Magnetic Declination Calculator

Magnetic declination is the angle between true (geographic) north and the direction a compass needle actually points, and it varies with location because Earth's magnetic field isn't perfectly aligned with its rotation axis. This calculator uses a simplified tilted-dipole model centered on the magnetic north pole's approximate 2025 position (86.5°N, 162.8°E) rather than the full 13th-degree spherical-harmonic expansion that real-world models like WMM or IGRF use, so treat the result as a reasonable approximation, not survey-grade precision. The core formula, tan(D) derived from the spherical relationship between your position's colatitude and the pole's colatitude, gives the instantaneous declination; a secular-variation term is then added to account for the fact that the magnetic pole drifts roughly 40 km per year, which the calculator approximates as about 0.1°/year of declination change at mid-latitudes, scaled by your Years-from-2025 input.

Once declination is known, converting between bearings is simple addition: true bearing = magnetic bearing + declination (east declination is positive), and the reverse subtracts it. The calculator also reports magnetic inclination (how steeply the field dips below horizontal, which increases toward the poles) and a rough grid-convergence estimate relevant to UTM mapping. The most common practical mixup is sign confusion — forgetting whether declination should be added or subtracted when converting a bearing — and the second is trusting the output for precision navigation or surveying, where an actual current WMM/IGRF lookup should be used instead of this simplified approximation.