Great Circle vs Rhumb Line Calculator compares shortest-path great-circle routing with constant-bearing rhumb-line routing between two latitude/longitude positions. It is useful for marine route distance planning, offshore passage briefings, navigation coursework, and assessing the tradeoff between distance efficiency and steering simplicity. The tool reports great-circle distance, initial bearing, ETA, rhumb-line distance, rhumb-line bearing, and ETA at the selected speed. Use it to frame route strategy, then validate the route against charts, weather routing, currents, traffic separation schemes, restricted areas, depth constraints, and port approach requirements.
Use this calculator as one part of a broader passage-planning workflow. The
Marine Navigation Calculators toolkit
explains how CPA/TCPA, UKC, tide windows, route distance, set and drift, fuel range, and manoeuvring checks fit together.
You can also browse the full
Marine & Navigation category.
Enter the vessel, route, tide, current, or clearance assumptions exactly as labelled, then verify the result against official data and onboard judgment.
How to use this tool
Enter departure latitude/longitude and destination latitude/longitude in decimal degrees, using negative values for south and west.
Enter expected vessel speed in knots for the planning ETA.
Click "Compare routes" and review great-circle distance, initial bearing, ETA, rhumb-line distance, rhumb-line bearing, and ETA.
Use the result as a geometry baseline, then adjust the actual route for weather, currents, hazards, traffic, and local rules.
Pair the route comparison with drift, ETA compensation, fuel endurance, tide-window, and CPA/TCPA checks before treating the passage plan as complete.
Route Inputs
Result
Great-circle distance: 4467.935 nm
Great-circle initial bearing: 303.36°
GC ETA: 319.14 h
Rhumb-line distance: 4713.825 nm
Rhumb bearing: 268.47°
Rhumb ETA: 336.70 h
Formula or method
Great-circle sailing gives the shortest path on a spherical Earth approximation, but the bearing changes along the route.
Rhumb-line sailing keeps a constant bearing on a Mercator chart, which can be simpler to steer or brief but may be longer.
ETA is distance divided by entered speed and does not include current, wind, route detours, manoeuvring, or speed restrictions.
Worked example
Comparing route distance before an offshore passage brief
Start: 37.7749, -122.4194
End: 34.0522, -118.2437
Vessel speed: 12 kn
Result: The calculator compares great-circle and rhumb-line distance, bearing, and ETA at the entered speed.
A shorter geometric path may still be operationally unsuitable if weather, traffic separation, exclusion zones, or routing guidance require a different track.
How to interpret the result
Route geometry is a planning baseline; the operational route still depends on navigation constraints, weather, traffic, and vessel capability.
Great-circle routing is the shortest path on a spherical Earth approximation.
Rhumb-line routing keeps a constant bearing, which can be simpler to steer or discuss in some planning contexts.
Differences are usually more important over longer distances and higher latitudes.
The output does not include weather routing, currents, hazards, traffic schemes, or regulatory constraints.
Near high latitudes, polar regions, or anti-meridian crossings, coordinate handling and route practicality deserve extra review.
Common mistakes
Treating the shorter geometric distance as the best operational route.
Forgetting that initial great-circle bearing changes along the route.
Entering latitude/longitude signs incorrectly for north/south or east/west.
Using ETA output without checking speed assumptions, current, wind, and route constraints.
Formula References
Great-circle distance via haversine relation on Earth radius in nautical miles.
Rhumb-line distance/bearing using Mercator latitude (`dpsi`) relation.
Assumptions
Earth treated as sphere for planning-grade approximation.
No currents, weather-routing detours, traffic separation, or exclusion zones included.
Review note and limitations
Method - spherical great-circle and rhumb-line planning formulas.
Uses a spherical Earth approximation and does not include ellipsoidal geodesic refinements.
Does not account for currents, wind, traffic separation schemes, exclusion zones, ice, depth, routing guidance, or port approaches.
Rhumb-line calculations become less stable close to the poles where Mercator geometry is extreme.
Navigation planning support only. Verify routes with official charts, weather routing, notices to mariners, local rules, and bridge-team procedures.
FAQ
What is the difference between great-circle and rhumb-line routing?
A great circle is the shortest path on a sphere. A rhumb line follows a constant bearing, which can be easier to plot but may be longer.
Does this include weather or current routing?
No. It compares geometric route types only. Weather, current, traffic, hazards, and local rules must be checked separately.
Why can the bearing differ between methods?
A great-circle route changes bearing along the path, while a rhumb line keeps a constant bearing.
Related tools and workflows
Route geometry belongs with ETA compensation, drift, tide-window, fuel endurance, CPA/TCPA, and under-keel-clearance tools for a fuller passage plan.