Magnetic Declination Calculator
Convert between true bearing and magnetic bearing using local magnetic declination.
Tip: Enter the declination value for your exact location and date (from NOAA, map margin, or aviation chart).
What this declination calculator does
This tool helps you convert headings between true north and magnetic north. If you hike, navigate by map and compass, fly, survey land, or do fieldwork, this conversion matters. Your compass points to magnetic north, but many maps and GPS outputs reference true north.
A declination calculator removes guesswork by applying the correct adjustment using your local declination value.
True north vs. magnetic north
True north
True north is the direction toward the geographic North Pole (Earth’s rotational axis). Grid systems and many digital mapping tools are based on true north.
Magnetic north
Magnetic north is where a compass needle points. It is controlled by Earth’s magnetic field and shifts over time.
Declination
Magnetic declination is the angular difference between true north and magnetic north at a given place and time. It is labeled East or West.
Formula used in this calculator
This page uses signed declination:
- East declination = positive
- West declination = negative
Then:
- Magnetic Bearing = True Bearing − Declination
- True Bearing = Magnetic Bearing + Declination
Final bearings are normalized to the range 0° to <360°.
How to use the calculator
- Select the conversion type (True → Magnetic or Magnetic → True).
- Enter the known bearing in degrees.
- Enter declination magnitude and pick East or West.
- Click Calculate to get the converted bearing.
Example conversions
Example 1: True to Magnetic
Suppose your map line is 100° true and local declination is 8° East:
Magnetic = 100° − 8° = 92° magnetic
Example 2: Magnetic to True
Suppose your compass gives 250° magnetic and local declination is 12° West:
West is negative in signed form, so declination = −12°.
True = 250° + (−12°) = 238° true
Common mistakes to avoid
- Using old declination values from outdated maps.
- Forgetting whether your value is East or West.
- Mixing up conversion direction (True → Magnetic vs Magnetic → True).
- Failing to wrap values that pass 0° or 360°.
Why declination changes
Declination varies because Earth’s magnetic field changes and the magnetic pole moves. That means your local value can drift year by year. In practical navigation, always verify the date associated with your source.
Quick FAQ
Is this the same as solar declination?
No. Solar declination describes the sun’s seasonal angle relative to Earth’s equator. This calculator is for magnetic declination in navigation.
Can I use decimal degrees?
Yes. Enter values like 7.3° or 121.75° and the calculator handles them directly.
Where do I find declination for my location?
Use recent map margins, aviation charts, or official geomagnetic tools such as NOAA or national geological services.