A forecast says 12 m/s, the harbourmaster says 23 knots, the road sign thinks in km/h, and the American sailing podcast quotes mph — all four are the same wind. Speed is one physical quantity wearing four different uniforms, and every conversion between them is exact, because each unit is defined as a fixed ratio to the SI metre. This converter moves one value between knots, mph, km/h, ft/s, and m/s simultaneously, so the numbers always reconcile.
The exact factors (no approximations needed)
Every unit here has an exact definition traceable to the metre — the factors below are definitions, not measurements:
1 nautical mile = 1852 m (exact, international definition)
1 statute mile = 1609.344 m (exact, international yard & pound agreement)
1 foot = 0.3048 m (exact)
1 knot = 1852 ÷ 3600 = 0.514444… m/s
1 mph = 1609.344 ÷ 3600 = 0.44704 m/s (exact)
1 km/h = 1000 ÷ 3600 = 0.277778… m/s
1 ft/s = 0.3048 m/s (exact)The tool converts your input to m/s once, then back out to each unit, so round-trips are lossless: 10 knots → km/h → knots returns exactly 10. The factor tables published in NIST Special Publication 811 are the authoritative reference for all of these.
Cross-reference table
| Description | Knots | mph | km/h | m/s |
|---|---|---|---|---|
| Light breeze (Beaufort 2) | 5 | 5.8 | 9.3 | 2.6 |
| Strong breeze (Beaufort 6) | 24 | 28 | 44 | 12.3 |
| Gale (Beaufort 8) | 37 | 43 | 69 | 19 |
| Hurricane threshold | 64 | 74 | 119 | 33 |
| Cruising yacht | 6–8 | 7–9 | 11–15 | 3–4 |
| Container ship | 20–24 | 23–28 | 37–44 | 10–12 |
| Airliner cruise (TAS) | ~480 | ~552 | ~889 | ~247 |
The 64-knot hurricane threshold is the operational definition used by the NOAA National Hurricane Center for the Saffir-Simpson scale — a good example of knots serving as the primary unit even in a scientific context.
Where the knot comes from — and why it survives
The name is literal: sailors paid out a log-line knotted at fixed intervals and counted knots slipping through their hands against a sandglass. The modern definition is tidier — one nautical mile per hour, where the nautical mile (1852 m exactly) approximates one minute of latitude. That link to the geometry of the globe is why aviation and shipping never metricated their speeds: on a chart, 60 knots for one hour ≈ one degree of latitude, distances measure directly with dividers, and great-circle calculations stay clean. A unit that makes navigation arithmetic easy beats a unit that is merely decimal.
Metres per second rules science and meteorology (wind profiles, turbulence, physics), km/h covers roads and public forecasts in most of the world, mph persists on US and UK roads, and ft/s appears in US ballistics and hydraulics. A UK pilot genuinely uses three of these in one day: knots for airspeed, mph on the drive home, m/s in the METAR-adjacent gust discussion.
Mental-math anchors worth memorising
- knots → mph: add 15% (20 kn ≈ 23 mph)
- knots → km/h: just under double (30 kn ≈ 55.6 km/h; ×1.852 exactly)
- m/s → km/h: multiply by 3.6 exactly (10 m/s = 36 km/h)
- m/s → knots: roughly double (10 m/s ≈ 19.4 kn)
- mph → ft/s: add ~47% (60 mph = 88 ft/s exactly — the old driving-test chestnut)
A worked chain: a 25-knot wind is 25 × 0.514444 = 12.86 m/s, ×3.6 = 46.3 km/h, ÷1.609344 km/mi = 28.8 mph, and ×1.46667 = 42.2 ft/s. Each step uses an exact factor, which is why every route between units gives the same answer.
Edge cases and gotchas
Airspeed flavours: aviation quotes indicated (IAS), calibrated (CAS), and true airspeed (TAS) — all in knots but numerically different; this converter changes units, not airspeed types. Mach number is not a fixed speed: sound at 20 °C sea level is ≈ 343 m/s (~667 kn), but it falls with temperature, so Mach conversions need altitude context. Historical UK/US nautical miles differed slightly (6,080 ft in the UK Admiralty version) before the 1929/1954 international adoption of 1852 m — old texts may not match modern conversions exactly. Wind averaging differs by agency (a 10-minute mean in WMO practice vs 1-minute sustained for NHC), so two agencies can report different numbers for the same storm before any unit conversion — check the averaging period, not just the unit.