Speed & velocity: the exact defined values
1 mph = 0.447 04 m/s exactly and 1 knot = 1852/3600 m/s, both following from exact length definitions.
How the base unit is defined
The metre per second derives from the metre and the second, both fixed by defining constants.
Exact values table
"Exact" means the value is fixed by definition and carries no measurement uncertainty.
| Quantity | Exact value | Notes |
|---|---|---|
| 1 mile per hour | 0.44704 m/s ≈ 1.609344 km/h | 1609.344 m ÷ 3600 s. Exact. Open converter → |
| 1 kilometre per hour | 1/3.6 m/s ≈ 0.2777… m/s | 1000 m ÷ 3600 s. Repeating decimal, so shown rounded. Open converter → |
| 1 knot | 1852/3600 m/s ≈ 0.514444 m/s | One nautical mile per hour; the nautical mile is exactly 1852 m. Open converter → |
| Speed of light in vacuum | 299792458 m/s | A fixed defining constant of the SI — it has no uncertainty because the metre is defined from it. |
Rounding traps that actually change your result
Knots are not 'nautical km/h'
A knot is one nautical mile per hour = 1.852 km/h exactly. Reading a knot as a km/h value understates speed by about 85%, which matters for aviation and marine figures.
Where these values come from
- BIPM SI Brochure, 9th edition
Defining constants including the speed of light.
- NIST Special Publication 811
Speed conversion factors, including mph to m/s as exactly 0.44704.
Frequently asked questions
Is 1 mph exactly 0.44704 m/s?
Yes. A mile is exactly 1609.344 m and an hour is exactly 3600 s, so 1609.344 / 3600 = 0.44704 m/s with no rounding.
How fast is one knot in km/h?
Exactly 1.852 km/h, because the nautical mile is defined as exactly 1852 metres.
Convert speed & velocity now
These converters use the exact factors listed above.