Speed Converter (km/h, mph, m/s, knot, Mach)
Type a speed and pick the source unit; see all other speed units update at once. Includes Mach (sea-level standard atmosphere) for aviation context.
How it works
How speed conversion works
All units pivot through meters per second internally. 1 km/h = 1/3.6 m/s, 1 mph = 0.44704 m/s (exactly), 1 knot = 1852/3600 m/s. These factors are derived from exact length and time definitions, so the conversion is numerically exact.
Mach is special: it's a ratio to the local speed of sound, which depends on temperature and altitude. We use 343 m/s (sea level, 20°C) for the conversion. Real-world Mach numbers in aviation differ slightly because cruise altitudes are colder.
Where each unit appears
Vehicles: km/h in most of the world, mph in the US and UK. Speed limits, speedometers, and vehicle specs use these.
Aviation: knots (1 nautical mile per hour ≈ 1.852 km/h) for ground speed and indicated airspeed. Mach for jets at high altitude.
Marine: knots for ship and boat speed, related to nautical miles for navigation.
Physics and engineering: m/s as the SI base unit. Wind speed in meteorology often uses m/s in scientific contexts and km/h or mph in consumer weather apps.
Useful reference points
Walking: ~5 km/h ≈ 3 mph ≈ 1.4 m/s. Running: 10 km/h ≈ 6 mph for a moderate jog. Highway driving: 100 km/h ≈ 62 mph ≈ 28 m/s. Cruising airliner: 850 km/h ≈ 530 mph ≈ Mach 0.78. Speed of sound: 343 m/s ≈ 1235 km/h ≈ Mach 1.0. Speed of light: 299,792,458 m/s exactly.
Frequently asked questions
›Is the Mach value altitude-dependent in real life?
Yes. Sound travels slower in colder, thinner air. We use 343 m/s (sea level, 20°C). At 35,000 ft cruise altitude, 1 Mach ≈ 295 m/s. Real cockpit displays compute it from local conditions.
›What's a knot exactly?
1 knot = 1 nautical mile per hour = 1852/3600 m/s ≈ 0.5144 m/s ≈ 1.852 km/h ≈ 1.151 mph. Standardized internationally since 1929.
›Why do aviation and marine use knots?
Nautical miles align with the geometry of the Earth (1 minute of latitude). Navigation calculations are simpler when speed and distance share the system.
›Are these conversions exact?
All except Mach. The non-Mach units are linked by exact integer ratios (km/h, mph, ft/s, knot). Mach depends on local temperature.
›What's the speed of light in km/h?
299,792,458 m/s × 3.6 = 1,079,252,848.8 km/h. The base unit is exact (defined this way since 1983); km/h is its conversion.
›How do I find my running pace?
Pace is time per distance, not speed. For a 5 min/km pace, that's 12 km/h or 7.46 mph. Divide 60 by pace minutes to get km/h.
›Is this accurate enough for aviation?
For learning and quick conversions, yes. For flight planning, use certified avionics.
›Does the data leave my browser?
No. All math runs locally.
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