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What it is
Magnetic declination is the angle between true north, the direction to the geographic North Pole, and magnetic north, the direction a compass needle actually points. It varies by place and changes slowly over time, and it is the reason a bearing taken from a map and a bearing read from a compass disagree unless you convert between them.
Maps are drawn to true north, while compasses point to magnetic north. If the declination where you stand is 10 degrees east, a true bearing of 90 degrees is a magnetic bearing of 80 degrees, and steering 90 on your compass would send you 10 degrees off course. Over a long leg, that error grows into kilometres.
How to use it
Enter the bearing you have, the declination in degrees, and whether the declination is east or west, then choose the system you have and the system you want. Press Calculate and you get the true bearing, the magnetic bearing, and the converted result, so you can check the conversion in both directions at a glance.
Get your declination from an official model or chart for your place and date. This tool never invents a declination for a location, because a guessed value is worse than no value: it looks precise while steering you wrong.
Worked example
Take a true bearing of 90 degrees with a declination of 10 degrees east. The magnetic bearing is 80 degrees: you subtract east declination when going from true to magnetic. Reverse it, starting from a magnetic bearing of 80 degrees with the same declination, and you get back to a true bearing of 90 degrees.
With a declination of 10 degrees west instead, the same true bearing of 90 degrees becomes a magnetic bearing of 100 degrees. West does the opposite of east, and that single sign flip is the source of most conversion errors.
How to read your result
The result shows all three numbers so you can audit the conversion: true bearing, magnetic bearing, and the one you asked for. If the two bearings differ by exactly your declination, the conversion is consistent. If they differ by anything else, re-check the east or west setting first, because that is the usual culprit.
Remember the handy rule: east is least and west is best when going from magnetic to true, meaning subtract east and add west. The tool applies the rule for you, but knowing it lets you catch a wrong input.
Common mistakes
The most common mistake is getting east and west backwards, which doubles the error instead of correcting it. Another is using an old declination from a map printed years ago, since declination drifts over time. A third is forgetting local magnetic interference from iron, vehicles, or electronics, which can deflect a compass far more than declination does.
Limits
This converter handles true and magnetic bearings only. It does not apply compass deviation, the error caused by magnetic material in your own vessel or vehicle, which must be measured separately with a deviation card. It also does not supply declination values: always take yours from an official model or chart for your place and date, and re-check it on long trips.
Questions people ask
What is the difference between declination and deviation?
Declination is the Earth’s field differing from true north, and it depends on where you are. Deviation is error from magnetic material near your own compass, like a steel hull or a phone, and it depends on your heading.
How do I find my local declination?
Use an official geomagnetic model or chart for your location and date, such as a national survey’s online calculator. Note the date, because declination drifts year by year.
Does declination matter on short walks?
On a short, well-marked trail, barely. On long legs, in poor visibility, or over water, a 10-degree error over 10 kilometres puts you nearly 2 kilometres off course, which is why navigators always convert.
Related calculators
- Spanish version
- Bearing Between Two Points Calculator – Initial Bearing and Distance from Coordinates