuck around despite being an approximation rather than an exact figure.
The formula
Multiply indicated airspeed by (1 plus 0.02 times the altitude in thousands of feet). At 8,000 feet, that is a multiplier of 1.16, or 16% above indicated airspeed.
| Term | Meaning |
|---|---|
| IAS | Indicated airspeed: the reading shown on the airspeed indicator. |
| TAS | True airspeed: the aircraft’s actual speed through the surrounding air mass. |
| Altitude | Height above sea level, in thousands of feet, used to scale the correction. |
The inputs explained
| Field | What to enter |
|---|---|
| Indicated airspeed (IAS) (knots) | The indicated airspeed reading from the airspeed indicator. |
| Altitude (ft) | The altitude at which that indicated airspeed was read. |
When to use it
Quick flight-planning estimates
Before a detailed flight plan is worked out, this gives a fast approximation of cruise true airspeed to sanity-check fuel and time figures.
Mental cross-check in the cockpit
A pilot can apply this rule of thumb mentally at any point in a flight to get a rough sense of true airspeed without reaching for an E6B or avionics readout.
Understanding why climbing changes your groundspeed
Even holding the same indicated airspeed throughout a climb, true airspeed (and therefore groundspeed, wind aside) increases steadily as altitude increases.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How true airspeed changes with altitude, at a fixed indicated airspeed
A fixed 120 kt IAS, across a range of altitudes.
| Altitude | Estimated true airspeed | Increase over IAS |
|---|---|---|
| 2,000 ft | 124.8 kt | 4.8 kt (4.0%) |
| 4,000 ft | 129.6 kt | 9.6 kt (8.0%) |
| 6,000 ft | 134.4 kt | 14.4 kt (12.0%) |
| 8,000 ft | 139.2 kt | 19.2 kt (16.0%) |
| 10,000 ft | 144.0 kt | 24.0 kt (20.0%) |
| 12,000 ft | 148.8 kt | 28.8 kt (24.0%) |
How true airspeed changes with indicated airspeed, at a fixed altitude
A fixed 8,000 ft altitude, across a range of indicated airspeeds.
| Indicated airspeed | Estimated true airspeed |
|---|---|
| 80 kt | 92.8 kt |
| 100 kt | 116.0 kt |
| 120 kt | 139.2 kt |
| 140 kt | 162.4 kt |
| 160 kt | 185.6 kt |
| 180 kt | 208.8 kt |
Questions
Why does this not ask for outside air temperature?
Because it is a simplified rule of thumb built around a standard-temperature day, not a full TAS computation. Actual air temperature shifts the true density altitude and therefore the real TAS correction, sometimes by a meaningful margin on unusually hot or cold days.
How accurate is the 2% per 1,000 ft rule?
Reasonably close on a standard-temperature day at moderate altitudes, which is why it remains in common use for quick estimates. It is not precise enough for performance planning that depends on an exact TAS figure.
What should I use instead for a precise TAS figure?
An E6B flight computer (physical or electronic), a dedicated TAS formula that incorporates actual outside air temperature and pressure altitude, or the TAS readout from onboard avionics if fitted.
Does this rule apply the same way at very high altitudes?
The same basic relationship holds, but the approximation was developed with typical light-aircraft cruise altitudes in mind; at high-altitude jet cruise levels, temperature deviation from standard becomes more significant and a proper TAS computation matters more.
To turn a true airspeed into an expected range, see the fuel burn endurance and range calculator. For how altitude and temperature together affect aircraft performance, see the density altitude calculator.