Speed at sea uses its own unit. A knot is a unit of speed equal to one nautical mile per hour, approximately 1.852 kilometers per hour or 1.151 statute miles per hour.

The term comes from the old chip log, in which knots in a line were counted as they ran out over a sandglass interval. Today speed logs and GPS provide speed through the water and speed over ground in knots. Wind speed is also commonly reported in knots in marine forecasts.

Navigators think in knots and nautical miles together, so that time, speed, and distance calculations stay consistent. A ship making 15 knots covers 15 nautical miles in one hour. Confusion between knots and kilometers per hour has no place on the bridge.

• Speed unit equal to one nautical mile per hour
• Used for ships, aircraft, and wind in marine contexts
• Derived historically from the chip log
• Keeps speed and distance calculations consistent at sea