Magnetic Declination Calculator

Background.

A compass needle does not point at the North Pole. It lines up with the horizontal part of the Earth's magnetic field, which in most places points some degrees east or west of true north, and the size of that offset — the magnetic declination — depends on where you are and when. Sailors and pilots call the same angle variation.

For anyone navigating with map and compass, the difference matters more than it sounds. A bearing measured off a map is referenced to true north; a compass measures from magnetic north. Follow an uncorrected map bearing on the compass and you drift off line by the full declination: at 8.5°, after one kilometre you are about 148 metres to one side of where you meant to be. The fix is a single addition or subtraction, but applying it with the wrong sign doubles the error instead of removing it.

The sign convention here is NOAA's: declination is positive when magnetic north lies east of true north and negative when it lies west. With that convention, true bearing = magnetic bearing + declination, and magnetic bearing = true bearing − declination. Answers are brought back into the 0° to 360° range, so a magnetic 355° with 8.5° east declination becomes a true 3.5° rather than 363.5°.

The page does not look up the declination for you. That needs the World Magnetic Model, which NOAA and the British Geological Survey issue for five-year periods because the field drifts continuously; take the value for your location and date from the official calculator or a recent map. Compass deviation — the error from nearby steel, electronics or a vehicle's own magnetism — is not included either.

What is magnetic declination calculator?

Magnetic declination is the horizontal angle between true north, the direction along a meridian to the geographic North Pole, and magnetic north, the direction a level compass needle settles in at that place. It is positive when magnetic north lies east of true north and negative when it lies west. It differs from place to place, from zero along the line where the two norths coincide to tens of degrees near the magnetic poles, and it shifts gradually from year to year.

How to use this calculator.

1. 01Choose which bearing you already have: a magnetic bearing read from a compass, or a true bearing measured from a map or chart.
2. 02Enter that bearing in degrees. Values outside 0–360, such as −10 or 370, are accepted and wrapped round to the equivalent direction.
3. 03Enter the local declination with its sign: east positive, west negative. Use a current value for your location from the World Magnetic Model calculator or a recent map, not the figure printed on an old one.
4. 04Read both bearings. The one you entered comes back normalised to 0–360°, and the other is its converted partner.

The formula.

True bearing = magnetic bearing + D, and magnetic bearing = true bearing − D, where D is the declination in degrees, east positive. The addition makes sense once pictured: if magnetic north sits 8.5° east of true north, a compass reading of 0° actually points to 8.5° true, so every magnetic bearing falls 8.5° short of its true equivalent. After the addition or subtraction, the result is reduced to the range from 0° up to but not including 360°, so a full turn reads as 0°. Navigators carry the same rule as a rhyme for going from true to magnetic — east is least, west is best — meaning subtract an easterly declination and add a westerly one. The arithmetic is exact. Any uncertainty lies in the declination you supply, which a model cannot know perfectly near magnetic rocks or during a magnetic storm.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NOAA National Centers for Environmental Information. World Magnetic Model 2025. Declination definition and official calculator. Retrieved 2026-08-06.
2. [2]OpenStax, Rice University. University Physics, 2016. Volume 1 mechanics and Volume 2 thermodynamics. Retrieved 2026-08-06.
3. [3]OpenStax, Rice University. Chemistry 2e, 2019. Chapters 4, 9, and 12. Retrieved 2026-08-06.