Speed Converter
Convert speed between mph, km/h, m/s, ft/s, knots, and Mach — the units of driving, running, sailing, aviation, and physics.
How to use this calculator
- 1Enter the speed value.
- 2Choose the unit it's currently in.
- 3Read the equivalent in mph, km/h, m/s, ft/s, knots, and Mach.
- 4Use metres per second as the common base for any physics work.
How it works
Speed conversion
1 mph = 0.44704 m/s, 1 km/h = 0.2778 m/s 1 knot = 0.5144 m/s (1 nautical mile per hour) 1 m/s = 3.6 km/h, 60 mph ≈ 96.6 km/h Mach = speed ÷ local speed of sound (~343 m/s)
Speed is distance divided by time, and it is measured in many units because different activities adopted their own. Cars are quoted in miles per hour or kilometres per hour depending on the country, physics uses metres per second as the SI standard, ships and aircraft use knots (nautical miles per hour), and high-speed flight uses Mach, a ratio to the speed of sound. Converting between any of them is done by routing through a common base — metres per second here — multiplying the input by its value in metres per second, then dividing by the target unit's value. A knot is worth knowing about because it descends from navigation: it is one nautical mile per hour, and a nautical mile is based on the geometry of the Earth, one minute of latitude, which is why it persists at sea and in aviation. Mach is unusual in that it is not a fixed speed at all but a comparison to the local speed of sound, which varies with temperature and altitude, so a given Mach number corresponds to different absolute speeds under different conditions.
Worked example
60 miles per hour converts to about 96.6 kilometres per hour, 26.8 metres per second, 88 feet per second, 52.1 knots, and Mach 0.078. The tidy 88 feet per second is why 60 mph is a common reference in physics problems — it is exactly 88 ft/s.
Speed Converter: the complete guide
Why so many units of speed
Speed is one of the most everyday physical quantities, and precisely because it matters in so many different domains, it accumulated a variety of units. Road speed is measured in miles per hour in the United States and the United Kingdom but kilometres per hour across most of the world, a split that causes constant confusion for travellers and requires conversion whenever a speed limit or a car's speedometer crosses a border. Science and engineering standardised on metres per second, the SI unit, because it combines cleanly with other metric units in equations. Each community picked the unit that suited its scale and history.
The maritime and aviation worlds went their own way with the knot, and high-speed flight with Mach, for reasons rooted in their specific needs. This variety means that converting speed is a genuinely common task — a pilot reading airspeed in knots, an engineer working in metres per second, a driver watching a speedometer in kilometres per hour, and a physics student solving a problem in feet per second may all be describing the same motion in different tongues. Routing every conversion through a single base unit, metres per second, makes translating between them straightforward, which is exactly what a speed converter does.
The knot and the nautical mile
The knot stands apart from other speed units because of its elegant connection to the geometry of the Earth. A knot is one nautical mile per hour, and a nautical mile is defined as one minute of arc of latitude — one sixtieth of a degree — along a meridian. This means that on a nautical chart, distance and angle are directly linked: sailing one nautical mile changes your latitude by one minute of arc. This built-in relationship to the Earth's coordinate grid made the nautical mile, and the knot, indispensable for navigation long before satellites, and it is why they remain the standard at sea and in the air.
The unit's name comes from an old method of measuring a ship's speed. Sailors would trail a rope with knots tied at regular intervals and a weighted board at the end; as the board dragged in the water, they counted how many knots ran out in a fixed time measured by an hourglass. The number of knots that passed gave the speed directly, and the term stuck. Today a knot is about 1.15 miles per hour or 1.85 kilometres per hour, and pilots and sailors think fluently in it, but for anyone outside those worlds it is one of the units most in need of conversion, which is why it is included here.
Mach: speed relative to sound
Mach number is the most conceptually different of the speed units, because it is not an absolute speed at all but a ratio. Mach 1 is the speed of sound, Mach 2 is twice the speed of sound, and so on. This matters for aircraft because the aerodynamic behaviour of an aircraft changes dramatically as it approaches and exceeds the speed of sound — shock waves form, drag rises sharply, and the physics of flight shifts. What matters for these effects is not the absolute speed but the speed relative to sound, which is exactly what Mach captures, making it the natural unit for high-speed aviation.
The subtlety is that the speed of sound is not constant. It depends on the temperature of the air, falling as temperature drops, which means it decreases with altitude up through most of the atmosphere. So an aircraft flying at a fixed Mach number is travelling at different absolute speeds depending on how high and how cold it is — Mach 0.85 at cruising altitude is a slower true speed than Mach 0.85 near the ground, because the air is colder and the speed of sound lower up high. This is why converting Mach to an absolute unit like miles per hour requires knowing the conditions; the calculator uses a representative sea-level value of about 343 metres per second. Understanding that Mach floats with the local speed of sound, rather than fixing a set speed, is the key to reading it correctly.
Frequently asked questions
How do I convert mph to km/h?
Multiply miles per hour by 1.609. So 60 mph is 60 × 1.609 ≈ 96.6 km/h. To go the other way, divide km/h by 1.609. The exact factor comes from a mile being 1.609 kilometres.
What is a knot?
A knot is one nautical mile per hour, used at sea and in aviation. A nautical mile is one minute of latitude, linking it to the Earth's geometry. One knot is about 1.15 mph or 1.85 km/h. The name comes from the knotted rope sailors once used to measure a ship's speed.
Is Mach a fixed speed?
No — Mach is a ratio to the local speed of sound, which changes with temperature and altitude. Mach 1 is the speed of sound, roughly 343 m/s (about 1,235 km/h) at sea level, but less at high altitude where the air is colder. So a Mach number corresponds to different absolute speeds under different conditions.
Why is 60 mph exactly 88 feet per second?
Because 60 miles is 316,800 feet, and an hour is 3,600 seconds, giving 316,800 ÷ 3,600 = 88 ft/s exactly. This tidy relationship makes 60 mph a favourite reference speed in physics problems and traffic engineering.