Marine Distance Calculator
Distance Over a Curved Ocean, Not a Flat Chart
Two points that look close together on a Mercator chart can sit much farther apart in reality, because the projection distorts distance at higher latitudes. This calculator uses the haversine formula, which accounts for the Earth's curvature directly, to compute the great-circle distance between two coordinates — then, in its second mode, converts a known distance and cruising speed into an estimated time of arrival.
The Formulas
Great-Circle Distance (Haversine):
a = sin²(Δφ/2) + cos(φ1) × cos(φ2) × sin²(Δλ/2)
c = 2 × atan2(√a, √(1−a))
Distance = R × c (R = 3,440.065 nautical miles)
ETA: Time = Distance / Speed
a = sin²(Δφ/2) + cos(φ1) × cos(φ2) × sin²(Δλ/2)
c = 2 × atan2(√a, √(1−a))
Distance = R × c (R = 3,440.065 nautical miles)
ETA: Time = Distance / Speed
Worked Examples
| Mode | Inputs | Result |
|---|---|---|
| Distance | Miami (25.7617°N, 80.1918°W) to San Juan (18.4655°N, 66.1057°W) | 896.73 nm (1,660.75 km) |
| ETA | 120 nm at 8 knots | 15 h 0 min |
Where This Matters
- Passage planning — an accurate great-circle distance is the starting point for fuel, provisioning, and watch-schedule planning on any offshore leg.
- Comparing routes — running the calculator against several waypoint options shows which routing actually shortens the trip, rather than just looking shorter on a flat chart.
- Setting realistic arrival windows — the ETA mode turns a distance and cruising speed directly into a time estimate for coordinating with marinas, crew changes, or customs.
How to Use This Calculator
- Choose the mode: Distance Between Coordinates or ETA (Time to Destination).
- For Distance: enter Latitude 1, Longitude 1, Latitude 2, and Longitude 2.
- For ETA: enter Distance (nm) and Speed (knots).
- Select Calculate to see the distance or estimated travel time.
Related Calculations
Once distance and speed are known, estimate fuel needs with the Boat Fuel Consumption Calculator, or convert the result with the Nautical Mile Converter.