Sea distance calculator
Pick two RoRo ports and a speed. See the distance by sea in nautical miles, the days at sea, and the route, including the canals and straits it passes.
Pick a port of departure and a port of arrival.
For planning: the shortest safe sea route, not a navigation route. The map line shows the general route. Sea routes: Eurostat SeaRoute (EUPL-1.2). Coastline: Natural Earth.
How the distance is worked out
The distance is the shortest safe sea route between the two ports: the same kind of figure the US National Geospatial-Intelligence Agency publishes in its Pub. 151 distance tables. Near the coast the route keeps a few miles off, further out it joins the shipping lanes Eurostat mapped from vessel traffic, and it is then straightened wherever a direct line stays at sea. It never uses the Corinth Canal, and goes round Denmark rather than through the Kiel Canal or the shallow southern Sound, which large car carriers cannot use.
The result is checked against 386 reference routes from Pub. 151. The median difference is 2.4 percent, and 83 percent of routes come within 3 percent on ocean passages of 500 miles or more, or within 10 percent (or 10 miles) on shorter hops.
Use it to compare routings, quote transit times and plan rotations. It doesn't know about weather, traffic separation schemes, pilot stations or the routing a master chooses on the day, so the sailed distance can differ.