The magnetic declination calculator above converts between magnetic and true bearings using a declination figure that you supply, and shows how far a published annual change would have moved that figure since the date it was valid for. It also gives the back bearing and, if you enter a grid convergence, the corresponding grid bearing.
Arb Digital publishes free physics and navigation calculators, and this one is deliberately built the other way round from most declination pages. It publishes no declination values for any location. Declination is not a constant, not a property of a place, and not something a static web page can responsibly assert. It changes measurably every year, it is modelled rather than tabulated, and the models themselves are replaced on a five-year cycle. A current chart or an official calculator run for today's date is the authority; this page only does the arithmetic once you have that number.
What This Magnetic Declination Calculator Does
A compass needle aligns with the horizontal component of Earth's magnetic field, which is not the geographic pole. The angle between magnetic north and true north at your position is the magnetic declination, sometimes called magnetic variation on nautical and aeronautical charts. Convert between the two references and you have solved the practical problem.
The tool applies the standard sign convention, adds the drift from your annual-change figure, and returns the converted bearing normalised to the range 0 to 360 degrees. It also reports the back bearing, which is the reciprocal, and applies a grid convergence if you supply one so you can work in grid bearings on a projected map.
What it does not do, and will not do, is tell you the declination at your location. That requires a geomagnetic field model evaluated at your latitude, longitude, altitude and today's date, and it is exactly the job the official calculators exist to perform.
How to Use It
- Get your declination from an authoritative source — an official geomagnetic calculator, your national geological survey, or the declination diagram printed on a current chart. Note the date it applies to.
- Enter the value and its direction, east or west. East means magnetic north lies clockwise of true north.
- Enter the published annual change and how many years have passed since the figure was valid, to see the drift.
- Enter the bearing you want to convert and say whether it is a magnetic reading or a true bearing from a map.
- Add the grid convergence if you are working on a projected grid, and read the converted, back and grid bearings.
The Formula: How Declination Is Applied
With declination D taken as positive east and negative west, the conversions are:
True = Magnetic + D · Magnetic = True − D
The mnemonic taught for it is "east is least, west is best" — meaning that going from a true bearing to a magnetic one you subtract an easterly declination and add a westerly one. Both forms say the same thing and both are easy to apply backwards, which is why the tool states the direction of the conversion explicitly rather than leaving you to remember it.
Every result is normalised into 0 to 360 degrees, so a conversion that would give −5 degrees is reported as 355. The back bearing is the result plus 180 degrees, normalised the same way.
Grid bearings need a second correction. Grid north on a projected map is the direction of the grid's northing lines, which coincides with true north only along the projection's central meridian and diverges from it elsewhere by the grid convergence C:
Grid = True − C, with C positive when grid north lies east of true north
The drift estimate is simply linear: Dnow = D + (rate in arcminutes per year × years) / 60. That is a first-order extrapolation of a quantity that does not actually change linearly, which is why it is presented as an indication of scale rather than as an answer.
Work the defaults through by hand. A placeholder declination of 12.5 degrees east, an annual change of 7 arcminutes per year eastward, and four years elapsed. The drift is 7 × 4 = 28 arcminutes, which is 0.4667 degrees, so the estimated present declination is 12.9667 degrees east. A compass bearing of 045 degrees becomes a true bearing of 45 + 12.9667 = 57.97 degrees, and the back bearing is 237.97 degrees. Note the size of that four-year drift: nearly half a degree, which is around 8 metres of lateral error per kilometre travelled.
Why This Page Refuses to Publish Declination Values
This is the important section. Declination is a modelled quantity that changes continuously, and the models are versioned.
The World Magnetic Model published by NOAA's National Centers for Environmental Information is the standard reference for declination, and NCEI states plainly that "a new version of the model is updated every five years to address changes in Earth's magnetic field", with each release valid for a five-year window before it expires. The International Geomagnetic Reference Field runs on the same kind of cycle. Between releases, the model is evaluated with a secular-variation term for the date you ask about — so declination is a function of latitude, longitude, altitude and time, not a lookup.
Three consequences follow, and they are why any published table goes stale:
- The number changes every year. Annual change of several arcminutes is common and can be much larger at high latitudes. Over a decade that accumulates into a degree or more in many places.
- The drift is not linear. Secular variation itself changes, which is precisely why a new model has to be issued rather than the old one simply extrapolated. The linear estimate this page offers is a scale indication, not a substitute.
- The gradient across a region can be steep. Declination varies with position as well as time, and near the magnetic poles or in areas of strong crustal anomaly it varies rapidly over short distances.
The correct workflow is to run an official calculator for your position and today's date. The NCEI geomagnetic calculators provide a declination calculator, a magnetic field calculator and a compass correction tool built on the current model. Your national geological survey or hydrographic office publishes equivalents, and aeronautical and nautical charts carry a declination diagram with the epoch and annual change printed on it.
Local Anomalies Beat Any Model
A global field model describes the main field from the core plus the large-scale crustal contribution. It cannot describe what is happening within a few metres of your compass, and that is where most practical errors come from.
Iron-rich rock, ore bodies and volcanic terrain produce local anomalies that can swing a compass by many degrees over a short distance. Charts and topographic maps sometimes carry an explicit warning where such anomalies are known, and in those areas a compass simply cannot be trusted for precise work.
Closer to hand, the problem is usually the observer. Vehicles, reinforcing bar, steel structures, buried services, power lines, mobile phones, camera magnets and even a steel belt buckle will deflect a needle. The standard practice is to step well away from metal and take the reading in the open. Smartphone compasses add their own failure mode: they use a small magnetometer that needs calibration and is easily disturbed by the device's own speaker and camera magnets, and many will report a heading without indicating that its accuracy has degraded.
Declination, Inclination and Deviation Are Three Different Things
The vocabulary trips people up, and mixing the terms produces real navigation errors.
Declination, called variation on marine and aviation charts, is the horizontal angle between magnetic north and true north. It is what this page applies.
Inclination, or magnetic dip, is the vertical angle the field makes with the horizontal. It is why compasses are balanced for a particular hemisphere or zone, and why a compass built for one latitude band can bind or read sluggishly in another. It plays no part in a bearing conversion.
Deviation is the error introduced by magnetic material in the vessel or aircraft carrying the compass. It is specific to that platform and that heading, it is measured by swinging the compass, and it is recorded on a deviation card. Applying declination without deviation on a steel vessel leaves a real error uncorrected.
Where This Sits Next to the Other Navigation Tools
This page handles the reference-frame correction. The azimuth calculator works out the bearing between two points from their coordinates, which gives you a true bearing that this page can convert to a compass heading. The coordinates converter moves between coordinate formats, and the angle converter handles degrees, radians, mils and gradians.
For the physics underneath, the magnetic field converter handles flux density and field strength units, the magnetic permeability calculator derives permeability from B and H, and the magnetic dipole moment calculator covers the moment and torque that make a compass needle work at all. The crosswind component calculator is the other correction that turns a heading into a track.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Using a declination figure without checking its date — the value changes every year and the underlying model is replaced every five, so an old chart can be degrees out.
- Applying the correction in the wrong direction — true equals magnetic plus an easterly declination, and the reverse subtracts it. Getting the sign backwards doubles the error.
- Confusing declination with deviation — declination is a property of the place, deviation is a property of the vessel or aircraft, and both have to be applied.
- Trusting the linear drift estimate as an answer — secular variation itself changes, which is exactly why models are re-issued rather than extrapolated.
- Taking a compass reading near metal — vehicles, reinforcement, power lines and phones deflect a needle far more than any declination correction will fix.
Related Free Tools From Arb Digital
Pair this with the azimuth calculator for the true bearing between two coordinates and the coordinates converter for position formats. The angle converter handles degrees, radians and mils, and the crosswind component calculator covers the wind correction. For the magnetism itself, use the magnetic field converter, the magnetic permeability calculator and the magnetic dipole moment calculator. Everything Arb Digital publishes sits on the free online tools hub.
Frequently Asked Questions
Because declination depends on latitude, longitude, altitude and date, and it changes every year. It comes from a geomagnetic field model that is re-issued on a five-year cycle, so any value printed on a static page starts going out of date immediately. Run an official calculator for your position and today's date instead, and enter that figure here.
Add the declination if it is east and subtract it if it is west. The reverse conversion, from true to magnetic, does the opposite. Normalise the result into the range 0 to 360 degrees. The mnemonic usually taught is that east is least and west is best, meaning you subtract an easterly declination when going from true to magnetic.
It varies by location. Annual change of a few arcminutes per year is common in mid-latitudes, and it can be considerably larger near the magnetic poles or in areas of strong secular variation. Over a decade that accumulates into a degree or more in many places, which is why the epoch printed alongside a declination figure matters as much as the figure itself.
Declination is the angle between magnetic north and true north at a place, caused by Earth's field. Deviation is the additional error caused by magnetic material in the vessel or aircraft the compass is mounted in, which varies with heading and is measured by swinging the compass. Both must be applied, and neither substitutes for the other.
Yes. Variation is the term used on nautical and aeronautical charts, declination is the term used in geomagnetism and land navigation, and they describe the same angle between magnetic north and true north. Charts print it in a declination diagram together with the epoch and the annual rate of change.
Grid north on a projected map is the direction of its northing lines, which matches true north only along the projection's central meridian. The angle between them is the grid convergence, and it varies across a map sheet. You need it whenever you are working in grid bearings; if you are working in true bearings, leave the field at zero.
Treat it with caution. Phone magnetometers are small, need regular calibration, and are easily disturbed by the device's own speaker and camera magnets as well as by nearby metal. Many will display a heading without clearly indicating that accuracy has degraded. For anything that matters, use a proper compass held well away from metal, and verify against a known bearing.
Because the rate of change is itself changing. Geomagnetic secular variation is not constant, which is exactly why a new model has to be issued every five years rather than simply extrapolating the previous one. A linear projection is useful for seeing how big the effect is over a few years, but it is not a substitute for evaluating the current model.
This tool is provided for educational use only and is not a navigation aid. It publishes no magnetic declination values and cannot determine the declination at any location. Declination changes continuously and is modelled by the World Magnetic Model and the International Geomagnetic Reference Field, both re-issued on a five-year epoch. Obtain your declination and its annual change from an official geomagnetic calculator, your national geological survey or a current chart, and treat that source as the authority. Do not rely on this page for navigation, aviation, marine or survey work.
