Ocean Crossing Time Knots Calculator

Ocean crossing time from knots

Crossing time is plain uniform motion: t(h) = d(nm) / v(kt), and you divide by 24 to get days. Take the North Atlantic: Lisbon to New York is roughly 3,000 nm, so at 18 kt you get 166.7 h, about 7 days. On a South Atlantic / coastal run, Salvador to Lisbon is around 4,000 nm, and at 22 kt that works out to 181.8 h, roughly 7.5 days. Keep in mind the model holds speed-over-ground constant. Real voyages have to deal with weather routing, where Beaufort 6+ can shave off 1–2 kt; ocean currents, since the Gulf Stream gives you up to 4 kt eastbound and takes the same back westbound; port pilotage; slow-steaming to save fuel; and queues at the Suez and Panama canals.

Applications

It comes up for international e-commerce ETAs on the sea-freight legs of Shein, AliExpress or Temu orders; for shipper bookings with Maersk, MSC and CMA CGM, whose voyage schedules publish "transit days" off this same arithmetic; for commodity traders sizing laycan windows on grain and iron-ore shipments; for cruise itinerary planning; for lay-day demurrage math; and for logistics dashboards comparing sea versus air freight on a cost-per-day basis.

FAQ

Why does my Maersk schedule show longer? Carriers tack on port calls (Santos, Algeciras and Rotterdam each eat up 1–2 days), leave a buffer for weather, and run "slow steaming" at 14–16 kt to burn less bunker fuel. Door-to-door always runs longer than pure sea time.

Should I use SOG or STW? Use speed-over-ground (SOG) for an ETA, because it already accounts for current. Speed-through-water (STW) tells you about engine performance instead.

Are great-circle and rhumb-line times different? Yes. The great-circle route is shorter on transoceanic east-west legs, so at the same speed it can knock 1–3% off the time.