Magnetic Variation Calculator: Find True Bearing

This calculator converts a magnetic bearing to a true bearing (or vice versa) using the local magnetic variation (declination) you enter. You provide the bearing you measured or plan to use, the variation value for your location (positive east, negative west), and the direction of conversion, and you get the corrected bearing in degrees. It is a simple, essential tool for navigation by map and compass, aviation, and marine use.

Formula

The relationship between magnetic and true bearings is simple arithmetic once you know the local magnetic variation (also called declination). The formula is:

True bearing = Magnetic bearing + Variation (East positive, West negative)

For the reverse conversion:

Magnetic bearing = True bearing – Variation

Where:

• True bearing (degrees) – the direction relative to geographic (true) north.
• Magnetic bearing (degrees) – the direction relative to magnetic north (what a compass reads).
• Variation (degrees) – the angle between true north and magnetic north at your location. East variation is positive; west variation is negative.

All bearings are measured clockwise from north (0° to 359°). The calculator normalizes the result to a 0–360° range.

Worked Example

Using the default values: you have a magnetic bearing of 90° (due east) and the local variation is 10° East.

1. Start with the magnetic bearing: 90°
2. Add the variation (East is positive): 90° + 10° = 100°

The true bearing is 100°. The calculator shows 100.00° as the headline result.

If you had a true bearing of 100° and wanted the magnetic bearing, you would subtract the variation: 100° – 10° = 90°.

What Is Magnetic Variation and Why Does It Matter?

Magnetic variation is the angle between true north (the geographic North Pole) and magnetic north (the point where the Earth’s magnetic field points downward, currently near northern Canada). This angle is not constant; it changes over time and varies by location. For example, in 2025, the variation in Seattle is about 13° East, while in New York it is about 12° West. If you ignore this offset, a 10-mile hike on a bearing of 90° magnetic would put you about 2 miles off your intended track – a significant error.

Maps are drawn relative to true north, but compasses point to magnetic north. To navigate accurately, you must convert between the two. This calculator does that conversion quickly, but you still need to obtain the correct variation for your area and date.

How to Find the Magnetic Variation for Your Location

You need the variation value for your specific location and the current date. Here are reliable ways to get it:

• On a topographic map: The declination diagram (usually in the margin) shows the offset for the map’s center and the year it was measured, plus an annual change note.
• Online magnetic field calculators: The National Centers for Environmental Information (NCEI) and the British Geological Survey offer free tools that provide the current declination for any coordinates.
• Aviation and marine charts: These charts show isogonic lines (lines of equal variation) and often list the annual change.
• GPS devices and navigation apps: Many units display the local declination in their setup or satellite page.

Always use the most current value, because variation can change by a fraction of a degree per year, and over a decade the shift can be several degrees.

Common Mistakes and Misconceptions

A frequent error is mixing up the sign of the variation. The mnemonic “East is least, West is best” helps: when converting from true to magnetic, subtract east variation and add west variation. But when converting from magnetic to true, you do the opposite. The calculator handles this automatically if you select the correct conversion direction.

Another mistake is using the wrong variation for the date. If you use a 20-year-old map without updating the annual change, your bearing could be off by several degrees. Also, some sources list variation as “magnetic declination” – they are the same thing.

Finally, remember that this calculator works with bearings in degrees, not with compass headings that include wind or current corrections. It only corrects for the magnetic offset, not for other navigation errors.

Limitations and Practical Tips

The formula assumes you are using a standard magnetic compass that points to magnetic north. Local magnetic anomalies (e.g., iron deposits, metal structures) can cause a compass to deviate from the true magnetic field. The calculator does not account for those anomalies.

Also, variation changes over time, so the value you enter is only valid for a certain date. For high-precision work (like instrument flying), you should use the exact variation for your route and date. For casual hiking, rounding to the nearest degree is usually sufficient.

When plotting a course on a map, you typically work with true bearings, then convert to magnetic for your compass. This calculator makes that step quick and error-free.

Frequently Asked Questions

What is the difference between magnetic variation and declination?

They are the same thing. “Declination” is the term used in geophysics and by many map makers; “variation” is the term used in aviation and marine navigation. Both refer to the angle between true north and magnetic north.

How often does magnetic variation change?

It changes continuously, but slowly – typically a few tenths of a degree per year. The rate varies by location. For most recreational navigation, updating your variation every 5–10 years is sufficient, but for precise work, use a current value.

Can I use this calculator for aviation navigation?

Yes, but be aware that aviation uses “magnetic variation” in the same way. However, for flight planning you also need to account for wind correction and instrument errors. This calculator only handles the variation conversion.

What if my bearing is exactly 360° or 0°?

The calculator normalizes the result to 0–360°, so a true bearing of 360° is shown as 0°. That is correct – 0° and 360° represent the same direction (north).

Do I need to adjust for magnetic variation when using a GPS?

Most GPS units are set to display true bearings by default, but they can be configured to show magnetic. If you are using a compass with a map, you still need to apply the variation. Always check your GPS settings to know which reference you are using.

Related

• Coefficient of Variation Calculator: CV Formula & Use
• Coefficient of Variation Calculator Online: Formula & Use
• Coefficient of Variation Formula Calculator

in bearing | Bearing (magnetic or true) | deg | 90 | 0
in variation | Magnetic variation (east +, west -) | deg | 10 | -180
sel conversion | Convert | Magnetic to True=1, True to Magnetic=2
out result | Corrected bearing | deg | bearing + variation * (conversion == 1 ? 1 : -1) | 2