About the Speed Converter

The speed converter translates between five common units of velocity: metres per second (the SI unit), kilometres per hour, miles per hour, feet per second, and knots. All conversions are expressed in m/s as the pivot unit, with the factors derived from the underlying length and time conversions.

The factor for km/h is 1000 metres per 3600 seconds, which equals 5/18 or 0.277778 m/s. The factor for mph is 1609.344 metres per 3600 seconds, which equals exactly 0.44704 m/s (since the mile is defined exactly as 1609.344 metres). The factor for ft/s is exactly 0.3048 m/s (since the foot is exactly 0.3048 metres). The factor for a knot is 1852 metres per 3600 seconds, which equals 0.514444 m/s (since the nautical mile is exactly 1852 metres).

Speed units combine a length unit with a time unit, so they inherit the definitions of both. The metre and second are SI base units; the second is defined by fixing the caesium-133 hyperfine transition frequency at exactly 9,192,631,770 Hz (SI Brochure 9th edition §2.3.1). The nautical mile, mile, and foot are defined exactly in metres per the 1959 International Yard and Pound Agreement.

The converter handles any real input, including negatives (which represent speed in the opposite direction in physics contexts) and zero. All factors are exact within the precision of the underlying definitions.

How It Works

The converter uses the two-step method through metres per second:

base_in_mps = value * factor[from]
result = base_in_mps / factor[to]

The factor table (in m/s) is:

m/s   = 1 m/s          km/h = 0.277778 m/s
mph   = 0.44704 m/s     ft/s = 0.3048 m/s
knot  = 0.514444 m/s

Each factor is the speed in m/s that one unit represents. For example, 1 km/h = 1000/3600 m/s = 0.277778 m/s, since one kilometre is 1000 metres and one hour is 3600 seconds. The factor for mph is exact because the mile is defined exactly as 1609.344 metres; the factor for km/h is a recurring decimal (5/18) but is exact in fractional form.

The two-step method through m/s is mathematically equivalent to direct conversion but avoids the need for a 25-entry pairwise table. The pivot unit also makes the formulas transparent: any conversion can be verified by computing the equivalent m/s value and dividing.

The displayed factor for km/h (0.277778) is rounded for readability; the actual computation uses the full-precision value. This is why very large conversions may show small differences from hand calculations using the rounded factor.

Worked Examples

Converting 60 mph to km/h: the input in m/s is 60 * 0.44704 = 26.8224 m/s, and the result is 26.8224 / 0.277778 = 96.5604 km/h. The popular approximation "60 mph is about 100 km/h" is therefore off by about 3.4 percent.

Converting 100 km/h to mph: the input in m/s is 100 * 0.277778 = 27.7778 m/s, and the result is 27.7778 / 0.44704 = 62.1371 mph. This explains why a 100 km/h speed limit on European highways corresponds to about 62 mph on a US speedometer.

Converting 30 knots to m/s: the input in m/s is 30 * 0.514444 = 15.4333 m/s, and the result is 15.4333 m/s directly. To convert to km/h, divide by 0.277778 to get 55.56 km/h. A ship sailing at 30 knots therefore moves at about 55.6 km/h, which is roughly 34.5 mph.

When to Use This Tool

Use the speed converter when you need to:

Convert speed limits between mph (US, UK) and km/h (rest of the world) when driving abroad.
Translate running pace from km/h to min/km or to miles per hour.
Convert aircraft speeds between knots and km/h or mph.
Convert ship speeds from knots to m/s for engineering calculations.
Translate wind speed between m/s (used in scientific reports) and mph or km/h (used in forecasts).
Verify a speedometer reading against a GPS speed measured in a different unit.
Compute terminal velocity or free-fall speed in different unit systems.

Limitations & Disclaimer

This converter uses exact conversion factors derived from SI definitions. It does not handle Mach number (which depends on the local speed of sound), acceleration units, or relativistic velocities. Results are subject to IEEE 754 floating-point precision. For very high speeds approaching the speed of light, special relativity applies and classical conversion is no longer appropriate. See our disclaimer for full details.

Frequently Asked Questions

Why is the knot used for ships and aircraft?

Because the knot is one nautical mile per hour, and a nautical mile corresponds to one minute of latitude on a nautical chart. This makes navigation simpler: a ship sailing at 20 knots covers 20 nautical miles (20 minutes of latitude) per hour, which can be plotted directly on a chart without unit conversion.

What is the difference between mph and km/h?

Both measure speed in distance per hour, but mph uses the statute mile (1609.344 metres) while km/h uses the kilometre (1000 metres). One mph equals about 1.609344 km/h. The US and UK use mph for road speed; the rest of the world uses km/h.

Is m/s the SI unit of speed?

Yes. The SI derived unit of speed is the metre per second, formed from the SI base units of length (metre) and time (second). Other units like km/h and mph are accepted for use with SI but are not SI units themselves.

Why is the factor for km/h a recurring decimal?

Because 1 km/h = 1000 m / 3600 s = 5/18 m/s, which is approximately 0.277778 but is a recurring decimal (the digit 7 repeats). The displayed factor 0.277778 is rounded; the actual computation uses the full-precision value, so results are accurate to the limits of double-precision arithmetic.

Does the converter handle negative speeds?

Yes. Negative speeds represent motion in the opposite direction in physics contexts (velocity is a signed quantity). The conversion factor is the same regardless of sign, so a negative input produces a negative result of the same magnitude.

What is Mach number and how does it differ from these units?

Mach number is the ratio of an object's speed to the local speed of sound, which varies with temperature and altitude. Mach 1 at sea level is about 340 m/s (1224 km/h, 761 mph), but at altitude it can be lower. Mach number is dimensionless, so it cannot be directly converted to m/s without knowing the local speed of sound.

Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team