What is a knot, and why is aircraft speed measured in knots?
What kt, kts and KIAS mean, why aviation uses knots, how to convert knots to mph or km/h, and why airspeed differs from GPS groundspeed.
A knot in aviation is one nautical mile per hour: exactly 1.852 km/h or about 1.151 mph. Aircraft speed is measured in knots because nautical miles suit latitude-and-longitude navigation, while using knots for both aircraft and wind speeds keeps chart, timing and wind-correction calculations consistent.
In Aviation & Real-World Flying, the crucial detail is the speed reference. Indicated airspeed, true airspeed and groundspeed may all be expressed in knots, yet each measures something different.
Why is plane speed measured in knots instead of mph?
Knots remain aviation’s standard speed unit because they pair directly with nautical-mile distances and the units used for most aviation wind reports.
- Chart-based navigation: one nautical mile is exactly 1,852 metres and approximately one minute of latitude, making it convenient for latitude-and-longitude charts.
- Simple timing: an aircraft travelling at a groundspeed of 180 kt covers 90 nautical miles in 30 minutes.
- Wind calculations: expressing aircraft and wind velocity in knots avoids an extra unit conversion when estimating headings and groundspeed.
- Shared terminology: pilots, controllers, dispatchers and weather services can communicate speeds consistently.
The word comes from maritime navigation, where sailors estimated speed by counting knots passing from a marked line during a timed interval. Aviation later adopted the established nautical system. Weather groups such as 24012KT mean wind from 240 degrees at 12 kt, although some states use metres per second; our guide to reading knots on airspeed displays and wind reports explains how to identify the reference and unit.
What does kts stand for in aviation?
Kts is commonly understood to mean knots, but it is not an acronym and the standard unit symbol is kt. A knot already means one nautical mile per hour, so “knots per hour” is incorrect for speed.
Labels add the type of speed being reported. KIAS means knots indicated airspeed, KCAS means knots calibrated airspeed, and KTAS means knots true airspeed. Writing only “120 kt” can be ambiguous when IAS, TAS and groundspeed differ.
How do I convert kts to mph or km/h?
Multiply knots by 1.15078 for mph or by 1.852 for km/h.
| Knots | Miles per hour | Kilometres per hour |
|---|---|---|
| 1 kt | 1.15078 mph | 1.852 km/h |
| 100 kt | 115.1 mph | 185.2 km/h |
One knot is also approximately 0.51444 metres per second. Converting a labelled KIAS value into mph is simple unit arithmetic, but converting indicated airspeed into true airspeed also requires atmospheric and aircraft data; see our explanation of KIAS conversions and true-airspeed estimation.
How is aircraft airspeed measured?
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 ambient static pressure plus the pressure associated with the aircraft’s motion through the air.
- Static ports sense ambient pressure. Their position is chosen to reduce disturbance from airflow around the airframe.
- The system finds dynamic pressure. The difference between pitot and static pressure varies with air density and the square of airspeed.
- The instrument converts pressure into a reading. This may be done mechanically or through sensors and an air-data computer.
At lower subsonic speeds, dynamic pressure is approximately described by q = ½ρV². Indicated airspeed therefore reflects aerodynamic loading well, but it does not directly show how fast the aircraft is moving through thinner air at altitude.
An air-data computer can combine pressure, temperature and altitude inputs to derive true airspeed and Mach. Groundspeed instead comes from a navigation source such as GNSS or an inertial system.
Which aircraft speeds are measured in knots?
Several speed types use knots, so the number’s label determines what it means and how it should be used.
| Speed | Meaning | Main use |
|---|---|---|
| IAS or KIAS | Indicated airspeed before correction for instrument and position error | Flying the aircraft, V-speeds and most operating limitations |
| CAS or KCAS | IAS corrected for instrument and position error | Performance data and precisely defined limitations |
| EAS or KEAS | CAS corrected for compressibility | Aerodynamic calculations at higher speed or altitude |
| TAS or KTAS | Actual speed through the surrounding air mass | Cruise performance and flight planning |
| Groundspeed | Speed across the Earth’s surface | Navigation, distance covered and arrival estimates |
| Mach | Speed relative to the local speed of sound; it has no unit | High-altitude and high-speed operation |
Near sea level at modest speed, IAS, CAS and TAS may be close. At altitude, TAS is normally higher than IAS for the same aerodynamic loading because the air is less dense. Fast aircraft consequently use both indicated speed and Mach; our guide explains when pilots change between IAS and Mach.
Why do airspeed and GPS groundspeed disagree?
Airspeed measures motion through the air mass, while GPS groundspeed measures motion over the Earth, so wind normally makes the readings different.
An aircraft flying at 120 KTAS into a directly aligned 20 kt headwind has a groundspeed of about 100 kt. With the same wind directly behind it, groundspeed is about 140 kt. Crosswinds require vector calculation rather than simple addition or subtraction.
GPS groundspeed is useful for route progress, but it is not the correct reference for stall margins, approach targets or aerodynamic limits. A mistake we see constantly in simulators is comparing an overlay’s groundspeed with an approach speed published in KIAS. Our detailed explanation of why indicated airspeed and GPS groundspeed differ covers the effects of wind, altitude and air density.
Do all aircraft display speed in knots?
No; knots are the prevailing convention, but some older aircraft use mph, while certain gliders and aircraft built for particular markets display km/h.
The aircraft’s gauge markings, placards and flight manual are authoritative. In a simulator, changing the general unit preference may alter maps or interface overlays without changing an analogue gauge face modelled in knots. Check the unit printed beside the scale rather than assuming every display follows the global setting.
Which speed should pilots and simmers use?
- Take-off, approach and aircraft limitations: use the specified IAS or CAS, accounting for configuration and operating conditions.
- Cruise and fuel planning: use TAS, then apply 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 as assigned.
Never substitute GPS groundspeed for a published KIAS limitation, assume IAS equals TAS at altitude, or treat an unlabelled number as knots. If indicated speed freezes or reacts implausibly, check for pitot icing, blocked pressure sources and simulated instrument failures rather than blaming wind alone.