Magnetic Declination Calculator
Convert a magnetic heading to a true heading, or the other way round, using the declination for your area.
What it does
A compass points at magnetic north, and a map points at true north, and the angle between the two is the magnetic declination. NOAA describes it as the angle between magnetic north and true north at a given place and time, with east declination positive and west declination negative. It is not a constant anywhere, because the earth's magnetic field is irregular and it drifts.
The correction is one line. Add the declination to a magnetic bearing to get a true bearing, and subtract it to go the other way, which is exactly what the NOAA bearing guide says. Set a compass by that rule, or read a map bearing with it, and the two agree.
How big the correction is depends on where you are. The World Magnetic Model 2025, the model NOAA publishes for the United States, puts Washington DC at 10.7 degrees west, Seattle at 14.9 degrees east and Los Angeles at 11.3 degrees east. A magnetic heading in Seattle is 25 degrees away from the same magnetic heading in Washington, which is the difference between roughly a mile and a half of ground error per mile travelled.
The declination also moves. The same model lists annual changes for those cities of about 0.01 to 0.12 degrees a year, and the direction of the change differs from place to place. A navigation setup that was correct when it was last set needs checking again after a few years, and a passage written down a decade ago may need a different number.
One thing this correction does not do is fix a compass that is badly installed. The NOAA resources are careful about that: the rule handles the difference between magnetic and true north, not the local deviation of a compass on a particular boat or a particular aircraft.
How to use
- Decide whether the heading you have is a magnetic heading or a true heading.
- Enter the heading in degrees.
- Look up your declination with the NOAA calculator and enter the value.
- Choose whether the declination is east or west.
- Read the converted heading, and use the reverse value if you need to set a compass from a map bearing.
Formula
- T
- True bearing, measured from true north.
- M
- Magnetic bearing, measured from magnetic north.
- D
- Magnetic declination, positive east and negative west.
The formula applies to a bearing taken from a compass, not to a magnetic course measured after the fact with a magnetometer, and it does not include the deviation of a particular compass. The declination value itself has to come from a current model for your location and date, which is what the NOAA calculator provides.
Worked example
Example: a magnetic heading of 090 with 12.5 degrees east declination
Defaults: magnetic heading of 90 degrees, declination 12.5 degrees, east.
- East declination is positive, so the correction is +12.5 degrees.
- True heading: 90 + 12.5 = 102.5 degrees.
- Going back the other way: 90 - 12.5 = 77.5 degrees magnetic.
- With 12.5 degrees of west declination the true heading would be 77.5 degrees instead.
- The tool shows 102.5 degrees true, 77.5 degrees magnetic and a correction of 12.5 degrees east.
Reference table
| City | Declination | Annual change | Model epoch |
|---|---|---|---|
| Washington, DC | 10.69° W | 0.01° E per year | 2025.0 |
| Denver, CO | 7.49° E | 0.08° W per year | 2025.0 |
| Seattle, WA | 14.87° E | 0.12° W per year | 2025.0 |
| Los Angeles, CA | 11.34° E | 0.08° W per year | 2025.0 |
Limits
- Does not include compass deviation, which is the error of a particular compass on a particular deck.
- Uses a single declination value, while the real value changes across a region.
- Does not update itself, so the value you enter goes out of date as the field drifts.
- Assumes a flat map, so it does not cover the convergence of magnetic and true meridians on a chart.
Common mistakes
- Subtracting east declination instead of adding it, which sends you the wrong way.
- Forgetting that the declination sign depends on where you are, so a rule learned in one country fails in another.
- Using a declination from a map when the value is several years old.
- Applying the correction twice, once on the chart and once on the compass.
- Confusing declination with the magnetic variation printed on an old chart, which is the same idea for a different model year.
Try it yourself
Quick checks. Answers sit tucked under each question.
Your compass reads 045 and the declination is 10 degrees west. What is the true heading?
West declination is subtracted, so the true heading is 45 - 10 = 35.0 degrees.
A chart gives a true bearing of 200 and the local declination is 14 degrees east. What does the compass read?
Subtract the east declination: 200 - 14 = 186.0 degrees magnetic.
Why does the same compass reading mean different things in Seattle and Denver?
Seattle has 14.87 degrees of east declination and Denver has 7.49 degrees of east declination, so the same magnetic heading points in very different true directions. Over ten miles that is several miles of difference on the ground.
Questions
What is magnetic declination?
The angle between magnetic north and true north at your location. It is positive to the east and negative to the west, and it is published by NOAA for any point on the map.
How do I find my declination?
The NOAA magnetic declination calculator takes a latitude and longitude, a model and a date, and returns the declination with an uncertainty figure. The World Magnetic Model covers the United States, and there is an international model for the rest of the world.
Does the declination change over time?
Yes, it drifts by a small amount each year, and the direction of the drift differs by place. The table on this page gives the annual change for four cities, and NOAA publishes the current values for the whole country.
What is the difference between declination and deviation?
Declination is the difference between magnetic and true north at your location. Deviation is the error of your own compass, caused by metal and electrical equipment nearby. The rule on this page handles declination only.
Why do old charts cause errors?
Charts print the magnetic variation for the year they were made. On an old chart that number is out of date, so the correction you apply from it no longer matches the real field.
Does this work for a magnetic course from a GPS?
It can be used, but a GPS gives a true course, so you subtract the declination to get what the compass would read. The difference between the two is the same correction in reverse.
Sources
- NOAA National Centers for Environmental Information: magnetic declination
- NOAA: magnetic and true bearing conversion
- NOAA National Centers for Environmental Information: World Magnetic Model