Knots, mph and km/h Compared

• Meter per second (m/s): the SI unit of speed.
• Kilometer per hour (km/h): 1000 meters in 3600 seconds, so 1 m/s = 3.6 km/h exactly.
• Mile per hour (mph): one international mile, exactly 1609.344 m, per hour.
• Knot (kn): one nautical mile per hour. The nautical mile is exactly 1852 m.

All four are defined exactly in meters and seconds, so every conversion between them is exact. Rounding only happens when a result is displayed.

The nautical mile was originally linked to one minute of arc of latitude, which makes it practical for navigation with charts. Today it is simply defined as 1852 m, about 1.15078 statute miles.

So one knot is about 1.852 km/h, 1.15078 mph or 0.514444 m/s.

• Roads: km/h in most countries; mph in the United States and the United Kingdom.
• Aviation and shipping: knots for airspeed, ground speed and boat speed, and nautical miles for distance.
• Science and engineering: m/s, the SI unit.
• Wind: weather services use km/h, mph, m/s or knots depending on the country and the audience. Marine and aviation forecasts usually give knots.

• m/s to km/h: multiply by 3.6. This one is exact.
• mph to km/h: multiply by 1.6. The result is about 0.6% too low.
• km/h to mph: multiply by 5 and divide by 8. The result is about 0.6% too high.
• Knots to mph: multiply by 1.15. The result is less than 0.1% too low.
• Knots to km/h: multiply by 2 for a rough value. The result is about 8% too high. Multiplying by 1.85 instead is only about 0.1% too low.
• m/s to mph: multiply by 2.24. The result is about 0.1% too high.

Use shortcuts for estimates only. For speed limits, flight planning or engineering, use the exact converter.

• Saying "knots per hour". A knot is already a speed, one nautical mile per hour, so "per hour" is doubled.
• Mixing up the nautical mile (1852 m) and the statute mile (1609.344 m). Distances on nautical and aviation charts are in nautical miles.
• Treating km/h and kph as different units. Both mean kilometers per hour; km/h is the SI-style symbol.
• Forgetting feet per second. It appears in US engineering and ballistics: 1 ft/s is exactly 0.3048 m/s, or about 0.681818 mph.

• A speed limit of 60 mph is 96.5606 km/h.
• A boat doing 25 knots moves at 46.3 km/h or 28.7695 mph.
• A wind of 15 m/s is 54 km/h, 33.554 mph or 29.1577 knots.