Magnetic Declination Calculator

About the Magnetic Declination Calculator

Magnetic declination (also called magnetic variation) is the angle between true north and magnetic north at any given point on Earth. Because a compass needle aligns with the local magnetic field rather than the geographic North Pole, navigators must correct their compass readings to determine true direction. This correction is essential for hiking, sailing, aviation, surveying, and any activity that depends on accurate directional measurement.

The magnitude and direction of declination varies dramatically with location. In parts of the eastern United States, declination may be 10-15° West, meaning magnetic north is west of true north. In parts of the western US, declination is East. Near the agonic line (zero declination, currently running through the central US), no correction is needed. In extreme locations like northern Canada, declination can exceed 30°.

This calculator uses a simplified model based on the International Geomagnetic Reference Field to estimate declination, inclination, and total field intensity for any location and date. It also converts between true and magnetic bearings, making it a practical tool for navigation planning and compass calibration.

When This Page Helps

Failing to account for magnetic declination leads to navigation errors that compound with distance. At 10° declination, you will be off by about 1,000 feet per mile traveled — enough to miss a trail junction, overshoot a waypoint, or navigate to the wrong location entirely.

It gives a declination estimate for any coordinates and includes the bearing conversion to make compass correction straightforward. The regional comparison table helps you understand how declination varies and plan for trips across different magnetic zones.

How to Use the Inputs

1. Enter your latitude and longitude (use negative values for South and West).
2. Select a preset city or enter coordinates manually.
3. Enter the date for the calculation (declination changes over time).
4. Read the magnetic declination — East means magnetic north is east of true north.
5. Enter a bearing to convert between true and magnetic headings.
6. Apply the correction: for East declination, subtract from true to get magnetic.
7. Reference the regional table to compare declination across major cities.

Example Calculation

Tips & Best Practices

• Always check current declination before a trip — values printed on older maps may be outdated by several degrees.
• Remember the mnemonic: "East is least, West is best" — subtract East declination from true, add West declination to true.
• GPS devices show true bearings unless configured otherwise, so convert when comparing to compass readings.
• Update your compass declination setting when traveling significant distances east or west.
• Near the magnetic poles (above ~70° latitude), compass reliability degrades significantly — use GPS or celestial navigation.
• For precision surveying, use the official NOAA calculator or WMM software for ±0.5° accuracy.

How the Earth's Magnetic Field Works

The Earth's magnetic field is generated by convection currents of molten iron in the outer core, a process called the geodynamo. This field roughly resembles that of a bar magnet tilted about 11° from the rotation axis, but with significant higher-order components that create regional variations. The magnetic poles do not coincide with the geographic poles and are constantly migrating — the north magnetic pole has moved more than 1,000 km in the last century.

The field at any point on Earth can be described by three components: the northward component (X), the eastward component (Y), and the downward component (Z). From these, we derive declination (the horizontal angle from true north) and inclination (the angle below horizontal).

Navigation Applications

For hikers and orienteers, magnetic declination is the most important correction to accurate land navigation. When transferring a bearing from a map to a compass or vice versa, the declination must be applied correctly. The consequences of omitting this correction scale with distance — a 15° error means you are nearly 1,600 feet off target for every mile walked.

Pilots use magnetic headings for VOR navigation and runway designations. Runway numbers are based on the magnetic heading rounded to the nearest 10° — this is why runways occasionally need to be renumbered as declination changes over decades.

Secular Variation and the World Magnetic Model

The Earth's magnetic field changes continuously, a phenomenon called secular variation. The World Magnetic Model (WMM), maintained by the US NOAA and the UK BGS, provides accurate forecasts of the geomagnetic field for 5-year epochs. The current model, WMM2025, is valid from 2025 to 2030. For most locations, the WMM is accurate to within ±0.5° for declination.

Frequently Asked Questions

• What is the difference between true north and magnetic north?

  True north (geographic north) is the direction toward the Earth's rotational axis at the North Pole. Magnetic north is where a compass needle points, determined by the Earth's magnetic field. These two directions rarely align exactly, and the angle between them is the magnetic declination.

• Does magnetic declination change over time?

  Yes, the Earth's magnetic field is constantly changing due to fluid motion in the outer core. Declination can change by 0.1° to 0.5° per year at most locations. The World Magnetic Model (WMM) is updated every 5 years to track these changes.

• What is the agonic line?

  The agonic line is where magnetic declination is zero — compass north equals true north. In North America, the agonic line currently runs roughly from Louisiana through the Great Lakes region. On this line, no compass correction is needed.

• How accurate is this calculator?

  This calculator uses a simplified dipole model. For precise navigation, use the official NOAA Magnetic Declination Calculator or the WMM model, which accounts for higher-order field components and is accurate to ±0.5° in most locations.

• Why is magnetic inclination important?

  Magnetic inclination (dip) is the angle between the magnetic field and the horizontal plane. Near the magnetic poles, inclination approaches 90°, which can cause compass needles to tilt and perform poorly. Special polar compasses are needed in high-inclination areas.

• How do I correct my compass for declination?

  Many compasses have an adjustable declination ring. Set it to your local declination and the compass will read true bearings directly. Without this feature, apply the correction manually: for East declination, subtract from true to get magnetic; for West, add.