Calculate Great-Circle Distance at Sea
Enter two coordinate points in decimal degrees to estimate the shortest distance over Earth’s surface (great-circle route).
How to use this nautical distance calculator
This tool helps mariners, sailors, pilots, cruisers, and students compute the great-circle distance between two points on Earth. Great-circle distance is the shortest path between coordinates on a sphere, which makes it ideal for open-water and long-range route planning.
- Enter start and destination latitude/longitude in decimal degrees.
- Choose your preferred display unit (nautical miles, kilometers, or miles).
- Click Calculate Distance to see route distance and initial bearing.
- Use Swap Points to reverse origin and destination instantly.
Why nautical miles are used for marine navigation
A nautical mile is defined as exactly 1,852 meters. It is tied directly to Earth geometry: one nautical mile corresponds to one minute of latitude. Because charts, GPS coordinates, and navigation systems are built around latitude and longitude, nautical miles are naturally aligned with marine and aviation operations.
Compared with land miles, nautical miles make chart plotting and dead-reckoning calculations more intuitive. Speed at sea is also measured in knots, where 1 knot = 1 nautical mile per hour.
How the calculator works
1) Great-circle distance (Haversine method)
The calculator uses the Haversine formula, which estimates distance across Earth’s curved surface:
distance = R × c, where R is Earth’s radius and c is the central angle between your two points.
For nautical output, Earth’s mean radius is set to 3440.065 NM. The result is then converted to kilometers and statute miles for comparison.
2) Initial bearing
In addition to distance, the calculator returns an initial course bearing (0°–360°) and a compass direction (for example, NE or WSW). This is useful for planning headings before adjusting for wind, current set/drift, and local restrictions.
Great-circle vs real-world route distance
Great-circle distance is the shortest theoretical path, but not always the path you sail. Actual passage length may be longer due to:
- Traffic separation schemes and shipping lanes
- Restricted or shallow water areas
- Weather routing and avoidance strategies
- Current systems, tides, and safety margins
Think of this calculator as a fast, reliable baseline for voyage planning. Then layer in chart constraints and seamanship judgment.
Example use case
Suppose you plan a route from Miami to San Juan. Enter both coordinates and you’ll get a great-circle estimate in nautical miles plus conversions. That lets you quickly estimate:
- Travel time at planned cruising speed in knots
- Fuel requirements and reserve margin
- Potential watch schedules and waypoint spacing
Conversion reference
- 1 nautical mile = 1.852 kilometers
- 1 nautical mile = 1.15078 statute miles
- 1 kilometer = 0.539957 nautical miles
Frequently asked questions
Does this include wind and current?
No. This is a geometric distance calculator. You should adjust route and ETA for weather, current, and vessel performance.
What coordinate format is supported?
Decimal degrees only (for example, 36.7783, -122.4194). If you have degrees/minutes/seconds, convert to decimal first.
Can it handle long routes or crossing hemispheres?
Yes. As long as values are within valid latitude/longitude ranges, the formula handles global routes, including large longitude differences.
Final note
Whether you’re passage planning offshore, preparing for a navigation exam, or just curious about distances between ports, this nautical distance calculator gives a quick and practical foundation for smarter decisions on the water.