Compass Declination Adjuster
Convert true bearing to/from magnetic bearing for any year and approximate location.
Inputs
Result
A natural placement between the calculator and the explanation.
What this solves — and what it doesn't
Magnetic north drifts over time and is offset from true north by an angle called declination. That offset is different in Seattle (-15 E) than in Miami (-7 W). If you set a bearing without correcting for declination, you can be off by 5 degrees per kilometer walked — way more than the trail width on most paths. This tool computes a declination estimate and converts a bearing between magnetic and true.
How to read the result
Pick the conversion direction. If you read a true bearing off the map and want to walk it with your compass, you subtract the declination (in E) or add it (in W). If you read a magnetic bearing from your compass and want to plot it on the map, you do the reverse.
Most handheld compasses have an adjustable declination ring — set it once for your area and forget it.
Limitations
- This is an approximation. For survey-grade work, use NOAA’s calculator.
- Doesn’t model local magnetic anomalies (igneous rock, ore deposits, large steel structures).
- Declination changes a few arc-minutes per year; a 30-year-old map may be wrong by 1°.
Frequently asked questions
Why does declination drift?
Earth’s magnetic field is generated by the moving outer core, which changes over decades. The pole also moves about 40 km per year.
What about local magnetic anomalies?
Some areas (Minnesota iron range, parts of the Pacific Northwest) have local declinations deviating 5 to 10 degrees from the regional value. Carry a corrected map or set your compass ring to match.
When is declination so small I can ignore it?
Within ~1 degree, the answer is "usually." On a 1 km bearing, that’s about 17 m of cross-track error — less than a trail width. On a 10 km traverse, it’s 170 m.