Speed Converter
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By Prof. Richard FosterReviewed by Dr. Marcus Webb, PhD (Aerospace Engineering)Published: Updated:
References
- [1]National Institute of Standards and Technology (NIST), Guide for the Use of the International System of Units (SI), 2008. https://physics.nist.gov/cuu/pdf/sp811.pdf
- [2]Bureau International des Poids et Mesures (BIPM), The International System of Units (SI) – 9th Edition, 2019. https://www.bipm.org/en/publications/si-brochure
- [3]International Organization for Standardization, ISO 80000-3: Quantities and Units – Part 3: Space and Time, 2019.
How to Use?
- 1
Enter the speed value
Type the numeric speed value into the Speed Value field.
- 2
Select the source unit
Choose the unit of your input value from the unit dropdown: m/s, km/h, mph, knots, or Mach.
- 3
Choose the display unit
Use the Display Result In dropdown to select which unit appears highlighted as the primary result.
- 4
Review all equivalent values
The result panel shows your value in the chosen display unit, with all other supported units listed as alternatives below.
- 5
Cross-check with manual calculation
Use the reference tables in the description section to verify key factors: 1 km/h = 0.278 m/s, 1 mph = 1.609 km/h, 1 knot = 1.852 km/h.
- 6
Account for Mach altitude variation
Remember that Mach 1 = 343 m/s at sea level only. At 11,000 metres, Mach 1 equals approximately 295 m/s. This converter uses the sea-level reference.
- 7
Use related converters
For distance or speed-time-distance calculations, use the Length Converter linked on this page. The Pressure Converter helps with aviation-related atmospheric calculations.
Worked Examples
1Convert 100 km/h to mph
Given Values
Speed Value:100
speedValue_unit:km/h
Results
Result:27.7778
2Convert 30 knots to km/h
Given Values
Speed Value:30
speedValue_unit:kn
Results
Result:15.4333
3Convert Mach 2 to m/s
Given Values
Speed Value:2
speedValue_unit:Mach
Results
Result:686
4Convert 10 m/s to km/h
Given Values
Speed Value:10
speedValue_unit:m/s
Results
Result:10
5Convert 500 knots to km/h
Given Values
Speed Value:500
speedValue_unit:kn
Results
Result:257.222
6Convert Mach 0.85 to mph
Given Values
Speed Value:0.85
speedValue_unit:Mach
Results
Result:291.55
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Frequently Asked Questions
Mach number is the ratio of an object's speed to the local speed of sound. Mach 1 at sea level and 20 °C equals 343 m/s or approximately 1,235 km/h. The Mach number is the standard speed reference in high-speed aeronautics because it determines the flow regime around the aircraft, affecting lift, drag, and structural loads.
One knot equals one nautical mile per hour, or 1.852 km/h exactly. Knots are the standard speed unit in aviation and maritime because the nautical mile corresponds to one minute of latitude, making chart-based navigation simpler. A vessel cruising at 20 knots travels about 37 km/h or 23 mph.
Divide by 3.6. For example, 100 km/h divided by 3.6 equals approximately 27.78 m/s. To convert in the opposite direction, multiply m/s by 3.6 to get km/h. This factor of 3.6 comes from 1,000 metres per kilometre divided by 3,600 seconds per hour.
Knots are based on nautical miles, which correspond to one minute of latitude. This makes navigation calculations simpler when plotting courses on charts — one minute of latitude equals one nautical mile of distance. Airspeed in knots, combined with distance in nautical miles, gives flight times in hours directly without unit conversion.
Yes. The speed of sound decreases with altitude as temperature drops. At sea level at 20 °C, Mach 1 = 343 m/s. At 11,000 metres where the temperature is approximately −56.5 °C, Mach 1 = about 295 m/s. This converter uses 343 m/s as the sea-level reference. Professional aviation uses altitude-compensated Mach conversion tables from the aircraft flight manual.
Speed is a scalar quantity that measures how fast something is moving — it has magnitude only. Velocity is a vector quantity that includes both speed and direction of travel. This converter handles speed only. For velocity calculations in physics problems, you need to account for direction using vector mathematics.
The speed of light in a vacuum is exactly 299,792,458 m/s, which equals approximately 1,079,252,849 km/h or about 671 million mph. This universal constant is the maximum speed at which any information or matter can travel, as established by Einstein's theory of special relativity.
The Beaufort scale is an empirical measure based on observed sea conditions: Force 0 (calm, <1 knot), Force 3 (gentle breeze, 7-10 knots), Force 6 (strong breeze, 22-27 knots), Force 8 (gale, 34-40 knots), Force 10 (storm, 48-55 knots), Force 12 (hurricane, 64+ knots). Use this converter to get the exact km/h or mph equivalent for any Beaufort force number.
The fastest land speed record is 1,227.985 km/h (763.035 mph), set by Andy Green in the Thrust SSC on 15 October 1997. This is approximately Mach 1.02 at the desert altitude where the record was set. Thrust SSC was the first land vehicle to officially break the sound barrier. Wheel-driven records are slower, with the current record at approximately 737 km/h (458 mph).
Police radar guns use the Doppler effect — they transmit a radio wave at a known frequency, which reflects off the moving vehicle at a shifted frequency. The frequency shift is proportional to the vehicle's speed relative to the radar gun. The gun calculates speed using the formula Δf = 2v/c × f₀, where v is vehicle speed, c is the speed of light, and f₀ is the transmitted frequency. The result is displayed in the local speed unit (mph in the US, km/h in most other countries).
Ground speed is the speed of an aircraft relative to the ground, measured by GPS. Airspeed is the speed relative to the surrounding air, measured by the pitot-static system. Airspeed determines aerodynamic performance (lift, stall speed), while ground speed determines navigation progress. A headwind reduces ground speed below airspeed; a tailwind increases it. The difference can be 100+ knots in strong jet stream conditions.
Modern commercial jetliners cruise at Mach 0.78 to 0.90, which corresponds to approximately 830 to 960 km/h (515 to 596 mph) true airspeed at typical cruise altitudes of 30,000 to 40,000 feet. The Boeing 787 cruises at Mach 0.85 (about 903 km/h or 561 mph). The Airbus A380 cruises at Mach 0.85 as well. Ground speed varies with wind conditions and can exceed 1,100 km/h with a strong tailwind.
Pace (minutes per kilometre or mile) and speed (km/h or mph) are reciprocals. To convert pace to speed, divide 60 by the pace in minutes. A pace of 5 min/km equals 60 / 5 = 12 km/h (about 7.5 mph). To convert speed to pace, divide 60 by the speed. Running at 10 km/h gives a pace of 60 / 10 = 6 min/km. Most running apps and treadmills display both, but this converter helps when you need to switch between the two conventions.
Earth's escape velocity is approximately 11.2 km/s (40,320 km/h or 25,054 mph). This is the minimum speed an object needs to escape Earth's gravitational pull without further propulsion. For comparison, low Earth orbit requires about 7.8 km/s (28,080 km/h or 17,450 mph). Using this converter, you can see that escape velocity equals about Mach 32.7 at sea level, though the concept of Mach number becomes less meaningful at altitudes where the atmosphere is thin.
Speed limits vary significantly by country and road type. In the European Union, motorway limits range from 110 km/h (68 mph) in Norway and Cyprus to 140 km/h (87 mph) in Poland and Bulgaria. The US interstate speed limit is typically 65 to 80 mph (105 to 129 km/h). Germany's Autobahn has no general speed limit on many sections, though 130 km/h (81 mph) is the advisory speed. Australia uses km/h with motorway limits of 100 to 130 km/h. The UK uses mph with 70 mph (113 km/h) on motorways. When driving abroad, use this converter to translate speed limit signs into familiar units.
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