Speed Converter

Using the Speed Converter

1. Pick your source unit from the "From" selector. The options cover meters per second (SI base), kilometers per hour, miles per hour, feet per second and knots - the five speed units you actually meet in daily life.
2. Type the value into the input. The output field reacts on every keystroke via the Preact onInput handler; there is no "Calculate" button to press.
3. Pick your destination unit from the "To" selector.
4. Swap direction with the arrow button when you want the inverse conversion - the button reassigns the two unit selections rather than recomputing from scratch.
5. Use Copy to push the numeric result to the clipboard through the browser Clipboard API.

What the Tool Computes

Every unit is stored internally as a ratio to meters per second, the SI coherent derived unit for speed (SI Brochure, BIPM 9th edition, Table 4). A conversion is a single multiplication: result = value * factorFrom / factorTo. The factor for km/h is exactly 1/3.6; mph uses the exact international mile (1609.344 m) and exact hour (3600 s), giving 0.44704 m/s per mph; feet per second uses 0.3048 m/s; knots use 1852/3600 m/s, which is exactly 0.5144444... m/s because the international nautical mile is defined as 1852 meters (First International Extraordinary Hydrographic Conference, Monaco 1929). Those exact definitions mean the only rounding you see is at display time.

Real Situations You Will Reach for It

• Interpreting a car review written in mph when your local speed limit signs are km/h.
• Matching a boat's GPS readout in knots against a harbor speed limit posted in km/h or mph.
• Converting a climbing or running pace reported in m/s on a research paper into more intuitive km/h.
• Reading aviation speed callouts - indicated airspeed is usually in knots, ground speed reports may be km/h or mph.
• Checking a weather report's wind speed ("sustained 25 knots, gusts to 40") against a boating forecast in meters per second.
• Translating a physics homework answer expressed in feet per second into SI units for a grading rubric.

Edge Cases and Pitfalls

Speed has two subtle traps. First, ground speed versus airspeed (or water speed): GPS tells you how fast your position is changing on the earth surface, whereas a pitot tube in a plane or a paddle-wheel on a boat tells you speed relative to the surrounding fluid. A 20-knot headwind gives 120 knots indicated airspeed but only 100 knots ground speed. This tool treats whatever number you enter as a scalar; you have to know which kind. Second, average versus instantaneous speed: a runner who completes 10 km in 40 minutes has an average speed of 15 km/h, but the instantaneous speed varies. If you are converting a Garmin or Strava entry, make sure you know which the device reported. Finally, very large magnitudes such as the speed of light (2.998 x 10^8 m/s) survive IEEE 754 double precision cleanly, but anything multiplied by 3.6 to get km/h starts giving unwieldy display strings - the formatter caps at 10 significant digits.

A Little Background on the Knot

The knot got its name from a device sailors used in the 17th century: a wooden log thrown overboard with a knotted line trailing behind. Counting the knots that ran out in half a minute while the line paid out at a fixed rate gave the ship's speed, and the unit stuck. The nautical mile it is built on originally equaled one minute of latitude along any meridian (about 1852 m near the equator, 1861 m near the poles). To get rid of that variation, the 1929 Monaco conference fixed it as exactly 1852 m, which is what ICAO (International Civil Aviation Organization) and IMO (International Maritime Organization) use today. The US retained a slightly different "US nautical mile" of 1853.181 m until 1954, and the UK held out until 1970; both now use 1852 m. NIST SP 811 lists the knot as "approved for use in marine and aeronautical applications" but not for science.

Alternatives and Tradeoffs

Google's inline converter is faster for one-off "how fast is 60 mph in kph" queries but sends the query to a server. The GNU units CLI supports speed of light (c), Mach (note that Mach is a ratio, not a unit, and depends on altitude and temperature), and hundreds of niche units; that is the right tool if you need them. Scientific environments such as Python with astropy.units or MATLAB are better when speeds feed into a longer equation because they carry units through the arithmetic and catch mismatches at runtime. This page is the right choice when you want five common units visible, no server round-trip and no dependency install.