The Unit Sea Boundaries and Passage Plans Are Written In
Nothing at sea is measured in kilometres. A country's territorial waters run out twelve nautical miles from the baseline, a passage plan is laid off in nautical miles against the latitude scale on the chart edge, and speed is quoted in knots, which are nautical miles per hour. The unit is not a legacy quirk — it is the one that makes chartwork arithmetic work.
Converting to kilometres is what you do when you have to explain a distance to someone ashore, plot it on a land map, or compare a passage against a road journey.
Why Nautical Miles Exist
Chart Navigation
Sea Boundaries
Passage Planning
Converting a Passage Leg or a Zone Limit
Take the distance off the chart
Step the leg off with dividers against the latitude scale at the side, not the longitude scale along the bottom — only latitude is a true nautical mile per minute.
Enter it and read kilometres
Type the nautical-mile figure in the upper field. The kilometre equivalent appears below, which is the form a land map or a shore contact will understand.
Turn the leg into hours
Keep the original nautical-mile figure for timing. Divided by boat speed in knots it gives hours directly, with no conversion at all.
The reverse case comes up whenever a distance arrives from a land source — a coastguard broadcast in kilometres, a satellite image scale bar, a marina's website. One press on the swap arrows puts kilometres on top.
Legal Limits and the Passages People Actually Sail
The first three rows are fixed in international law; the last three are the distances that shape a cruising plan.
| Limit or passage | Nautical miles | Kilometres |
|---|---|---|
| Territorial sea | 12 nmi | 22.224 km |
| Contiguous zone | 24 nmi | 44.448 km |
| Exclusive economic zone | 200 nmi | 370.4 km |
| Comfortable day sail | 50 nmi | 92.6 km |
| Overnight hop | 120 nmi | 222.24 km |
| Ocean passage | 1,000 nmi | 1,852 km |
A Leg Converted as Fast as You Step It Off
The kilometre figure follows each keystroke, so a passage broken into six legs is done in the time it takes to read them from the plan.
Reverse It for a Distance Given Ashore
The swap arrows take a kilometre figure from a land map or a marina website and hand back the nautical miles the plot needs.
Copy a Distance Into the Log
Each field copies the bare number, which is what a deck log, a passage spreadsheet or a mileage record for a qualification wants.
Reach Metres for Depth and Clearance
Soundings, drying heights and bridge clearances are printed in metres on modern charts. Search the unit list and the same fields handle those too.
Sailing and Sea Boundary Questions
How far offshore is the twelve-mile limit?
22.224 km from the baseline, which is normally the low-water line along the coast. Inside it a state's law applies in full; outside it, the contiguous zone at 24 nmi allows only limited enforcement over customs and immigration.
Why is the unit tied to latitude?
Because it makes a paper chart self-measuring. One minute of latitude is one nautical mile, so a navigator with dividers reads distance off the same scale that gives position, with no separate ruler and no scale factor to remember.
What exactly is a knot?
One nautical mile per hour, so 1.852 km/h. The name is literal: crews once streamed a log line knotted at regular intervals and counted how many ran out while a sand glass emptied. Saying "knots per hour" is a common slip, since the time is already inside the unit.
How much longer is it than a land mile?
About 15% longer. A statute mile is 1.609344 km against the nautical mile's 1.852 km. Over a 500-mile passage that difference is more than 120 km, which is why the two must never be mixed in a fuel or provisioning calculation.
What does an exclusive economic zone cover?
Rights over fish, seabed minerals and energy out to 200 nmi — 370.4 km — from the baseline. It is not sovereignty: other vessels keep freedom of navigation through it, which is why a coastal state can regulate fishing there but not passage.
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