Knots vs MPH: Why the Same Speed Reads 15% Higher in Miles

A knot is one nautical mile per hour, and mph is one ordinary (statute) mile per hour. A nautical mile is about 15% longer than a statute mile, so the same speed always reads about 15% higher in mph: 1 knot equals 1.15078 mph.

Neither unit is faster or more accurate. Pilots and sailors use knots because a nautical mile is tied to latitude, and that makes chart math easy. Drivers and most US weather forecasts use mph.

We compared the two as of October 2026 using NIST’s exact unit definitions, NOAA, the FAA’s speed rules, ICAO’s units annex and the National Hurricane Center’s wind scale.

How big the difference is: knots to mph

Multiply knots by 1.15078 to get mph. Multiply mph by 0.86898 to get knots. Because the factor never changes, the gap grows with speed: 1.5 mph at 10 knots, but 75.4 mph at 500 knots.

For mental math, add 15% to the knot figure. That shortcut is off by less than 0.07%, so 200 knots becomes 230 mph against an exact 230.2.

So 20 knots is not the same as 20 mph. It is 23 mph, and a boat or airplane doing 20 knots is moving 3 mph faster than a car doing 20 mph.

The other direction works the same way. A car doing 60 mph on the highway is doing 52.1 knots, and a 100 mph wind is 86.9 knots.

The aviation numbers show why the gap matters. A Cessna 172 Skyhawk in its 172S version must never exceed 163 knots indicated, which is 187.6 mph.

Read that limit as 163 mph and you stop 24.6 mph short of the real line. The risky mistake is the reverse: an mph limit flown as knots is overshot by about 15%.

Our guide to how fast airplanes go gives typical speeds for each type of aircraft.

Takeaway: one knot is 1.15078 mph, so add 15% to go from knots to mph and expect the gap to grow with speed.

Why a nautical mile is longer than a mile

The two miles come from different jobs. NOAA describes the nautical mile as “one minute of latitude,” which makes it “slightly longer than a mile on land, equaling 1.1508 land-measured (or statute) miles.”

The statute mile is a land measure of 5,280 feet with no link to the shape of the Earth. Since 1959 the foot has been exactly 0.3048 m, which fixes the mile at 1,609.344 m.

The nautical mile is now fixed at exactly 1,852 m, or about 6,076 feet. It is a set length, but it still matches one minute of latitude closely enough for navigation.

Before 1929, countries did not even agree on its length. The US used 1,853.248 m (6,080.2 feet) until 1954, and Britain’s Admiralty mile was 6,080 feet until 1970, both about 4 feet longer than today’s international mile.

Our page on statute miles vs nautical miles covers the distance side of the same split.

Here is the geometry in one number. A degree of latitude has 60 minutes, so it spans 60 nautical miles, or 69 statute miles. An aircraft flying due north at 60 knots covers about one degree of latitude every hour.

Takeaway: a nautical mile is longer because it was built from the Earth’s own grid, one minute of latitude, while the statute mile is a land measure.

Where the knot comes from

The name is literal. NOAA says the term dates from the 17th century, when sailors measured speed with a “common log”: a piece of wood on a rope “with knots at regular intervals,” let out behind the ship “for a specific amount of time.”

The spacing was chosen to make the count read straight in nautical miles per hour. With knots 47 feet 3 inches apart and a 28-second sand glass, each knot that ran out works out to 0.9998 knots in today’s unit, within 0.02% of the modern definition.

Takeaway: the knot started as a count of knots on a rope and has been an exact metric unit since 1929, adopted by the US in 1954.

Why pilots and sailors use knots instead of mph

Navigation charts are drawn on latitude and longitude. Because one nautical mile is one minute of latitude, a navigator can measure a distance straight off the latitude scale on the side of the chart, then divide by the speed in knots to get the time. In mph, every leg would need a conversion first.

A 300-nautical-mile leg flown at 150 knots takes exactly two hours. Written in statute units, it is 345.2 miles at 172.6 mph: the same two hours, after two extra conversions.

Wind uses the same unit. A METAR, the routine airport weather report, gives wind speed “in whole knots,” always followed by KT, as in 21016G24KT for wind from 210 degrees at 16 knots gusting 24. With airspeed and wind both in knots, working out a headwind or tailwind is simple subtraction.

US rules are written in knots first. The FAA’s speed rule, 14 CFR 91.117, says no one may fly below 10,000 feet “at an indicated airspeed of more than 250 knots (288 m.p.h.).” The limit near busy airports is “200 knots (230 mph).”

Those are indicated speeds, the knots shown on the airspeed indicator, not speed over the ground. Our explainer on KIAS and the other kinds of airspeed covers why indicated and true airspeed drift apart as an airplane climbs.

Ships use knots for the same chart reason. NOAA notes that knots “are used to measure speed” at sea, where the nautical mile began.

Takeaway: knots fit the latitude grid on every chart and match the wind reports, so they save a conversion on every calculation.

Where mph still shows up

The weather on your phone is converted. The National Weather Service measures wind in knots “in accordance with international practice,” then converts to mph for the public, WGN-TV’s weather team explained in 2019.

That conversion leaves holes. If every reading starts as a whole number of knots and is rounded to whole mph, some mph values can never come out: 9 knots is 10 mph, but 10 knots is already 12 mph.

Hurricane warnings use both units. The National Hurricane Center publishes the Saffir-Simpson scale in mph, knots and km/h side by side: Category 1 starts at 74 mph (64 knots) and Category 5 at 157 mph (137 knots).

Older light-aircraft paperwork is the other holdout. The General Aviation Manufacturers Association’s handbook standard, GAMA Specification No. 1, was first issued on February 15, 1975, and says “KNOTS are used throughout to avoid the confusion between knots (KTS) and miles per hour (MPH) in performance charts and tables.”

That standard covers handbooks written to it, so an airplane’s book and airspeed indicator can still read in mph on older types. Pilots check which unit the dial shows before using any speed from the book.

Takeaway: mph survives where the audience is the public, on roads and in forecasts, and in some older aircraft paperwork, while the measurement underneath is usually in knots.

The difference in one sentence

A knot is one nautical mile per hour and mph is one statute mile per hour, so because a nautical mile (about 6,076 feet) is longer than a statute mile (5,280 feet), 1 knot equals 1.15078 mph and the same speed always reads about 15% higher in mph.