Knots, MPH, and KM/H: Why Speed Units Differ by Land, Air, and Sea
Why ships and aircraft measure speed in knots, how knots relate to mph and km/h, and why some countries still use miles per hour for road speeds while most of the world uses kilometers per hour.
Published
Speed is measured in different units depending on the domain: mph or km/h for road travel, and knots for maritime and aviation navigation. A knot equals one nautical mile per hour, which is approximately 1.15 mph or 1.852 km/h — a unit that persists in ships and planes mainly because of its direct link to navigational charts rather than any advantage in raw speed measurement.
Why knots exist
The nautical mile was defined so that one minute of latitude on the Earth's surface equals one nautical mile, tying distance directly to a ship's position on a chart. Speed measured in nautical miles per hour — knots — therefore let sailors relate their speed directly to distance covered on the chart, without converting between different distance units. The name comes from an older method of measuring ship speed using a knotted rope let out over a fixed time.
Why aviation kept using knots
Modern aviation adopted the same nautical mile and knot convention used at sea, since long-distance flight navigation relies on the same latitude/longitude chart system. Air traffic control, flight plans, and airspeed indicators worldwide generally use knots, regardless of whether the country uses metric or imperial units on the ground.
Road speeds: mph vs. km/h
On land, most countries measure speed limits in kilometers per hour, consistent with their broader use of the metric system. The United States uses miles per hour. The United Kingdom is a notable partial exception: it uses metric units in most contexts but has kept miles and mph for road distances and speed limits, largely due to the cost and disruption of replacing existing road signage nationwide.
| Unit | Equal to |
|---|---|
| 1 knot | ≈ 1.15 mph ≈ 1.852 km/h |
| 1 mph | ≈ 1.609 km/h |
Frequently asked questions
- Why do ships and planes use knots instead of mph?
- A knot is one nautical mile per hour, and a nautical mile is defined so that one minute of latitude equals one nautical mile — a relationship that ties speed directly to position on a navigational chart. That made knots far more convenient for navigation than statute miles, and the convention has persisted in maritime and aviation use ever since.
- How many mph is one knot?
- One knot is approximately 1.15 mph (or about 1.852 km/h).
- Why does the UK use mph on road signs but km/h elsewhere?
- The UK partially metricated in the late 20th century but kept miles and mph for road distances and speed limits due to the cost and disruption of replacing road signage, even though it uses metric units in most other contexts. Most of the rest of the world, including the rest of Europe, uses km/h for road speeds.
- What is the difference between a nautical mile and a statute mile?
- A nautical mile (about 1.852 km) is longer than a statute (land) mile (about 1.609 km). The nautical mile is based on the Earth's circumference, while the statute mile has its roots in old Roman and English land-measurement units.