Marine Distance Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: August 19, 2026Distance 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
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.
Principles of Marine Navigation and Nautical Distance Calculations
A marine distance calculator computes the nautical distance, transit voyage time, and fuel consumption for maritime vessels (commercial container cargo ships, oil tankers, fishing trawlers, and sailing yachts). In marine geodesy and nautical chart navigation, marine distances are measured in Nautical Miles (NM), and vessel speed is expressed in Knots (kt).
The Geodetic Definition of the Nautical Mile
The Nautical Mile is historically and mathematically defined based on the curvature of the Earth:
International Standard: 1 Nautical Mile = Exactly 1,852 Meters (approx. 6,076.12 Feet)
1 Knot = 1 Nautical Mile per Hour ≈ 0.5144 Meters/Second
Mercator Projections and Rhumb Line Sailing
Unlike aviation where autopilots continually adjust heading along Great Circle arcs, maritime navigators frequently steer a constant compass course known as a Rhumb Line (Loxodrome) for coastal and regional passages. On standard navigational Mercator projection charts, a Rhumb Line plots as a straight line, allowing mariners to maintain a fixed magnetic rudder heading.
Radar and Visual Horizon Distance at Sea
Because the Earth's ocean surface curves downward, the maximum distance at which a ship's lookout or radar scanner can detect another vessel or lighthouse is limited by the geometric line of sight:
Combined Detection Distance = 1.17 × [ √(hobserver) + √(htarget) ]
Step-by-Step Worked Calculation Example
Example: Calculating Voyage Transit Time and Fuel Consumption for a Motor Yacht
Problem: A coastal cruising motor yacht plans a 180.0 Nautical Mile passage between two ports. The vessel cruises at a steady cruising speed of 15.0 Knots and burns diesel fuel at an average rate of 18.0 Gallons per hour. Calculate: (1) Total voyage transit duration in hours; (2) Total marine diesel fuel consumed; and (3) The fuel efficiency in Nautical Miles per Gallon.
Step 1: Calculate voyage transit duration (Time = Distance / Speed):
Transit Time = 180.0 NM / 15.0 Knots = 12.00 Hours
Step 2: Calculate total fuel consumed (Fuel = Time × Burn Rate):
Fuel Consumed = 12.0 hrs × 18.0 Gallons/hr = 216.0 Gallons of Diesel
Step 3: Calculate vessel fuel economy:
Fuel Economy = 180.0 NM / 216.0 Gallons = 0.833 Nautical Miles / Gallon (1.20 Gallons / NM)
Conclusion: The passage requires exactly 12 hours of underway cruising and consumes 216.0 gallons of diesel fuel.
Dead Reckoning (DR) Navigation and Tidal Current Vectors
In coastal navigation, a vessel's Course Over Ground (COG) and Speed Over Ground (SOG) diverge from its heading due to oceanic tidal currents and leeway wind drift. Navigators solve 2D vector triangles: Velocity Over Ground = Velocity Through Water + Tidal Current Vector.
COLREGs Closest Point of Approach (CPA) and TCPA
In marine radar navigation and collision avoidance (governed by the International Regulations for Preventing Collisions at Sea, COLREGs), vessel captains and Automatic Radar Plotting Aids (ARPA) compute two vital safety distance vectors:
- Closest Point of Approach (CPA, Nautical Miles): The minimum distance at which another vessel will pass if both ships maintain their current course and speed vectors.
- Time to Closest Point of Approach (TCPA, Minutes): The exact time remaining before the CPA minimum distance occurs:
TCPA = - ( R · Vrel ) / ||Vrel||²Where R is the relative position vector and Vrel is the relative velocity vector. If CPA < 1.0 NM and TCPA < 15 minutes, risk of collision exists, requiring immediate course alteration.
Oceanic Route Optimization: Great Circle vs. Composite Sailing
On transoceanic passages (such as San Francisco to Tokyo, 4,536 NM), sailing a true Great Circle route arcs far northward into high latitudes (50°N), exposing vessels to severe North Pacific winter gales and rogue waves. Navigators implement Composite Sailing: following a Great Circle arc until reaching a maximum safe latitude limit (e.g., 42°N), sailing a constant-latitude Rhumb line, and resuming a Great Circle descent to destination.
Nautical Depth Soundings and Bathymetric Underkeel Clearance
In marine pilotage and channel navigation, calculating passage distance is constrained by Underkeel Clearance (UKC). Deep-draft commercial vessels (such as Ultra Large Crude Carriers ULCC with drafts exceeding 20 meters) cannot navigate direct Great Circle paths across shallow continental shelves, requiring routing through established deep-water traffic separation schemes and dredged channels.
Fathom, Cable, and Nautical Distance Conversions
1 Cable Length = 0.10 Nautical Mile = 185.2 Meters (608 Feet)
Marine GPS Waypoints and Cross-Track Error (XTE)
Modern marine electronic chartplotters (ECDIS) guide autonomous navigation systems along programmed waypoint routes, calculating real-time Cross-Track Error (XTE) — the perpendicular distance deviation of the vessel's current position from the planned navigational track line.
Bathymetric Contour Navigation
Marine navigators cross-reference digital echo sounder depth readings with bathymetric depth contour lines on electronic nautical charts to verify GPS navigation fixes and maintain safe channel fairway clearance.