Magnetic Declination Calculator

Q: What is the difference between variation and deviation?

Variation (the same thing as declination) is the Earth-wide error. It depends only on where you are and the date, not on your vehicle. 2 Deviation is an error caused by your own vehicle: the engine, electrical circuits, radios, lights, tools, and magnetized metal parts. It is different for each aircraft and can change with the heading you are flying. 2 You fix variation with a model. You fix deviation by moving metal away or by using a deviation card.

Introduction

A compass does not point to true north. It points to magnetic north, which sits in a different spot. The gap between them is called magnetic declination (pilots often call it magnetic variation).¹ This calculator tells you that gap for any place on Earth.

Pick your spot in one of three ways: search for a city, type in latitude and longitude, or just click the map. Then choose a date. The tool uses the World Magnetic Model (WMM2025) or IGRF-14 to work out the answer.

You get the declination in degrees, minutes and seconds, or NATO mils. You also get the full magnetic field: inclination (dip), horizontal and vertical strength, and total strength. A compass rose shows the offset, and a chart shows how it will drift over the next few years.

This matters for flying, sailing, hiking, surveying, and setting up antennas. Runway numbers are based on magnetic north, and that north keeps moving.³ If you know the declination, you can turn a magnetic bearing into a true bearing, or the other way around. The step-by-step math is shown too, so you can check the work or learn how it is done.

How to use our Magnetic Declination Calculator

Give the tool a spot on Earth and a date. It shows you the magnetic declination there (how far a compass needle points away from true north), plus the full magnetic field values, a compass picture, a drift chart, and the math steps.

City / Place Search tab: Type a city, town, or landmark name, then press Search & Place. The tool finds the spot and fills in the coordinates for you.

Country: Pick a country to keep the search inside it. Leave it on "Any country" to search the whole world.

U.S. state / territory: This box shows up only when you choose the United States. Pick a state to narrow the search even more.

Manual Coordinates tab: Use this if you already know the exact spot, like an airport or a waypoint.

Coordinate system: Choose Geodetic (WGS84) for normal maps, charts, and GPS. Choose Geocentric only if you are measuring distance from the center of the Earth.

Latitude: Type how far north or south the spot is. You can write 40.7486, 40°44'55"N, or 40 44 55 N. North is positive, south is negative.

Longitude: Type how far east or west the spot is. You can write -73.9864, 73°59'11"W, or 73 59 11 W. East is positive, west is negative.

Altitude (or Radius): Type your height above the ellipsoid. Use 0 for ground level, or your cruise altitude for flying. Pick the unit next to it: km, m, ft, mi, or nmi.

Interactive Map: Click or tap the map to drop the marker, or drag it. You can also focus the map and use the arrow keys, then press Enter.

Date format: Pick Calendar date to use a normal YYYY-MM-DD date, or Decimal year to type something like 2026.5.

Date (or Decimal year): Enter the day you need the declination for. Stay between 2025 and 2030, or the answer gets less accurate.

Geomagnetic model: Keep WMM2025 for flying, sailing, and charts. Pick IGRF-14 for science and research work.

Model resolution: Leave the switch off to use degree 12, the resolution of the World Magnetic Model.¹ Turn it on to use every degree the model carries, which is degree 13 for IGRF-14. With WMM2025 both settings give the same answer.

Display declination as: After you calculate, choose decimal degrees, degrees/minutes/seconds, or NATO mils to match your charts or gear.

Field intensity unit: Pick nT, µT, mG, or G for the magnetic field strength shown in the components table.

Calculate and Reset: Press Calculate to see your results. Press Reset to clear everything and start over. You can then download the results as JSON or CSV, or copy them to your clipboard.

What Is Magnetic Declination?

A compass needle does not point to true north. It points along the horizontal part of Earth's magnetic field, toward magnetic north. Magnetic declination (also called magnetic variation) is the angle between true north and the horizontal direction of the magnetic field at your spot, measured clockwise from true north.¹ If the needle points to the right of true north, the declination is east (a plus number). If it points to the left, it is west (a minus number).¹

Why Declination Matters

Courses on aeronautical charts are measured from true north.² Your compass does not use true north. If you ignore the difference, you drift off course. A 5° error grows into about 1 mile of drift for every 11 or 12 miles you travel. Pilots, sailors, hikers, and surveyors all correct for it.

In aviation the same angle is called variation. It is printed on most aeronautical charts as broken magenta isogonic lines that connect points of equal variation.² Runway numbers are based on magnetic headings, not true headings, so as the local variation drifts, a runway's magnetic heading slowly changes too.

How to Use the Number

Magnetic to true: add east declination, subtract west declination.
True to magnetic: subtract east declination, add west declination.²
An old pilot memory trick: "East is least, West is best". Subtract east and add west when going from true to magnetic.²

Secular Variation: The Yearly Drift

Most of Earth's magnetic field is made by the liquid-iron outer core, and it changes from year to year.¹ This slow change is called secular variation. The World Magnetic Model treats it as a straight-line change in each coefficient, which is why the model must be updated every five years.¹ That is why an old chart can be wrong today, and why you should always use a current date when you check declination.

The Models Behind the Numbers

Declination is not measured at every point on Earth. It is calculated from a math model of the whole field:

WMM (World Magnetic Model), built by NOAA and the British Geological Survey. It is the standard model for navigation used by the U.S. Department of Defense, the U.K. Ministry of Defence, NATO, and the International Hydrographic Organization, and it is also used widely in civilian navigation.¹ It is updated every five years.¹
IGRF (International Geomagnetic Reference Field), a standard description of Earth's main field developed by the International Association of Geomagnetism and Aeronomy and used widely in research on Earth's interior, crust, ionosphere, and magnetosphere.⁶

Both models describe the field as a sum of spherical harmonic terms, wave-like patterns over the globe, with coefficients that change in a straight line over time.¹ Adding them up rebuilds the field at any latitude, longitude, altitude, and date.

Other Field Values You Will See

Inclination (I) is how steeply the field dips into the ground. At the magnetic poles the field lines are vertical, so a compass needle tries to point down and does not work well.¹
Horizontal intensity (H) is the sideways pull that actually turns a compass needle. Low H means a weak, slow needle.
X, Y, Z are the north, east, and downward parts of the field.¹
Total intensity (F) is the full strength of the field, which ranges from about 23,000 to 67,000 nanotesla at Earth's surface.¹

Things That Can Throw a Compass Off

Declination is only the Earth-wide part of the error. Magnetic influences in your own vehicle, such as electrical circuits, radios, the engine, and magnetized metal parts, deflect the needle too. That extra error is called deviation.² Near the magnetic poles, the World Magnetic Model marks Blackout Zones where its declination values are inaccurate and compasses cannot be trusted.¹ The agonic line is the line where there is no variation.² Along it, magnetic north and true north line up.

Formulas used

Time-varying Gauss coefficients ¹

g_n^m(t) = g_n^m(t_0) + (t - t_0)\,\dot{g}_n^m, \quad h_n^m(t) = h_n^m(t_0) + (t - t_0)\,\dot{h}_n^m

Geodetic to geocentric conversion ¹

R_c = \frac{a}{\sqrt{1 - e^2\sin^2\phi}},\quad p = (R_c + h)\cos\phi,\quad z = \left(R_c(1-e^2) + h\right)\sin\phi,\quad r = \sqrt{p^2 + z^2},\quad \phi' = \arcsin\!\left(\frac{z}{r}\right)

Spherical harmonic field synthesis (geocentric components) ¹

\begin{aligned} X' &= \sum_{n=1}^{N}\left(\frac{a}{r}\right)^{n+2}\sum_{m=0}^{n}\left(g_n^m\cos m\lambda + h_n^m\sin m\lambda\right)\frac{dP_n^m(\theta)}{d\theta} \\ Y' &= \frac{1}{\sin\theta}\sum_{n=1}^{N}\left(\frac{a}{r}\right)^{n+2}\sum_{m=0}^{n} m\left(g_n^m\sin m\lambda - h_n^m\cos m\lambda\right)P_n^m(\theta) \\ Z' &= -\sum_{n=1}^{N}\left(\frac{a}{r}\right)^{n+2}\sum_{m=0}^{n}(n+1)\left(g_n^m\cos m\lambda + h_n^m\sin m\lambda\right)P_n^m(\theta) \end{aligned}

Rotation to the local horizon frame ¹

\psi = \phi' - \phi,\quad X = X'\cos\psi - Z'\sin\psi,\quad Y = Y',\quad Z = X'\sin\psi + Z'\cos\psi

Magnetic declination and inclination ¹

D = \arctan\!\left(\frac{Y}{X}\right),\qquad I = \arctan\!\left(\frac{Z}{H}\right)

Horizontal and total field intensity ¹

H = \sqrt{X^2 + Y^2},\qquad F = \sqrt{H^2 + Z^2} = \sqrt{X^2 + Y^2 + Z^2}

Secular variation of declination and inclination ¹

\dot{D} = \frac{X\dot{Y} - Y\dot{X}}{H^2},\qquad \dot{I} = \frac{H\dot{Z} - Z\dot{H}}{F^2},\qquad \dot{H} = \frac{X\dot{X} + Y\dot{Y}}{H},\qquad \dot{F} = \frac{X\dot{X} + Y\dot{Y} + Z\dot{Z}}{F}

Decimal year from calendar date

t = Y + \frac{\tau - \tau_{\text{Jan 1},Y}}{\tau_{\text{Jan 1},Y+1} - \tau_{\text{Jan 1},Y}}

Frequently asked questions

Is magnetic north the same as the geographic North Pole?

No. The geographic North Pole is a fixed point where all the lines of longitude meet. The north magnetic pole (the north dip pole) is where Earth's magnetic field points straight down. In 2025 it was at about 85.76°N, 139.30°E, hundreds of miles from the true pole.¹ Your compass needle lines up with the field, so it points toward the magnetic pole, not the true one. The angle between the two directions at your spot is the magnetic declination.

How fast is the magnetic north pole moving?

It moves every year, and the speed changes. The most recent survey found the North Magnetic Pole moving north-northwest at about 55 km a year.⁴ For 2025 to 2030, the World Magnetic Model predicts a slower drift of about 35 km a year, still slowing down.¹

Because of this drift, the declination where you live changes too, usually by a few arcminutes each year.

How do I set declination on my compass?

Many baseplate and lensatic compasses have a small screw or a turning ring for declination:

Look up the declination for your area first.
Use the tiny key (often on the lanyard) to turn the screw until the orienting arrow sits at your declination angle: right of north for east, left of north for west.
Now you can read true bearings straight off the compass with no math.

If your compass cannot be adjusted, do the math yourself: add east declination to a magnetic bearing, or subtract west declination, to get the true bearing.

Does my phone compass show true north or magnetic north?

It depends on the setting. The sensor inside your phone measures the magnetic field, and many compass apps can then show either true north or magnetic north. Check the app's settings to see which one you are reading.

Keep the phone away from magnets, metal cases, speakers, and car dashboards. Those cause errors that no model can fix.

Does GPS use magnetic north or true north?

GPS itself works in true north. It finds your position from satellites, and direction of travel is computed from real positions, not from a magnet.

Many handheld GPS units and aircraft displays then show you a magnetic bearing anyway, because charts, runways, and ATC headings are magnetic. The unit simply applies the declination from a stored model. Check your device's north reference setting so you know which one you are reading.

Why do airports change their runway numbers?

A runway number is the whole number nearest one-tenth of the runway's magnetic heading.³ A runway pointing 184° magnetic is Runway 18.

Magnetic north keeps drifting, so over the years that heading slowly changes. When the drift pushes it past the rounding point, the airport has to repaint the numbers, redo the signs, and update the charts.

Does altitude change magnetic declination?

Yes, a little. The magnetic field gets weaker as you go up, and the direction shifts slightly too. In the WMM2025 test values, going from the surface to 100 km up changes the declination at 80°N, 0°E from 1.28° to 0.85°, and at the equator at 120°E from −0.16° to −0.15°.¹

Altitude matters much more for satellites and high-flying research work, where the field strength drops a lot.

Where on Earth is magnetic declination the biggest?

Near the magnetic poles. Declination can be anywhere from −180° to 180°, so close to a pole magnetic north can be 90°, or even 180°, away from true north.¹

In the United States, the compass points east of true north on the west coast and west of true north on the east coast.²

What is the difference between variation and deviation?

Variation (the same thing as declination) is the Earth-wide error. It depends only on where you are and the date, not on your vehicle.²

Deviation is an error caused by your own vehicle: the engine, electrical circuits, radios, lights, tools, and magnetized metal parts. It is different for each aircraft and can change with the heading you are flying.²

You fix variation with a model. You fix deviation by moving metal away or by using a deviation card.

What is a compass deviation card?

It is a small card mounted next to the magnetic compass in a plane or boat. It lists the heading you should steer to actually fly or sail a given magnetic heading, for example "For 030 steer 032."

It is made during a compass swing: the aircraft is lined up on a marked compass rose with the engine and radios running, and the compass reading is recorded at 30° intervals.² Landing shocks, vibration, mechanical work, and equipment changes can change the deviation, so it should be checked from time to time.²

Why do pilots near the poles use true north instead of magnetic?

Near the magnetic poles the field lines are vertical, so the sideways pull that turns a compass needle is weak and the needle may spin. The World Magnetic Model marks Blackout Zones where the horizontal intensity is under 2,000 nT; there its declination values are inaccurate and compasses cannot be trusted.¹ Each is surrounded by a Caution Zone where the horizontal intensity is under 6,000 nT.¹

That is why navigation near the poles relies on true or grid directions from other systems rather than the magnetic compass.

How often is the World Magnetic Model updated?

Every five years¹. The current version is WMM2025, and it is valid from 2025.0 to 2030.0.¹ Each release includes yearly change rates so it can predict the field between updates.

Once, the field changed faster than the model predicted, and an out-of-cycle update, WMM2015v2, was released in early 2019, before the normal 2020 version.⁵ That is why it pays to check declination with a current date instead of trusting an old chart.

Sources

Chulliat A, Brown W, Nair M, et al. The US/UK World Magnetic Model for 2025-2030: Technical Report. NOAA National Centers for Environmental Information. 2025;§ 1.1, Table 1, § 1.2 eqs. 7-20, § 1.6, § 1.8, Table 6. Accessed September 29, 2026.

Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 16: Navigation. Federal Aviation Administration. 2023;Variation and Deviation, pp. 16-6 to 16-8. Accessed September 29, 2026.

Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 14: Airport Operations. Federal Aviation Administration. 2023;Runway Designation Marking, p. 14-8. Accessed September 29, 2026.

Wandering of the Geomagnetic Poles. NOAA National Centers for Environmental Information. Accessed September 29, 2026.

Chulliat A, Brown W, Alken P, et al. Out-of-Cycle Update of the US/UK World Magnetic Model for 2015-2020: Technical Note. NOAA National Centers for Environmental Information. 2019;§ 1. Accessed September 29, 2026.

International Geomagnetic Reference Field (IGRF). NOAA National Centers for Environmental Information. Accessed September 29, 2026.

Location Input

Interactive Map

Date, Model & Resolution

Rate this calculator

Introduction

How to use our Magnetic Declination Calculator

What Is Magnetic Declination?

Why Declination Matters

How to Use the Number

Secular Variation: The Yearly Drift

The Models Behind the Numbers

Other Field Values You Will See

Things That Can Throw a Compass Off

Formulas used

Frequently asked questions

Is magnetic north the same as the geographic North Pole?

How fast is the magnetic north pole moving?

How do I set declination on my compass?

Does my phone compass show true north or magnetic north?

Does GPS use magnetic north or true north?

Why do airports change their runway numbers?

Does altitude change magnetic declination?

Where on Earth is magnetic declination the biggest?

What is the difference between variation and deviation?

What is a compass deviation card?

Why do pilots near the poles use true north instead of magnetic?

How often is the World Magnetic Model updated?

Sources

Magnetic Declination Calculator

Location Input

Interactive Map

Date, Model & Resolution

Rate this calculator

Introduction

How to use our Magnetic Declination Calculator

What Is Magnetic Declination?

Why Declination Matters

How to Use the Number

Secular Variation: The Yearly Drift

The Models Behind the Numbers

Other Field Values You Will See

Things That Can Throw a Compass Off

Formulas used

Frequently asked questions

Is magnetic north the same as the geographic North Pole?

How fast is the magnetic north pole moving?

How do I set declination on my compass?

Does my phone compass show true north or magnetic north?

Does GPS use magnetic north or true north?

Why do airports change their runway numbers?

Does altitude change magnetic declination?

Where on Earth is magnetic declination the biggest?

What is the difference between variation and deviation?

What is a compass deviation card?

Why do pilots near the poles use true north instead of magnetic?

How often is the World Magnetic Model updated?

Sources

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