Magnetic Declination Calculator

Three different "norths"

Navigation has historically conflated three distinct reference directions. Understanding which one your compass, map, and GPS each use is the first step in not getting lost.

The World Magnetic Model (WMM)

The WMM is the standard mathematical representation of Earth's main magnetic field, jointly produced by NOAA NCEI (US National Centers for Environmental Information) and the British Geological Survey. It's a spherical-harmonic series — currently to degree 12 — fitted to data from satellite missions (Swarm, CHAMP, Ørsted) and ground observatories. The coefficients update every five years; the current generation is WMM 2025, valid 2025-01-01 through 2029-11-13.

Inside the validity window, the model represents the main field— what's generated by molten-iron convection in Earth's outer core — to about ~30 arc-minutes RMS declination accuracy at the surface globally. Local crustal anomalies(mineralised bedrock, iron ore deposits) can produce additional meter-scale variations that WMM doesn't model; for those you need EMM (Enhanced Magnetic Model) which adds crustal terms to degree 720.

What each output means

The calculator returns four primary scalar quantities plus three vector components. The full set:

The relationship: F² = X² + Y² + Z²; H² = X² + Y²; tan(D) = Y / X; tan(I) = Z / H. So the three vector components determine all four scalar quantities. The report surfaces every one of them.

The arithmetic of compass-to-map conversion

The single thing most field-navigation users want from this tool: how to read a compass bearing and translate it to a bearing on a map (or vice versa). Two simple rules:

Worked example: you're in New York City with a declination of about −12.5°. A compass tells you to walk due north (magnetic bearing 0°). The true bearing is then 0 + (−12.5) = −12.5° = 347.5° — about 12° west of true north. The deep report has the complete table for the eight cardinal directions automatically filled in for the point you queried.

Ten worked examples — declination at major cities (2026-06-15)

The declination pattern follows the agonic line— the contour where declination = 0. It currently snakes from the Arctic across western Europe, down through eastern Africa, and across the south Indian Ocean. East of the agonic line declination is positive; west of it, negative. Cape Town's strongly west declination and Sydney's strongly east are typical of points far from the agonic line; Singapore sits almost on it.

Misconceptions worth getting straight

"Magnetic north is the same as the magnetic pole"

Roughly, but not exactly. The north magnetic pole is where the field points directly downward (I = +90°). The geomagnetic poleis the axis of the best-fit magnetic dipole, which doesn't coincide with the magnetic pole. A compass needle's horizontal direction points along the local field, which generally doesn't lead directly to either pole. "Magnetic north" in everyday use means "the direction a compass points" — that's the direction this tool gives you.

"Declination is the same everywhere in a country"

Only if the country is small. In the conterminous US, declination ranges from about −17° in Maine to +14° in Washington state — a 31° spread. UK, France, and Germany are small enough to have nearly uniform declination, but anywhere in continental-scale geography you need to compute for the specific point.

"The compass error doesn't matter for short walks"

It matters whenever the path is long enough that a 1° bearing error pushes you off the target. 1° of bearing error over 1 km is 17 m of cross-track error; over 10 km it's 170 m. For backcountry navigation in featureless terrain, always apply the declination — even a 5–10° error can put you onto a different valley.

"Altitude doesn't affect declination"

For surface use, true — the change between sea level and 5,000 m is fractions of a degree. For aviation work above 10 km, or satellite operations, declination changes measurably with altitude. The WMM expresses the field as a 3D spherical-harmonic series, so the tool can be evaluated anywhere from −1 km to 850 km above the WGS-84 ellipsoid. Default the altitude input to 0 for surface; set it explicitly for aviation.

"A GPS receiver gives me magnetic bearing"

Modern GPS receivers report bearings in true north by default — they know the geometry of the satellites and the ground exactly. A magnetic-bearing reading on a GPS unit is a calculated value: it took the GPS-derived true bearing and subtracted WMM declination. The same calculation runs inside any phone with a compass app. This tool is the stand-alone version of that calculation.

When to use this tool, when to upgrade

How to verify the result

1. Cross-check against NOAA's online calculator. ngdc.noaa.gov/geomag/calculators/magcalc.shtml. Should agree to four decimal places — both use WMM 2025.
2. Compare to a printed declination chart. NOAA publishes annual world declination maps. Your point should fall in the right contour band.
3. Sanity-check the sign. In the eastern US and most of Asia, declination is negative (west). In the western US, eastern Europe, and Australia, positive (east). If the sign looks wrong for your region, double-check the coordinate input.

How this tool is built

WMM evaluation runs server-side via the bundled geomagnetismnpm package — a pure-JavaScript port of NOAA's reference C implementation. The coefficient set is the official WMM 2025 published by NOAA NCEI on 2024-11-13. The page route in src/app/tools/[tool]/page.tsx calls geomagnetism.model(date).point([lat, lon, altKm])and passes the result to the client widget. Secular variation is computed by sampling the model again one year later and diffing. The nearest-place and elevation lookups in the report fetch from server-side proxies with Cache-Control: no-store — no caching, no logging, no retention.