Understanding Magnetic Declination and Bearing Conversion
Magnetic declination (also called magnetic variation) is the angle, measured in degrees, between true north — the direction to the geographic North Pole along a meridian — and magnetic north — the direction a compass needle points, toward Earth's north magnetic pole. By convention, declination is positive when magnetic north lies east of true north and negative when it lies west of true north. This calculator applies that sign convention to convert a magnetic (compass) bearing into a true bearing, or a true bearing into the equivalent magnetic bearing.
The conversion formula and sign convention
The standard navigation relationship is True Bearing = Magnetic Bearing + Declination, where an easterly declination is entered as a positive value and a westerly declination as a negative value. Rearranging gives the reverse conversion: Magnetic Bearing = True Bearing − Declination. This matches the classic navigator's rule "variation east, magnetic least; variation west, magnetic best" — with east declination the magnetic reading is numerically smaller than the true bearing, and with west declination it is larger. If a converted bearing falls outside 0°–360°, the calculator wraps it back into that range (for example, −10° becomes 350°).
Finding your local declination value
Magnetic declination is not a fixed constant — it depends on your location and slowly changes over time (secular variation) as Earth's magnetic field shifts. It ranges from roughly 0° in parts of the central United States to more than 15–20° near the magnetic poles or in some coastal regions. Look up a current value from NOAA's National Centers for Environmental Information World Magnetic Model (WMM) calculator, the declination diagram printed in the margin of a topographic map, or a recent GPS/compass app, then enter that value here — avoid reusing an old map's value in areas where declination changes quickly.