Speed & Velocity Converter
Convert speed and velocity between metres per second, kilometres per hour, miles per hour, feet per second, knots and Mach. Every conversion uses an exact defined ratio — a knot is exactly one nautical mile per hour, and mph and km/h derive from the exact 1959 international foot and mile. Mach is included on the standard-atmosphere basis (the speed of sound at 15 °C, sea level, about 340.29 m/s); the real speed of sound falls as air gets colder, so a given Mach number is a fixed fraction of that reference here rather than a live atmospheric calculation.
How to use this tool
- Enter a speed value.
- Choose the unit you are converting from.
- Choose the unit you want the answer in.
- Read the converted speed; switch on “show every equivalent unit” to see all of them at once.
- For Mach, note the result assumes the standard-atmosphere speed of sound at sea level.
The formula
Speed is distance over time, so its base unit is the metre per second. Every other speed unit is a fixed multiple of it, and the metric and imperial definitions are exact, so only Mach — tied to the speed of sound — carries an assumption.
metres_per_second = value × (m/s per unit)
result = m/s ÷ (m/s per target)- knot
- One nautical mile (1852 m) per hour
- Mach
- Speed ÷ speed of sound (340.29 m/s at ISA sea level)
Multiplying m/s by 3.6 gives km/h, since an hour has 3600 seconds and a kilometre has 1000 metres.
Worked examples
Motorway speed
- Given
- 100 km/h
- Result
- ≈ 62.14 mph
One kilometre per hour is 0.621 mph, so 100 km/h is about 62 mph.
Aircraft cruise
- Given
- Mach 0.8 at sea-level ISA
- Result
- ≈ 272 m/s
At the standard speed of sound (340.29 m/s), Mach 0.8 is about 272 m/s — though at altitude the true speed is lower.
Wind speed at sea
- Given
- 20 knots
- Result
- ≈ 37.04 km/h
A knot is one nautical mile per hour, so 20 knots is about 37 km/h.
Frequently asked questions
Speed is how fast something moves; velocity is speed together with direction. As a magnitude they convert identically, which is what this tool does — the units are the same whether or not you are tracking direction.
A knot is defined as one nautical mile (exactly 1852 metres) per hour. That gives 1.852 km/h exactly, which looks untidy in metric but is a clean definition rooted in a minute of latitude at sea.
Mach number is speed divided by the local speed of sound, and the speed of sound depends mainly on air temperature. This tool uses the standard-atmosphere sea-level value (340.29 m/s at 15 °C). Higher up, where the air is colder, the same true speed is a higher Mach number.
The metric and imperial ratios are exact by definition, since the metre, foot, mile and nautical mile all have exact fixed relationships. Only Mach carries an assumption — the reference speed of sound — which the tool states.
Multiply by 3.6. One metre per second is 3.6 kilometres per hour, because there are 3600 seconds in an hour and 1000 metres in a kilometre. So 10 m/s is 36 km/h.