# Speed Converter: km/h, mph, Knots, m/s and Mach with Exact Factors

Page: https://hirantha.me/tools/speed-converter
Part of: Hirantha Sandaruwan Lakshan free tools (https://hirantha.me/tools)
Updated: 2026-10-13

Convert km/h, mph, knots, m/s, ft/s and Mach in one go. Every factor is exact, and Mach uses a stated speed of sound that you can change.

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Type a speed, choose its unit, and see it in km/h, mph, knots, metres per second, feet per second, km/s, Mach and the speed of light at once. Every factor is exact: a mile is 1,609.344 m, a knot is 1,852 m per hour and a foot is 0.3048 m. Mach is the only unit that needs an assumption about the air, so the page states the speed of sound it uses and lets you change it. Nothing you type leaves your device.

The tool turns your number into metres per second, then divides by the size of the unit you asked for. The sizes are exact fractions. One km/h is 1,000 m in 3,600 s, which is 5/18 m/s. One mph is 1,609.344 m in 3,600 s, which is 0.44704 m/s, because the international mile is 5,280 feet of 0.3048 m. One knot is one nautical mile an hour, and the international nautical mile is exactly 1,852 m, so a knot is 1,852/3,600 m/s, about 0.5144 m/s. NIST lists these relations in its Guide to the SI (SP 811). The speed of light in a vacuum is exactly 299,792,458 m/s. All the arithmetic is done with whole-number fractions, so converting there and back returns exactly what you typed.

Mach is not a fixed speed. One Mach is the speed of sound in the surrounding air, and sound travels faster in warm air. In the International Standard Atmosphere, air at sea level is at 15 °C and the speed of sound is 340.294 m/s, which is the square root of 1.4 x 287.053 x 288.15 (the ratio of specific heats of air, its gas constant in J/kg/K, and the temperature in kelvin). That is 1,225.06 km/h, 761.22 mph or 661.48 knots; Wikipedia's Mach number article gives the same figure rounded to 340.3 m/s. The page uses 340.294 m/s by default.

Height matters only through temperature. The speed of sound in air depends on the temperature, not directly on the pressure or altitude. In the standard atmosphere the air cools to -56.5 °C (216.65 K) from 11 km and stays there up to 20 km, so the speed of sound there is about 295.07 m/s, which is 1,062.3 km/h. You can pick that reference, calculate the speed of sound from any air temperature between -100 and 100 °C, or type your own value. In 30 °C air the speed of sound is 349.04 m/s, so Mach 1 is faster on a hot day in Colombo than at jet cruising height.

Answers show up to 10 significant digits, rounded half up. You can type commas between groups of three digits (1,250), a full stop for the decimal point, and e-notation (1.2e3). Values from 0 to 1e30 are accepted; a negative speed is refused because speed here has no direction. A table of familiar speeds, each with its source, sits below the tool.

Familiar speeds, each tied to its source:
| Speed | m/s | km/h | mph | Source |
| --- | --- | --- | --- | --- |
| Usain Bolt, 100 m world record (9.58 s), average over the whole race | 10.44 | 37.58 | 23.35 | 100 m in 9.58 s, World Athletics, Berlin, 16 August 2009 |
| Speed of sound in dry air at 15 °C, sea level (Mach 1) | 340.294 | 1,225.06 | 761.22 | International Standard Atmosphere |
| Escape velocity from Earth | 11,186 | 40,269.6 | 25,022 | NASA Earth Fact Sheet: 11.186 km/s |
| Earth around the Sun, mean orbital speed | 29,780 | 107,208 | 66,616 | NASA Earth Fact Sheet: 29.78 km/s |
| Light in a vacuum | 299,792,458 | 1,079,252,848.8 | 670,616,629.4 | Exact by the definition of the metre (NIST) |

Q: How do I convert km/h to mph?
A: Divide by 1.609344. For example 100 km/h is 62.137 mph and 120 km/h is 74.565 mph. To go from mph to km/h, multiply by 1.609344. The factor is exact because the international mile is defined as exactly 1,609.344 metres.

Q: How many km/h is one knot?
A: Exactly 1.852 km/h, because a knot is one nautical mile (1,852 m) per hour. So 10 knots is 18.52 km/h and 30 knots is 55.56 km/h. A knot is also 1.150779 mph, and a mph is 0.868976 knots.

Q: What speed is Mach 1?
A: It depends on the air. At sea level and 15 °C in the standard atmosphere, Mach 1 is 340.294 m/s, which is 1,225.06 km/h, 761.22 mph or 661.48 knots. In colder air it is slower (about 295.07 m/s at -56.5 °C) and in warmer air it is faster (349.04 m/s at 30 °C). Altitude changes it only because the temperature changes with altitude.

Q: Why does the Mach reference change the answer?
A: Mach is a ratio: speed divided by the local speed of sound. The same aircraft speed in km/h is a higher Mach number in cold air than in warm air. The tool therefore shows which speed of sound it is using, and you can switch it, enter an air temperature, or type your own value.

Q: How fast is the speed of light in km/h?
A: Light in a vacuum travels 299,792,458 m/s exactly, which is 1,079,252,848.8 km/h or about 670,616,629 mph. The metre is defined from this speed, so the number is exact and not a measurement.

Q: Is km/h the same as kph or kmph?
A: Yes. They all mean kilometres per hour. The standard symbol is km/h. Likewise mph, mi/h and miles per hour are the same unit, the statute mile an hour.

Q: Does this tool send my numbers anywhere?
A: No. The conversion runs in your browser and nothing is uploaded or stored.

Sources: NIST Guide to the SI, Appendix B.9: factors for units by kind of quantity (knot, nautical mile 1,852 m, mile per hour, km/h), checked 2026-10-13 (https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9); NIST Guide to the SI, chapter 5: 1 nautical mile = 1852 m and 1 knot = (1852/3600) m/s, checked 2026-10-13 (https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-5-units-outside-si); NIST CODATA: speed of light in vacuum 299 792 458 m/s (exact), checked 2026-10-13 (https://physics.nist.gov/cgi-bin/cuu/Value?c); NASA Glenn: the speed of sound in air depends on the temperature, a = sqrt(gamma R T), checked 2026-10-13 (https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/speed-of-sound-interactive/); Wikipedia, International Standard Atmosphere (15 °C at sea level, -56.5 °C at 11 km, ISO 2533:1975), checked 2026-10-13 (https://en.wikipedia.org/wiki/International_Standard_Atmosphere); Wikipedia, Mach number (340.3 m/s at 15 °C and mean sea level), checked 2026-10-13 (https://en.wikipedia.org/wiki/Mach_number); NASA Earth Fact Sheet: escape velocity 11.186 km/s and mean orbital velocity 29.78 km/s, checked 2026-10-13 (https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html); World Athletics: Usain Bolt's 100 m world record of 9.58 in Berlin, checked 2026-10-13 (https://worldathletics.org/news/news/bolt-again-958-world-record-in-berlin-updat)

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