Ship Speed Knots MPH Converter

Ship speed: knots, km/h and mph

At sea and in the air, speed is measured in knots (kt), where 1 kt = 1 nautical mile per hour = 1.852 km/h = 1.151 mph. The name goes back to the old "chip log", a line knotted at fixed intervals that sailors paid out over the stern for 28 seconds. The math is straightforward: km/h = kt × 1.852 and mph = kt × 1.151. So a cargo ship at 18 kt is doing 33.3 km/h, or 20.7 mph. For context, container ships and bulk carriers cruise around 12–25 kt, nuclear submarines push past 30 kt submerged, a frigate or "naval Concorde" clears 30 kt, and the SS United States took the Blue Riband at 35.6 kt back in 1952.

Applications

ECDIS electronic chart navigation. COLREG collision-avoidance rules, which spell out speed-related give-way obligations. AIS broadcasts, which report in kt. ANTAQ port-arrival reports in Brazil. Fuel-consumption planning, where drag scales somewhere between v² and v³, so easing off from 22 to 18 kt can shave burn by 30% or more. And ETA calculations for cargo and cruise lines.

FAQ

Why not just use km/h at sea? One nautical mile is one minute of arc along a meridian, so 1 kt on a chart maps directly onto angular position. For celestial and GPS navigation, that turns out to be far handier than working in metric units.

Is "knots per hour" correct? No. A knot already means nautical miles per hour, so "knots per hour" would describe an acceleration. It's a common slip.

Are aviation knots the same? They are. 1 kt = 1.852 km/h whether you're on a ship or in a cockpit, and that shared unit is exactly why ATC and ship bridges speak the same language.