Convert any speed unit in one place
Speed is distance over time, and the world uses many units for it: scientists prefer metres per second, drivers use km/h or mph, sailors and pilots use knots, and aerospace engineers express extreme velocities as a multiple of the local speed of sound (Mach). This reference converts between all of them from a single input, and also shows the value as a fraction of the speed of light.
How it works
Every unit has an exact factor to the SI base unit, metres per second. Your input is converted to m/s first, then out to each target:
1 km/h = 0.277778 m/s (1000 ÷ 3600)
1 mph = 0.44704 m/s (1609.344 ÷ 3600)
1 knot = 0.514444 m/s (1852 ÷ 3600)
1 ft/s = 0.3048 m/s (exact foot definition)
Mach 1 = 340.29 m/s (dry air, 15 °C, sea level ISA)
c = 299,792,458 m/s (speed of light, BIPM definition)Converting through a single base avoids the error that accumulates when you multiply a chain of unit-to-unit factors.
Why Mach number is not a fixed speed
Mach number is a ratio, not a constant — it expresses a vehicle’s speed relative to the local speed of sound, which changes with air temperature and density. At sea level in the International Standard Atmosphere (15 °C, 1013.25 hPa), the speed of sound is 340.29 m/s. At a cruising altitude of about 11 km, the air temperature drops to around −56.5 °C and the speed of sound falls to roughly 295 m/s. A jet flying at Mach 0.85 covers very different ground speeds at sea level versus cruise altitude.
The value used in this tool — 340.29 m/s — is the sea-level ISA standard, appropriate for general comparisons. It is not the right figure for a high-altitude calculation.
Knots and why maritime navigation still uses them
A knot is one nautical mile per hour. Since one nautical mile is defined as exactly 1,852 metres (one arc-minute of latitude on the Earth’s meridian), a knot directly links speed to angular position on the globe. A ship or aircraft travelling at 1 knot covers one minute of latitude every hour — which makes computing range and position by dead reckoning intuitive. Knots are not an archaic unit; they are the preferred unit in aviation and maritime navigation worldwide.
Quick conversion: 1 knot ≈ 1.852 km/h ≈ 1.151 mph. In reverse, 100 km/h ≈ 54 knots.
Reference speeds for context
| Speed | m/s | km/h | mph | Mach (ISA sea level) |
|---|---|---|---|---|
| Walking pace | ~1.4 | ~5 | ~3 | ~0.004 |
| Highway (100 km/h) | 27.8 | 100 | 62.1 | ~0.082 |
| Commercial airliner | ~250 | ~900 | ~560 | ~0.85 |
| Sound (Mach 1, ISA) | 340.29 | 1225 | 761 | 1.0 |
| Low Earth orbit | ~7,900 | ~28,400 | ~17,600 | ~23 |
Practical tips
- A fast mental shortcut: 1 m/s is exactly 3.6 km/h. Divide km/h by 3.6 to get m/s.
- mph to km/h: multiply by 1.609344 (exact). A rough check: double and subtract 20% — 60 mph × 2 = 120, minus 12 = ~96 km/h (actual: 96.56 km/h).
- For aviation fuel planning, pilots typically work in knots; airspeed indicators, flight plans, and weather reports all use knots, so convert ground speed into knots rather than km/h when working with aviation data.