Set and Drift Calculator
Set and drift describe the effect of current on a vessel. Set is the direction the current is flowing (true bearing) and drift is the current's speed in knots. This calculator compares your dead reckoning position to your actual GPS fix after a known period to determine the current that affected you. Enter both the DR position and the actual fix position, plus the elapsed time. The calculator computes the displacement vector from DR to fix, which equals the current's effect on your track. Use this information to correct future courses using the course-to-steer calculator.
Set and drift formula
dLat (nm) = (Fix lat - DR lat) * 60
dLon (nm) = (Fix lon - DR lon) * 60 * cos(mean lat)
Displacement = sqrt(dLat^2 + dLon^2) (nm)
Set = atan2(dLon, dLat) converted to 0-360 deg true
Drift = Displacement / Elapsed time (knots)
Drift in knots equals the displacement in nautical miles divided by the elapsed time in hours.
Using set and drift in navigation
- Record set and drift observations in your log to build a picture of local current patterns.
- NOAA publishes tidal current predictions at tidesandcurrents.noaa.gov for hundreds of US ports.
- Ocean current atlases (e.g., Pilot Charts) give statistical current expectations by season and region.
- Comparing multiple DR-to-fix pairs over time reveals systematic current patterns in your cruising area.
Set and drift: frequently asked questions
What are set and drift?
Set is the direction toward which the current is flowing, expressed as a true bearing. Drift is the speed of the current in knots. Together, set and drift describe the effect of the current on a vessel's actual track over the ground.
How do I measure set and drift?
Compare your dead reckoning position (based on course and speed through water) with your actual GPS fix after a known time period. The vector between the DR position and the actual fix represents the total current displacement. Dividing displacement by hours elapsed gives drift in knots.
How is set calculated?
Set is the true bearing from the DR position to the actual fix position. It represents the direction the current has pushed you. For example, if the fix is northeast of your DR position, the set is approximately 045 degrees.
Why does set and drift vary through the day?
Tidal currents change direction and speed with the tidal cycle. Wind-driven currents change with wind shifts. Ocean currents are more steady but still variable. Always use current predictions (NOAA Current Predictions) for the specific time and location.
How do I correct my course for known current?
Use the current correction (course to steer) formula. Point your bow upwind/upcurrent to compensate. The course to steer calculator solves this problem using vector triangle arithmetic.
Official sources
- NOAA Tides and Currents: NOAA Current Predictions.
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.