Dead Reckoning Position Calculator
Dead reckoning (DR) is the fundamental navigation technique of estimating your current position by advancing a known starting position using your true course, speed through the water, and elapsed time. This calculator converts your DR run into a new estimated latitude and longitude. It uses the plane sailing approximation, which is accurate for passages up to about 200 nautical miles. Enter your last known position, the true course steered (0-360 degrees), boat speed in knots, and the elapsed time in hours. Accounts only for water track; to account for current, use the set and drift calculator.
Dead reckoning formula
Distance = Speed * Time
dLat (nm) = Distance * cos(course)
dLon (nm) = Distance * sin(course)
New latitude = Start lat + dLat / 60
New longitude = Start lon + dLon / (60 * cos(mean lat))
1 minute of latitude = 1 nautical mile. Longitude uses the departure correction: 1 minute of longitude = cos(latitude) nautical miles.
Practical dead reckoning tips
- Always label DR positions with time on the chart, distinguishing them from fixed positions.
- Update DR whenever you take a new GPS fix or visual bearing to maintain situational awareness.
- Current (set and drift) causes the actual track to diverge from the DR track. Use an EP when current data is known.
- Leeway (sideways drift from wind) also causes error in the DR track. Add leeway angle to course before applying DR.
Dead reckoning: frequently asked questions
What is dead reckoning?
Dead reckoning is the process of estimating your current position by starting from a known fix and advancing it using your course, speed, and elapsed time. It does not account for current or leeway, so position error accumulates over time without new fixes.
How accurate is dead reckoning?
Accuracy depends on the precision of your course, speed log, and time. Without accounting for current and leeway, errors of 2-5% of distance run are common. For this reason, mariners take frequent fixes using GPS, landmarks, or celestial navigation to correct DR positions.
What is the difference between DR position and estimated position?
A DR position uses only course and speed. An estimated position (EP) additionally accounts for known current (set and drift). An EP is therefore more reliable than a DR position when current data is available.
Why does my new latitude or longitude seem slightly off?
This calculator uses flat-Earth approximations suitable for short passages (under about 200 nm). For longer passages, use the great circle or Mercator sailing formulas. At high latitudes, the cos(lat) correction for longitude becomes significant.
How do I update my DR position?
DR positions should be updated whenever you take a new fix (GPS, visual bearing, celestial). The new fix becomes the origin for the next DR leg. Regular DR practice builds situational awareness even when GPS is available.
Official sources
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator (Pub. 9).
- US Coast Guard Navigation Center: USCG Navigation Center.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.