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Magnetic Declination Calculator

Magnetic declination is the angle between true north and the direction your compass needle actually points. This varies by location and changes gradually over time. Navigate accurately by knowing your local declination—essential for orienteering, surveying, aviation, and backcountry travel. Enter your latitude, longitude, elevation, and date to find the precise correction needed for your area.

Last updated:

Creators Steven Wooding, BSc Physics
5,723 people find this calculator helpful

Understanding Magnetic Declination

Earth's magnetic field does not align perfectly with its rotational axis. True north sits at the geographic North Pole, where Earth's spin axis meets the surface. Magnetic north, by contrast, is where the planet's magnetic field lines point vertically downward in the Northern Hemisphere—currently located in the Canadian Arctic, roughly 1,600 km away from true north.

Your compass needle aligns with magnetic field lines, not true north. Depending on where you stand, this difference can exceed 20 degrees. In parts of Australia, the declination reaches −12°; in Alaska, it reaches +21°. This angular offset is magnetic declination. Navigation errors accumulate quickly when declination is ignored: a 10° error compounds to 176 metres of drift over just 1 kilometre of travel.

Declination also changes over time. The magnetic pole migrates roughly 55 kilometres per year, so a declination value from a ten-year-old map may be noticeably inaccurate.

Magnetic Declination Components

The World Magnetic Model calculates declination using measurements of Earth's magnetic field at your exact location. The model outputs several related values:

Declination (D) = angle between true north and magnetic north

Inclination (I) = angle between horizontal and magnetic field direction

Horizontal intensity (H) = horizontal component of Earth's magnetic field

Total intensity (F) = total magnetic field strength

North (X), East (Y), Down (Z) components of field vector

  • Declination (D) — The angle in degrees by which magnetic north deviates from true north (positive = east, negative = west)
  • Inclination (I) — The angle below horizontal at which the magnetic field points, used in geophysics and mineral exploration
  • Horizontal Intensity (H) — The strength of the horizontal component of Earth's magnetic field, measured in nanoteslas
  • Total Intensity (F) — The complete strength of Earth's magnetic field at your location

Elevation and Coordinate Systems

Accurate declination requires knowing your elevation above the WGS84 ellipsoid (Height Above Ellipsoid, or HAE)—the same standard GPS devices use. This differs from elevation above mean sea level (AMSL), which varies geographically. Most GPS receivers, smartphones with GPS, or online mapping tools report HAE by default.

If your elevation source only shows AMSL, the difference is typically 30–50 metres in most populated regions, but can exceed 100 metres in mountainous areas. For most navigation purposes, this small difference has negligible effect on declination calculations.

You'll also need precise latitude and longitude. Six decimal places (0.000001°) provides accuracy to within 0.1 metres, which is more than sufficient.

Using Declination for Navigation

Armed with your local declination, adjust compass bearings when navigating by map. If your declination is +12°, your compass reads 12° too far east. To follow a true bearing of 0° (due north), aim your compass at 348° (0° − 12°). Conversely, if declination is −8°, add the absolute value to convert true to magnetic.

In the field, place your compass on a map with the edge connecting your starting point to your destination. Rotate the compass housing until the orienting lines align with the map's north–south grid lines. The bearing marked on the compass is your true bearing. Adjust this by your local declination to get the magnetic bearing to dial into your compass needle.

Modern GPS units often perform this conversion automatically if you input your location's declination in settings. Aviation charts and topographic maps typically print the current declination in the map margin, but these become outdated; the calculator provides real-time values.

Common Pitfalls and Caveats

Avoid these frequent mistakes when working with magnetic declination.

  1. Sign confusion — Positive declination (East) means magnetic north is east of true north: subtract it when converting true to magnetic bearings. Negative declination (West) means magnetic north is west of true north: add its absolute value. Double-check the sign on your calculator output.
  2. Using outdated maps — Topographic maps printed more than 5 years ago may show declination values off by 2–3° or more. Always verify current declination for your location and year, especially for precision surveying or long-distance backcountry navigation.
  3. Confusing elevation types — The calculator requires elevation above the WGS84 ellipsoid (HAE), not elevation above mean sea level. If you enter AMSL by mistake, results will be inaccurate. Check your source—GPS devices report HAE, but traditional survey benchmarks often use AMSL.
  4. Assuming declination is constant — Declination changes measurably over months and years as the magnetic pole drifts. Recalculate annually for critical navigation work. The calculator is valid from 2020 to 2029; outside this range, results become progressively less reliable.

Frequently Asked Questions

What's the difference between magnetic declination and magnetic inclination?

Magnetic declination is the horizontal angle between true north and magnetic north—the deviation you notice on a compass face. Magnetic inclination (or dip) is the angle at which Earth's magnetic field tilts below the horizontal plane. Near the equator, inclination approaches 0° (field runs horizontally). At the poles, it approaches 90° (field points straight down). Inclination is crucial for compass design and geophysics but less relevant for basic navigation.

Why does magnetic declination change over time?

Earth's magnetic poles are not fixed. The magnetic north pole migrates roughly 55 kilometres per year, currently drifting northwest from the Canadian Arctic toward Siberia. Additionally, the strength and direction of the magnetic field fluctuate slightly due to changes in liquid iron flow in Earth's outer core. Over a decade, declination can shift by 1–2° at most locations. This is why navigation charts and maps must be updated regularly.

How do I convert a true bearing to a magnetic bearing using declination?

The rule is: <strong>Magnetic Bearing = True Bearing − Declination</strong> (keep the sign). If declination is +10° and your true bearing is 90°, the magnetic bearing is 80°. If declination is −10°, the magnetic bearing is 100°. A positive declination means magnetic north lies east of true north, so you subtract it. A negative declination means magnetic north lies west of true north, so you're subtracting a negative number (effectively adding).

Can I use this calculator for aviation or surveying?

The calculator provides declination values derived from the World Magnetic Model 2020, the same standard used by aviation authorities and land surveyors. For professional surveying, especially over large areas, consult local geospatial agencies for site-specific variations and historical correction models. For aviation, always cross-reference declination with current NOTAMs and aeronautical charts. The calculator is accurate but should complement, not replace, official guidance for safety-critical work.

Why does elevation affect magnetic declination?

Earth's magnetic field varies in strength and direction with altitude. At higher elevations, you are closer to the magnetosphere and further from certain subsurface anomalies, both of which influence the measured field. The change is small—typically less than 0.1° per thousand metres—but precise geophysical models, including the World Magnetic Model, account for it to ensure accuracy.

What if my location is in the Southern Hemisphere?

The calculator works identically in the Southern Hemisphere. Magnetic south (rather than magnetic north) points toward the magnetic pole in Antarctica. Declination is still defined relative to true north, but in the Southern Hemisphere, positive easterly declination still means the magnetic direction is east of true north—the convention is universal. Enter your latitude as negative (e.g., −33.5° for Sydney) and longitude according to the Eastern or Western Hemisphere convention.

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