Magnetic declination calculator – true north vs magnetic north

A compass does not point to true north. It points along the Earth's magnetic field, which in late 2026 bends about 12° west of true north in New York, 7° east in Denver, 15° east in Seattle, 5° east in Berlin and almost exactly north in London. That angle is the magnetic declination, also called magnetic variation, and you need it whenever you combine a map with a compass: to walk a bearing taken from a map, to plot a bearing taken in the field, to set the declination screw on a compass, to align a satellite dish or a sundial, or to orient a solar panel or a survey. Search a town, paste coordinates, use your location or click anywhere on the map, and the panel shows the declination in large type, in degrees and in degrees and minutes, with the direction it is moving and by how much each year. Below it come the inclination (how steeply the needle dips), the field strength, and for map readers the grid convergence and grid magnetic angle of the UTM zone, and inside Great Britain of the Ordnance Survey National Grid. Type a bearing to convert it between true, magnetic and grid north. Choose a date to see the declination in the past or future, open the table to see it year by year to 2030, and turn on the world map of isogons, the lines of equal declination, with the magnetic poles. The lines can be downloaded as GeoJSON or KML, and the share link keeps the point and the date.

How to use it

1. Search for a town, paste coordinates, press "Use my location" or click anywhere on the map.
2. Read the declination in the panel, east or west, with its yearly change, inclination and field strength.
3. If you use a paper map, read the grid magnetic angle for its UTM zone or the OS National Grid.
4. Type a bearing under "Convert a bearing" and choose whether it is true, magnetic or grid to get the other two.
5. Change the date, show the lines of equal declination, download them as GeoJSON or KML, or copy the share link.

How accurate is it?

How the declination is calculated

The tool evaluates the World Magnetic Model 2025 (WMM2025), published by the US National Centers for Environmental Information and the British Geological Survey, in your browser. The model describes the main magnetic field from the Earth's core as a spherical-harmonic series of degree and order 12, with a linear change per year, valid from 2025.0 to 2030.0. The code was checked against two independent implementations of the same model, with differences below a millionth of a degree, and the magnetic dip poles it finds for 2025.0 agree with the model report. The model itself is accurate to about 0.3–0.5° of declination for most of the world at sea level.

What the model cannot know

WMM describes the field from the Earth's core, not local disturbances. Iron ore, volcanic rock, power lines, cars, railways, reinforced concrete and even a phone or a belt buckle can turn a compass needle several degrees, and some regions have magnetic anomalies of 5° or more that no global model shows. Magnetic storms can shift the declination by a fraction of a degree for hours, more at high latitudes. Take a bearing away from metal, and where the map shows a local magnetic anomaly, trust the map's note over any calculator.

Near the magnetic poles

Close to the magnetic poles the horizontal part of the field becomes weak, a compass needle swings slowly and the declination changes quickly over short distances. Where the horizontal intensity is below 6,000 nT the tool warns that compass readings are degraded, and below 2,000 nT that the compass is unreliable; this covers parts of northern Canada, Greenland, the Arctic Ocean and Antarctica. The north magnetic pole is now moving from the Canadian Arctic towards Siberia by about 35 km a year.

Grid convergence and grid magnetic angle

On a map printed with a UTM or OS grid, grid north is the direction of the vertical grid lines, which differs from true north by the grid convergence: zero on the central meridian of the projection and up to about 3° at the edge of a UTM zone in mid-latitudes. The grid magnetic angle is the declination minus the convergence, the angle you need to go between grid bearings measured on the map and compass bearings. The convergence is calculated with the transverse Mercator series on the WGS84 ellipsoid for UTM and on Airy 1830 with the central meridian 2° W for the British National Grid, accurate to better than 0.001°.

Good to know

East or west, add or subtract

With an east declination, magnetic north lies east of true north, so the compass bearing is smaller than the true bearing: subtract the declination from a map bearing to walk it with a compass, and add it to a compass bearing to plot it on the map. With a west declination it is the other way round. The old rhyme is "east is least, west is best". The bearing converter does the sum for you.

The declination printed on your map is old

Topographic maps print the declination for the year of the survey, often decades ago, with a yearly change. In Britain the declination grows about 0.2° a year and in Iceland about 0.3°, so a 20-year-old map can be 4–6° off. Use today's value, or the date of your trip, rather than the margin of an old map.

Phone compasses can show true or magnetic north

Most phone compass apps can show either, and many switch to true north when location is on, using the same model as this tool. If your phone and a hand compass disagree by roughly the declination, one shows true north and the other magnetic north. Phones are also easily disturbed by a metal case, a magnet in a cover or a car.