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.
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 bearing to a true bearing, subtract the declination: True Bearing = (Compass Bearing − Declination + 360) % 360. The +360 and modulo operation keep the result within the 0°–360° range. Positive declination means magnetic north is east of true north (common in western North America); negative means it is west (common in eastern North America and Europe). 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 + 360) % 360. Step 2 — Calculate: (75 − 12) = 63; 63 + 360 = 423; 423 % 360 = 63°. Your true bearing is 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 Many modern topographic maps also print the local declination in the map legend, though printed maps may be outdated.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. East declination (positive) means magnetic north is east of true north, so you subtract it from your compass bearing to get true bearing, as the formula (Bearing − Declination) shows. West declination (negative) means magnetic north is west of true north; subtracting a negative value effectively adds it, moving the true bearing clockwise. A simple mnemonic is 'East is least, West is best' — east declination decreases the bearing, west increases it. This calculator handles the sign automatically when you enter a negative value for west declination.