Sea Distance Calculator

Ships can’t sail through continents, so sea distance depends on the route. Pick a departure and arrival port, add the canals, straits and capes you’ll pass, and this calculator adds up the legs in nautical miles and tells you how many days the voyage takes at your chosen speed.

Nautical miles and knots

A nautical mile is 1,852 metres — about 1.15 statute miles — and was originally one minute of latitude, which makes it convenient on charts: one degree of latitude is 60 nautical miles. A knot is one nautical mile per hour. A ship at 14 knots covers 336 nautical miles a day; at 20 knots, 480.

Why the route matters

The shortest path between two points on a globe is a great circle, but at sea that line often crosses land. A voyage from Europe to Asia must go through the Mediterranean and the Suez Canal, or all the way around Africa via the Cape of Good Hope; from the US East Coast to the West Coast, through the Panama Canal or around South America. Add waypoints at the canals, straits and capes you will pass, in order, and the calculator sums the legs between them.

Worked example

Rotterdam to Singapore through the English Channel, the Strait of Gibraltar, the Mediterranean, the Suez Canal, the Red Sea and the Malacca Strait sums to about 8,273 nautical miles with the example waypoints, compared with a great-circle distance of 5,711 nm straight across Europe and Asia. At 14 knots, plus a 5% allowance for routing and weather, the voyage takes about 25.9 days, excluding the Suez Canal transit and any waiting time.

Canal and strait shortcuts

Use the calculator to compare: build the route both ways and see the difference in days at your speed.

Great circle vs rhumb line

A great-circle route is the shortest path on a sphere but constantly changes compass heading; a rhumb line keeps one heading and appears straight on a Mercator chart but is longer, especially on east–west passages at high latitudes. Across the North Pacific the difference can be a few hundred nautical miles, which is why ocean crossings usually follow a great circle broken into rhumb-line legs between waypoints. Near the equator, or on mainly north–south passages, the two are almost identical.

Speed, time and fuel

Fuel use rises steeply with speed — for many ships, roughly with the cube of speed per hour of steaming — so slowing down a few knots can cut fuel per voyage substantially even though the trip takes longer. Enter your vessel’s daily fuel use to estimate the total for the passage.

Time zones at sea

Voyage planning is normally done in UTC to avoid confusion as a ship crosses time zones. The ETA here is in UTC; convert it to the destination’s local time for port bookings. Ships’ clocks are usually adjusted by an hour at a time on long passages, and crossing the International Date Line changes the date.

Planning a voyage

• Start from the departure port and add waypoints in the order you will pass them.
• Add an allowance for real routing — traffic separation schemes, pilot stations and weather.
• Include canal transit and waiting times separately; they can add a day or more.
• Check fuel range and bunkering ports on long legs.
• Use official charts and sailing directions for actual navigation.

What’s in the template pack

• Passage plan template with waypoint table (lat/long, course, distance, ETA)
• Voyage log spreadsheet with daily noon positions and distance run
• Fuel and speed calculator workbook
• Port call checklist
• Nautical mile and knot conversion card

The calculator is free; the pack is a one-time download.

AI-assisted content

This page and the templates were drafted with AI assistance and reviewed by Kedop. Port and waypoint positions are approximate and the results are for planning and education — not for navigation.