Article — Knots to MPH Converter
Knots to mph: convert the speed of wind, ships, and aircraft
One knot equals 1.15078 miles per hour. That works out because one knot is one nautical mile per hour, and one nautical mile is 1852 metres exactly — a defined value set by the International Hydrographic Conference in 1929. The statute mile (the regular land mile) is 1609 metres, so a nautical mile is about 15% longer, and so a knot is about 15% faster than mph. Every aviation, marine, and weather speed quoted in knots can be converted to mph by multiplying by 1.15078, or simply adding 15% for a close approximation.
The calculator above handles both directions and shows you km/h and m/s at the same time. The article below covers where the knot came from, why pilots and sailors still use it after 400 years, and how the Beaufort and Saffir-Simpson scales map wind speed in knots to weather conditions you can see and feel.
What is a knot?
A knot is a unit of speed equal to one nautical mile per hour. It is the standard speed unit for ships, aircraft, and wind speed across most of the world. International aviation authorities (ICAO) and the major weather agencies (NOAA, WMO, the UK Met Office) report aircraft speeds, ocean currents, and wind forecasts in knots as the default unit.
The abbreviation is "kt" (singular and plural). "kts" is also seen but is less standard. The unit is not part of the SI system but is one of the small set of non-SI units the International Bureau of Weights and Measures (BIPM) accepts for continued use because it is so deeply embedded in aviation and maritime practice.
The conversion: 1.15078
The factor 1.15078 comes from comparing the nautical mile to the statute mile. A nautical mile is 1852 m. A statute mile is 1609.344 m. The ratio is 1852 / 1609.344 = 1.15077944802..., which rounds to 1.15078 for everyday use.
mph = knots × 1.15078knots = mph × 0.86898km/h = knots × 1.852 (exact)m/s = knots × 0.51444The cleanest of those is knots to km/h, where the conversion is exactly 1.852 because the nautical mile is defined as 1852 m. Knots to mph has no exact rational form because the statute mile (1609.344 m) is itself defined by the 25.4 mm inch, and the ratio 1852/1609.344 is irrational beyond a handful of decimals.
The nautical mile and one minute of latitude
The nautical mile was originally defined as the length of one minute of arc along a meridian — one-sixtieth of one degree of latitude. The Earth is not a perfect sphere, so the actual length of one arc-minute varies slightly with latitude (it is about 1842 m at the equator and 1862 m at the poles). Navigators in the 17th to 19th centuries used the average value, which sits at almost exactly 1852 m.
In 1929, the International Hydrographic Conference in Monaco standardised the nautical mile at exactly 1852 m, regardless of where on Earth the measurement was taken. The US held out and continued to use a slightly different value (1853.248 m) until 1954, and the UK until 1970. Since then, the 1852 m figure has been universal.
The reason the nautical mile equals one minute of latitude is that it makes chart reading instant. On a paper chart with a latitude scale on the side, the distance between any two points can be measured by stepping a divider across the latitude markings: each minute of latitude is exactly one nautical mile, regardless of scale. No conversion. No formula. Just count the minutes. That single property kept the unit alive long after the metric system arrived for everything else.
Why aviation and shipping use knots
The simplest answer: navigation maths is easier in knots. One knot times one hour equals one nautical mile, and one nautical mile equals one minute of latitude on the chart. A pilot or navigator can read distance directly off the chart's latitude scale without any conversion. Speed in knots × time in hours = distance in nautical miles = arc-minutes of latitude. Three values, one unit family, no conversion factors.
That convenience is why the International Civil Aviation Organization (ICAO) standardised knots as the global default for airspeed reporting. Most countries report aircraft cruise speeds, approach speeds, and reference speeds in knots. China and Russia run some of their domestic operations in km/h for military aircraft, but international commercial aviation is uniformly in knots. The FAA Aeronautical Information Manual treats knots as the default unit for indicated airspeed across all US controlled airspace.
For shipping, the same logic applies. Charts use latitude minutes for distance. Speed-over-ground readings in knots map directly onto chart minutes. Switching to km/h or mph would require every chart reading and every speed-time-distance calculation to add a conversion step. Nobody wanted that, and so the unit persists.
The Beaufort scale
The Beaufort scale rates wind speed from 0 (calm) to 12 (hurricane force), with each rating tied to a band of knot values and a description of what the sea or land looks like at that speed. Francis Beaufort, a captain in the British Royal Navy, created the scale in 1805 because ship logs of the era were full of subjective wind descriptions ("a fresh gale," "moderate breeze") that did not match up between captains. Beaufort's innovation was to anchor each rating to an observable effect: at Force 3 the leaves on a tree are in constant motion; at Force 6 it is difficult to use an umbrella; at Force 10 trees are uprooted.
The Royal Navy adopted the scale in 1838 and required its use in all official ship logs. The World Meteorological Organization (WMO) extended it in 1946 with formal knot ranges for each force level. Beaufort's scale is still the default wording when marine forecasts describe wind conditions: "force 6 to 7" rather than "25 to 30 knots." Both are correct, but the Beaufort wording carries the visual cue along with the number.
Hurricane categories
The Saffir-Simpson Hurricane Wind Scale rates tropical cyclones from Category 1 to Category 5 based on sustained wind speed. Category 1 begins at 64 knots (74 mph), and Category 5 begins at 137 knots (157 mph). The National Hurricane Center (part of NOAA) uses this scale for every Atlantic and East-Pacific cyclone forecast.
- Tropical depression: less than 34 kt (less than 39 mph)
- Tropical storm: 34-63 kt (39-73 mph) — named storms
- Hurricane Cat 1: 64-82 kt (74-95 mph) — minimal damage
- Hurricane Cat 2: 83-95 kt (96-110 mph) — extensive damage
- Hurricane Cat 3: 96-112 kt (111-129 mph) — major hurricane
- Hurricane Cat 4: 113-136 kt (130-156 mph) — catastrophic
- Hurricane Cat 5: 137+ kt (157+ mph) — total destruction
The strongest tropical cyclone ever reliably measured by aircraft was Hurricane Patricia in 2015, which peaked at 185 knots (213 mph) of one-minute sustained wind in the eastern Pacific. The strongest Atlantic hurricane on the same metric was Allen (1980) at 165 knots (190 mph). The scale stops at 5 because the damage at that level is essentially total — there is no engineering case for distinguishing higher categories.
The chip log: where the name came from
The word "knot" comes from a literal knot tied in a rope. From the 1500s to the 1800s, sailors measured a ship's speed using a "chip log" — a triangular wooden float (the "chip") attached to a long rope marked with knots tied at regular intervals. The standard spacing was 47 feet 3 inches (about 14.4 m) between knots.
The procedure: throw the chip overboard from the stern. The chip stayed in place in the water while the moving ship paid out the rope. A second sailor turned a 28-second sand-glass timer. When the sand ran out, the rope was clamped and the knots were counted. Each knot that had passed through the sailor's hands represented one nautical mile per hour.
The maths is straightforward. 14.4 m × (3600 / 28) = 1851.4 m per hour, which rounds to one nautical mile per hour. The chip log was crude but cheap — any ship could carry one — and it was accurate enough to be standard equipment for over 200 years. The name "knot" outlasted the technology by centuries. Modern ships measure speed with GPS, Doppler logs, and impeller logs, but the unit is still called by the name of the device that no longer exists.
If you want to convert knots to mph in your head, add 15% to the knot value. 20 kt + 3 = 23 mph (true: 23.02). 60 kt + 9 = 69 mph (true: 69.05). 100 kt + 15 = 115 mph (true: 115.08). The accuracy is within 0.07% across the whole range. Going the other way — mph to knots — subtract about 13%. 70 mph - 9 = 61 kt (true: 60.83).
Common mistakes
"Knots per hour" is a tautology. A knot is already a rate — nautical miles per hour. Saying "20 knots per hour" is like saying "20 mph per hour." The correct phrasing is "20 knots" or "a wind speed of 20 knots." This mistake shows up in news copy and casual speech but is technically wrong.
Confusing knots with the nautical mile. The knot is a speed; the nautical mile is a distance. "We sailed 100 knots" is a category error — you sail 100 nautical miles, at some speed in knots. The distance is one thing, the rate of covering it is another.
Assuming a nautical mile equals a regular mile. It doesn't. A nautical mile is 1852 m; a statute mile is 1609 m. The nautical mile is 15% longer. A 100-nautical-mile passage is 115 statute miles of distance covered. This catches out sailors who try to convert between coastal charts (which use nautical miles) and road maps (which use statute miles).
Using "knots" for ground speed when you mean airspeed. In aviation, "indicated airspeed" (IAS) is the speed shown on the cockpit instrument, measured by air pressure against the pitot tube. "True airspeed" (TAS) corrects for altitude and temperature. "Ground speed" (GS) is what GPS shows — speed over the ground including wind effects. All three can be quoted in knots and can differ from each other by tens of knots at high altitude. Pilots specify which one they mean.