Magnetic Declination Adjuster
Convert a compass (magnetic) bearing to a true bearing by applying the local magnetic declination. Essential for accurate navigation in hiking, sailing, and surveying where map bearings are true north.
Last updated: September 2026
Formula below · 2 sources (usgs.gov, Wikipedia) · Updated Sep 2026
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About this calculator
A compass points to magnetic north, which differs from true (geographic) north by an angle called magnetic declination. Declination varies by location and changes slowly over time. To convert a magnetic (compass) bearing to a true (map) bearing, add the declination, entering east declination as positive and west as negative: True Bearing = (Compass Bearing + Declination) mod 360. The modulo keeps the result within the 0°–360° range. Positive (east) declination means magnetic north lies east of true north, so the needle reads a smaller number than the true bearing and the declination must be added back; this is the case in most of western North America. Negative (west) declination means magnetic north lies west of true north, as in most of eastern North America, so the correction is subtracted. Always look up the current declination for your specific location from a source such as NOAA's World Magnetic Model before navigating.
How to use
Scenario: You are hiking and your compass reads a bearing of 75° (ENE). Your location has a magnetic declination of +12° (east). Step 1 — Apply the formula: True Bearing = (75 + 12) mod 360. Step 2 — Calculate: 75 + 12 = 87°. Your true bearing is 87°, almost due east. With a west declination of −12° instead (enter −12), the true bearing would be 75 − 12 = 63°. Enter 75° as the Compass Bearing and 12° as the Magnetic Declination above to confirm the result instantly.
Frequently asked questions
What is magnetic declination and why does it affect compass navigation?
Magnetic declination is the horizontal angle between true north (the geographic North Pole) and magnetic north (where a compass needle points). This difference exists because Earth's magnetic field is generated by its molten outer core, which does not align perfectly with the rotational axis. The declination value varies from roughly −30° to +30° depending on your location on Earth. If you ignore declination, your compass-based route will deviate from your intended true-north path, which can lead to significant positional errors over long distances.
How do I find the magnetic declination for my location?
The most reliable source is NOAA's National Centers for Environmental Information, which provides a free online declination calculator based on your latitude, longitude, and the current date. Many modern topographic maps also print the local declination in the map legend, though printed maps may be outdated. Smartphone navigation apps often apply declination automatically. Because Earth's magnetic field shifts over time, always use a recent source — declination can change by a degree or more per decade in some regions.
When should I add versus subtract magnetic declination from a compass bearing?
The direction of adjustment depends on whether declination is east or west. It depends on the direction of the conversion. Going from a compass (magnetic) bearing to a true bearing, as this calculator does, you add east declination and subtract west declination: True = Magnetic + Declination, with east positive. Going the other way — from a true map bearing to the bearing to set on your compass — you do the opposite, which is what the mnemonic 'East is least, West is best' describes (subtract east, add west when converting true to magnetic). Applying the correction in the wrong direction doubles the error instead of removing it, so at 12° declination you would be 24° off course. This calculator handles the sign automatically when you enter a negative value for west declination.