What this calculator does
Ground speed is how fast an aircraft actually moves over the ground, which is not the same as its airspeed through the surrounding air. A headwind slows ground speed below airspeed, a tailwind pushes it above airspeed, and a crosswind pushes the aircraft sideways without changing speed along the flight path by much on its own. This calculator combines true airspeed with a wind speed and direction to find the resulting ground speed.
The wind direction is entered relative to the aircraft heading, not a compass bearing: 0° means the wind blows straight from ahead (a pure headwind), 180° means it blows from directly behind (a pure tailwind), and 90° or 270° gives a pure crosswind with no headwind or tailwind component at all.
The formula
The wind is split into two components using the angle between the wind and the aircraft heading: a headwind/tailwind component along the direction of travel, and a crosswind component at right angles to it. True airspeed combines with the along-track component directly, and the crosswind component is then added using the Pythagorean relationship (vector addition) to get the resulting ground speed and drift angle.
| Term | Meaning |
|---|---|
| True airspeed (TAS) | The aircraft's speed through the air mass itself, unaffected by wind. |
| Ground speed | Actual speed over the ground, after the wind's effect is combined with true airspeed. |
| Drift angle | The angle between the aircraft's heading and its actual track over the ground, caused by a crosswind component. |
The inputs explained
| Field | What to enter |
|---|---|
| True airspeed (kt) | True airspeed, the speed shown on an airspeed indicator corrected for altitude and temperature. |
| Wind speed (kt) | Wind speed at altitude. |
| Wind direction relative to heading (0° = headwind, 180° = tailwind) (°) | The wind's direction relative to the nose of the aircraft: 0° is a straight headwind, 180° is a straight tailwind, 90°/270° is a pure crosswind. |
When to use it
Planning a flight time estimate
Ground speed, not airspeed, determines how long a flight actually takes over a given distance, so a headwind or tailwind component changes the estimated time even though the aircraft's airspeed hasn't changed.
Understanding a crosswind's effect
A pure crosswind barely changes ground speed but does push the aircraft off its intended track, which is why pilots need a drift correction angle to stay on course.
Comparing headwind and tailwind legs
The outbound and return legs of the same route can have noticeably different ground speeds and flight times if the wind direction stays roughly the same relative to the ground.
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 does ground speed change with wind direction, at a fixed 120kt airspeed and 20kt wind?
The same airspeed and wind speed, with the wind direction relative to heading varied from a straight headwind to a straight tailwind.
Questions
What is the difference between ground speed and air speed?
Airspeed is how fast the aircraft moves through the surrounding air mass. Ground speed is how fast it actually moves over the ground below, which differs from airspeed whenever there is wind, since the air mass itself is moving relative to the ground.
Does a crosswind slow an aircraft down?
Only slightly, and mostly through a small increase in the distance actually flown if a heading correction isn't applied. A pure 90° crosswind has almost no headwind or tailwind component, so it barely changes ground speed, but it does push the aircraft sideways off its intended track.
Why is the wind angle measured from the heading and not from north?
Because what matters for ground speed is the wind's effect relative to the direction the aircraft is actually pointed, not its compass direction. The same wind has a completely different effect on an aircraft heading north than on one heading east.
Is this the same as a pilot's E6B flight computer?
It calculates the same underlying wind-triangle relationship, using vector addition of true airspeed and the wind's along-track and crosswind components, though a full E6B or flight-planning tool also handles magnetic variation and true-to-magnetic heading conversions this calculator leaves out.
For converting the resulting speed into other units, see the knots to km/h converter. For a completely different kind of vector combination, see the angle between vectors calculator.