Magnetic Declination Calculator — True vs Compass Bearing
Apply your local magnetic declination to convert a compass bearing to a true geographic bearing, or find the compass heading that corresponds to a known true direction.
°
°
Geographic bearing after applying the declination correction
- 1
Add declination to compass bearing
45 + 12 = 57East declination is positive; West is negative. - 2
Normalize to 0–360°
(57 + 360) mod 360 = 57
How does this calculator work?
Apply the rule True = Magnetic + Declination (East positive, West negative) to convert a compass reading to a geographic bearing. Enter your local declination — look it up from NOAA's geomagnetic tool for your location — along with the compass bearing, and get the true bearing and the reverse conversion instantly.
Formula
How this is calculated
A magnetic compass aligns with Earth's magnetic field, which does not point exactly toward the geographic North Pole. The horizontal angle between geographic (true) north and magnetic north at a given location is called magnetic declination (or magnetic variation). East declination means magnetic north lies east of true north and is given a positive sign; West declination is negative. Declination varies by location and changes slowly over years — typically by 0.1–0.2° per year in mid-latitudes — so always use a value current to within a few years.
The correction rule is simple: True bearing = Magnetic bearing + Declination, where East declination is positive. For example, if your compass reads 80° and your local declination is +12° East, your true bearing is 92°. To go the other way — say you have a 92° true bearing on a map and want to know which compass direction to follow — subtract: Magnetic = True − Declination = 80°. Both results are normalised to the 0–360° range so they stay within compass convention.
This calculator does not compute declination from coordinates; that requires the International Geomagnetic Reference Field (IGRF) or World Magnetic Model (WMM) datasets. Look up your current value from NOAA's online geomagnetic calculator (ngdc.noaa.gov/geomag/calculators), a GPS app, or a national mapping authority, then enter it here. The diagram shows the angular offset between true and magnetic north.
Frequently asked questions
Magnetic declination is the horizontal angle between geographic north (the direction to the North Pole) and the direction a freely suspended compass needle actually points (magnetic north). It varies by location — from near 0° on certain meridians to over 20° in parts of Alaska or Siberia — and drifts slowly as Earth's internal magnetic field changes.
The most reliable source is NOAA's online World Magnetic Model calculator at ngdc.noaa.gov/geomag/calculators — enter your latitude, longitude and today's date to get declination to the nearest tenth of a degree. Most GPS receivers and navigation apps also display or automatically apply it.
Yes. Earth's liquid outer core slowly moves, causing the magnetic poles to drift. At typical mid-latitude locations declination changes by about 0.1–0.3° per year. For accurate navigation, use a declination value dated within the last 2–3 years rather than one from an old map.
TG we-Calculate Editorial Team. (2026). Magnetic Declination Calculator — True vs Compass Bearing [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/magnetic-declination-calculator
TG we-Calculate Editorial Team. "Magnetic Declination Calculator — True vs Compass Bearing." TG we-Calculate. 2026. https://we-calculate.com/calculator/magnetic-declination-calculator.
TG we-Calculate Editorial Team, "Magnetic Declination Calculator — True vs Compass Bearing," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/magnetic-declination-calculator
@misc{wecalculate_magnetic_declination_calculator, title = {Magnetic Declination Calculator — True vs Compass Bearing}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/magnetic-declination-calculator}}, year = {2026}, note = {TG we-Calculate} }
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