Speed Converter

Every speed here is a distance unit divided by a time unit, so each conversion inherits its factor directly from the underlying length. mph to km/h: multiply by 1.609344 — the same factor as miles to kilometers, because the hour cancels out on both sides. 60 mph = 96.6 km/h. km/h to m/s: divide by 3.6. 72 km/h = 20 m/s. knots: one knot is one nautical mile per hour, which is 1.852 km/h exactly, or 1.151 mph. ft/s to m/s: multiply by 0.3048, the exact definition of the foot. One relationship is worth memorizing because it comes out exact: 60 mph is exactly 88 ft/s. A mile is 5,280 feet and an hour is 3,600 seconds, and 5,280 ÷ 60 lands on a whole number. Traffic engineers use the shortcut constantly — multiply mph by 1.467 for ft/s — and it makes stopping-distance arithmetic much easier, since a car at 60 mph covers 88 feet in every second of reaction time.

The 3.6 looks like a magic number and is not. It falls straight out of the definitions. One kilometer per hour is 1,000 meters in 3,600 seconds. Divide: 1,000 ÷ 3,600 = 0.2778 m/s. Flip it around and one m/s is 3,600 ÷ 1,000 = 3.6 km/h. That is the whole derivation — the 3.6 is just the ratio between an hour and a kilometer expressed in seconds and meters. So km/h → m/s is divide by 3.6, and m/s → km/h is multiply by 3.6. Handy pairs: 36 km/h = 10 m/s; 72 km/h = 20 m/s; 108 km/h = 30 m/s. This matters more than it seems, because physics and engineering work almost exclusively in m/s while road signs, weather apps and vehicle specs use km/h or mph. Kinetic energy scales with the *square* of speed, so a value converted incorrectly does not produce a proportional error — it produces a quadratic one.

A knot is one nautical mile per hour. The nautical mile is exactly 1,852 meters, and that number was chosen to approximate one minute of arc of latitude on the Earth's surface. That definition is why the unit refuses to die despite being metric-hostile. On a nautical chart, a distance stepped off with dividers can be read directly against the latitude scale in the margin: one minute of latitude is one nautical mile, everywhere. No conversion, no scale bar, no arithmetic. For centuries that was the difference between an easy fix and a hard one, and marine and aviation navigation still run on it. A usage note: "knots per hour" is wrong. The per-hour is already inside the unit, so a ship makes 12 knots, never 12 knots per hour — saying so would describe an acceleration. Conversions worth carrying: 1 knot = 1.852 km/h = 1.151 mph = 0.514 m/s. 10 knots = 18.5 km/h = 11.5 mph. 20 knots = 37.0 km/h = 23.0 mph. Wind matters here too — the threshold for a hurricane is 64 knots, which is 74 mph or 119 km/h, and those three numbers describe one wind.

Mach numbers are a ratio, not a unit: Mach 1 is the local speed of sound, whatever that happens to be. And the speed of sound in air depends mainly on temperature. At sea level in standard conditions, around 15 °C, sound travels at about 340 m/s — 1,225 km/h, or 761 mph. In the cold thin air near typical airliner cruising altitude, roughly −57 °C, it drops to about 295 m/s, which is 1,062 km/h or 660 mph. So Mach 1 at cruise is about 13% slower than Mach 1 at sea level. An aircraft flying Mach 0.85 at altitude is doing roughly 903 km/h (561 mph), not the 1,041 km/h that the sea-level figure would suggest. Two aircraft at the same Mach number can be moving at genuinely different ground speeds. This is exactly why aviation quotes Mach at altitude at all: what threatens an airframe is the airflow relative to the local speed of sound, not the raw velocity. Sound also travels faster in denser media than in air — much faster in water, faster still in steel — so a Mach number is only meaningful with the medium and its temperature specified.

Anchors across all four units. A comfortable walk is 5 km/h = 3.1 mph = 1.4 m/s. A casual run is 10 km/h = 6.2 mph = 2.8 m/s. A world-class sprinter peaks near 12 m/s, which is about 43 km/h or 27 mph. A US residential street is posted at 25 mph = 40 km/h; an interstate at 70 mph = 113 km/h; a European motorway at 120 km/h = 75 mph. Runners and cyclists often want the reciprocal instead. Pace is time per distance, so it is 60 divided by the speed: 10 km/h is 6:00 min/km, and 12 km/h is 5:00 min/km. In imperial terms, 6 mph is exactly 10:00 min/mile. Because pace is a reciprocal, equal speed gains are not equal pace gains — going from 8 to 10 km/h cuts 1:30 off each kilometer, while going from 16 to 18 km/h cuts only 0:40, even though both are a 2 km/h improvement. It is the same non-linearity that makes MPG a misleading measure of fuel savings.