What is a knot in aviation, and how is speed measured?
A knot is one nautical mile per hour: exactly 1.852 kilometres per hour, or about 1.151 miles per hour. Aircraft speed is not a single value; pitot-static instruments derive indicated airspeed, corrections yield calibrated and true airspeed, and navigation systems calculate groundspeed relative to the Earth.
For Aviation & Real-World Flying, the crucial distinction is the reference: through the air, across the ground, or relative to the local speed of sound. Two instruments can therefore show different speeds while both are correct.
How many mph or km/h is one knot?
One knot equals 1.852 km/h, 1.15078 mph or approximately 0.51444 metres per second. For example, 100 knots is 185.2 km/h or about 115.1 mph.
The standard abbreviation is kt. Labels such as KIAS and KTAS mean knots indicated airspeed and knots true airspeed. Saying “knots per hour” is incorrect because a knot already means one nautical mile per hour.
The nautical mile fits chart-based navigation and is defined as exactly 1,852 metres. Our explanation of why aviation uses nautical miles and knots covers the historical and navigational reasons for retaining the unit.
How does an aircraft measure airspeed?
An aircraft normally measures airspeed by comparing total pressure from a pitot tube with ambient pressure from static ports.
- The pitot tube senses total pressure. Facing the airflow, it receives static pressure plus the pressure produced by the aircraft’s movement through the air.
- Static ports sense ambient pressure. They are positioned to sample outside atmospheric pressure with as little airflow disturbance as practical.
- The system compares the pressures. Their difference represents dynamic pressure, which depends on air density and the square of airspeed.
- An instrument converts pressure into speed. A mechanical airspeed indicator uses a pressure-sensitive diaphragm; newer aircraft use sensors and an air-data computer.
At lower subsonic speeds, dynamic pressure is approximately represented by q = ½ρV². This explains why indicated airspeed is useful for judging aerodynamic loading, but it does not directly equal the aircraft’s true speed through thin air at altitude.
An air-data computer can combine pressure information with temperature and altitude data to derive true airspeed and Mach. Groundspeed comes from a navigation source such as GNSS or an inertial system, not from the pitot tube.
Which aircraft speed does the instrument show?
The primary cockpit airspeed display normally shows indicated airspeed, while other displays or flight-management pages may show true airspeed, groundspeed or Mach.
| Speed | What it means | Main use |
|---|---|---|
| IAS | The airspeed-indicator reading before correction for instrument and position error | Flying the aircraft and following most operational speed references |
| CAS | IAS corrected for instrument and position error | Performance data and precisely defined limitations |
| EAS | CAS corrected for compressibility effects | Aerodynamic calculations at higher speed or altitude |
| TAS | Actual speed through the surrounding air mass, corrected for density | Cruise performance and flight planning |
| Groundspeed | Speed across the Earth’s surface | Navigation, arrival times and distance covered |
| Mach | True airspeed divided by the local speed of sound | High-altitude and high-speed operations |
At low altitude and modest speed, IAS, CAS and TAS can be fairly close. At altitude, TAS is normally higher than IAS for the same aerodynamic loading because the air is less dense. Airliners consequently use both indicated speed and Mach; we explain how pilots select IAS or Mach at different altitudes.
Why do airspeed and GPS groundspeed disagree?
Airspeed measures movement through the air mass, whereas GPS groundspeed measures movement over the ground, so wind creates a normal difference between them.
If an aircraft has a TAS of 120 knots with a directly aligned 20-knot headwind, its groundspeed is about 100 knots. With the same wind from behind, groundspeed is about 140 knots. Crosswinds require vector calculation rather than simple addition or subtraction.
A steady wind changes groundspeed for a given TAS; it does not make GPS speed the correct reference for stall margins or aerodynamic limits. A mistake we see constantly in simulators is comparing groundspeed from an overlay with an approach speed published in KIAS.
Which speed should pilots and simmers use?
Choose the speed reference that matches the task and always retain the unit stated by the aircraft documentation or simulator display.
- Take-off, approach and aircraft limitations: use the specified IAS or CAS value, taking configuration and operating conditions into account.
- Cruise performance and fuel planning: use TAS, then apply the wind to estimate groundspeed.
- Navigation and arrival time: use groundspeed.
- High-altitude jet operation: observe both indicated-speed and Mach limits.
- ATC speed instructions: use IAS or Mach exactly as assigned.
Common knot and airspeed mistakes
The safest way to interpret any speed value is to check its label rather than assuming every number means IAS.
- Do not use “knots per hour”; write knots or kt.
- Do not substitute GPS groundspeed for a published KIAS limitation or V-speed.
- Do not treat IAS and TAS as interchangeable at altitude.
- Do not rely on the rough “TAS increases by 2% per thousand feet” rule when accurate temperature and pressure data are available.
- In a simulator, check whether the HUD, telemetry panel or external view is displaying IAS, TAS, groundspeed, mph or km/h.
If indicated speeds freeze, diverge or respond implausibly with altitude, wind is not the only possible cause. Pitot icing, blocked pressure sources and simulated instrument failures can produce false readings; our guide to pitot-static faults and unreliable speed indications explains the symptoms and appropriate simulator checks.