About this tool
The Compass Declination & Bearing Converter is a free, in-browser tool that handles the three bearing conversions navigators use most: reversing a bearing, correcting magnetic readings to true north, and finding the direction and distance between two coordinates. Pick a mode and the answer updates as you type — you supply the declination yourself, so no live magnetic-model data is fetched and nothing is uploaded.
In back-bearing mode it applies backBearing = (bearing + 180) mod 360, so a bearing of 90° reverses to 270° — the direction to look back along your line of travel. In declination mode it converts a magnetic compass reading to true north with true = magnetic + declination (east declination positive, west negative), reports the true back bearing, and optionally applies grid convergence to give a grid bearing. In coordinate mode it computes the initial great-circle bearing with θ = atan2(sin Δλ · cos φ₂, cos φ₁ · sin φ₂ − sin φ₁ · cos φ₂ · cos Δλ), plus the great-circle distance from the haversine formula on a 6,371 km sphere.
Declination changes with location and drifts over years, so read the current value from your map legend or a magnetic-model source and enter it here. Use back bearings to retrace a route, declination correction to align a map with the field, and the coordinate mode to plan a heading between two waypoints.