Nautical Distance & ETA Calculator
Measure the great-circle distance between two positions in nautical miles, then turn a distance and a speed of advance into a passage time and an ETA.
Distance, speed, and time at sea
The whole of passage planning turns on one relationship: time equals distance over speed. Get a good distance from the chart and an honest speed of advance, and the ETA falls out of it. The great-circle figure gives you the shortest possible distance between two positions; your own routeing then adds the miles the real world demands.
Working from a forecast wind or a log speed in another unit? Convert it first with the Knots Speed Converter, then bring the knots back here.
Frequently Asked Questions
How is the distance between two positions calculated?
It uses the haversine formula for the great-circle distance — the shortest path over the surface of a sphere — with the Earth's mean radius expressed in nautical miles (about 3,440 nm). You enter each position in decimal degrees, with south latitudes and west longitudes as negative numbers, and the result comes back in nautical miles, kilometres, and statute miles.
Why is the great-circle distance shorter than the route I actually steam?
The great-circle track is the geometric shortest path and ignores the real world. An actual passage follows rhumb-line legs between waypoints, keeps clear of land and shoal water, respects traffic-separation schemes, and diverts through canals and channels — all of which add miles. Treat the great-circle figure as the theoretical minimum and add your routeing allowance on top.
What is 'speed of advance' and how does it change the ETA?
Speed of advance (SOA) is the average speed you expect to make good over the whole passage, allowing for weather, current, and any slow steaming — not the instantaneous speed through the water. Time equals distance divided by SOA, so 3,000 nautical miles at 14 knots is about 8 days 22 hours. Use a realistic SOA, because a knot or two makes hours of difference over an ocean passage.
Can I chain this with the speed converter?
Yes. If your forecast or log gives a speed in km/h or mph, convert it to knots first with the Knots Speed Converter, then enter that figure as the speed of advance here so the distance in nautical miles divides cleanly into a time.
Great-circle distances and simple time/speed estimates for planning and learning. Not an approved passage plan — appraise and plan every passage on the ship’s charts and publications.