It is never a straight line
A great-circle line between two ports is the shortest theoretical path on a sphere — and it routinely runs straight through land. Real sea distance is the sum of great-circle legs between the ports and every chokepoint the voyage actually has to pass through, which is a materially different, and usually longer, number.
The chokepoints that decide the answer
A small set of straits and canals connects the world's ocean basins, and which one a voyage uses is the single largest variable in a distance calculation.
- Suez Canal and Bab-el-Mandeb — Europe/Mediterranean to Asia and the Gulf
- Cape of Good Hope — the alternative to Suez, materially longer
- Panama Canal and Cape Horn — Atlantic to Pacific
- Strait of Malacca — Indian Ocean to the South China Sea
- Strait of Hormuz — into and out of the Arabian Gulf
- Strait of Gibraltar and the Dover Strait — Mediterranean and Northern Europe
Why the canal-vs-Cape choice matters so much
Singapore to Rotterdam is roughly 7,900 nautical miles via the Suez Canal. Via the Cape of Good Hope, the same voyage is around 10,400 nautical miles — a 30% difference on one routing decision. Any distance figure quoted without stating which of these was assumed is not a comparable number, regardless of how precisely it is stated.
The other two reasons calculators disagree
Beyond the routing assumption, two further sources of disagreement are structural rather than accidental. First, the port reference point: a berth, a pilot boarding ground and a port-limit position for the same port can differ by tens of miles, and providers rarely publish which one they use. Second, coastal correction: pure great-circle routing cuts across land, so every provider inserts waypoints to prevent that — and no two providers use exactly the same waypoint set, so the legs differ even when the routing assumption is identical.
What a distance figure is useful for
Planning-grade distance figures are appropriate for voyage estimation, freight and bunker budgeting, and laycan feasibility. They are not appropriate for a contractual distance under a charter party, for navigational passage planning, or for a demurrage or deviation claim — those require the distance table named in the contract, or a licensed navigational routing source, not a general-purpose calculator.
Zeaclub publishes a free port-to-port distance calculator that shows the routing behind the figure.
Frequently asked questions
Why do two sea distance calculators give different answers for the same route?
Three compounding reasons: the routing assumption (canal versus Cape can differ by thousands of miles), the port reference point used (berth vs. pilot station vs. port limits), and different waypoint sets used for coastal correction. All three vary by provider.
How much longer is the Cape of Good Hope route than Suez?
On a lane like Singapore to Rotterdam, roughly 30% longer — about 10,400 nautical miles via the Cape versus about 7,900 via Suez.
Can I use a planning distance figure for a charter party?
No. Planning-grade distances from a general calculator are for voyage estimation and budgeting. A contractual distance should come from the distance table named in the charter party itself.
How is sailing time calculated from distance?
Hours equal nautical miles divided by speed in knots, since one knot is one nautical mile per hour. That figure is steaming time only and excludes port time, canal waiting, weather deviation and any ordered speed changes.