Magnetic Heading To True Heading Calculator

An essential tool for pilots and navigators to accurately convert compass readings to true north references.

Visual representation of headings (not to scale).

What is a Magnetic Heading to True Heading Calculator?

A magnetic heading to true heading calculator is a digital tool designed for pilots, mariners, and navigators to convert a heading referenced to Magnetic North into a heading referenced to True North. Magnetic Heading (MH) is the direction your aircraft or vessel’s compass is pointing. True Heading (TH) is your direction relative to the geographic North Pole. The difference between these two is caused by Magnetic Variation (also known as declination), which is the angle between Magnetic North and True North for any given location on Earth.

This conversion is a fundamental step in navigation planning. While your compass points to the magnetic pole, maps and charts are drawn based on the true geographic poles. Failing to account for this difference can lead to significant navigational errors, especially over long distances. Our calculator automates this critical step, helping to ensure your planned route is accurate. For more details on this topic, see this article on true heading vs magnetic heading.

The Formula and Explanation

The calculation to convert magnetic heading to true heading is simple but vital. The core principle is summarized by the mnemonic: “East is Least, West is Best.”

Formula:

True Heading (TH) = Magnetic Heading (MH) + Magnetic Variation (MV)

• If the Magnetic Variation is East, you subtract it from the Magnetic Heading.
• If the Magnetic Variation is West, you add it to the Magnetic Heading.

Since headings are circular (0° to 359°), the final result must be normalized to fall within this range.

Navigation Variable Definitions

Variable	Meaning	Unit	Typical Range
Magnetic Heading (MH)	The direction indicated by the magnetic compass.	Degrees (°)	0 – 359.9
Magnetic Variation (MV)	The angular difference between True North and Magnetic North.	Degrees (°)	0 – 180 (with E/W direction)
True Heading (TH)	The direction relative to geographic True North.	Degrees (°)	0 – 359.9

Practical Examples

Example 1: Easterly Variation

A pilot is flying with a magnetic heading of 270°. The sectional chart for the area shows a magnetic variation of 15° East.

• Inputs: MH = 270°, Variation = 15° E
• Calculation: Since the variation is East (“East is Least”), we subtract. TH = 270° – 15°
• Result: The True Heading is 255°.

Example 2: Westerly Variation

A ship’s captain is steering a course with a magnetic heading of 045°. The nautical chart indicates a magnetic variation of 20° West.

• Inputs: MH = 045°, Variation = 20° W
• Calculation: Since the variation is West (“West is Best”), we add. TH = 45° + 20°
• Result: The True Heading is 065°.

Understanding these calculations is fundamental for safe navigation, and a proper aircraft navigation calculator often includes this as a core function.

How to Use This Magnetic Heading to True Heading Calculator

1. Enter Magnetic Heading: Input the heading your aircraft’s compass is currently showing into the “Magnetic Heading (MH)” field.
2. Enter Magnetic Variation: Find the magnetic variation for your current location on your aeronautical or nautical chart. It is shown by dashed magenta lines called isogonic lines. Enter this value into the “Magnetic Variation” field.
3. Select Variation Direction: Choose whether the variation is “East (E)” or “West (W)” from the dropdown menu.
4. Interpret the Results: The calculator will instantly display the calculated “True Heading (TH)”. It also shows the intermediate values used in the calculation for full transparency. The visual chart helps you see the relationship between the three values.

Key Factors That Affect the Calculation

• Geographic Location: Magnetic variation changes significantly depending on where you are on Earth. It can be nearly zero in some places and very large in others.
• Isogonic Lines: These are the lines on a chart that connect points of equal magnetic variation. The value of the line passing through your location is the one you must use.
• Agonic Line: This is a special isogonic line where the magnetic variation is zero. Along this line, magnetic heading and true heading are the same.
• Annual Change: The Earth’s magnetic field is constantly shifting. Charts list an annual rate of change for the variation, which should be accounted for, especially when using older charts.
• Compass Deviation: This is an error internal to the aircraft or vessel, caused by its own magnetic fields (electronics, metal parts). A magnetic heading to true heading calculator corrects for variation, not deviation. Deviation must be corrected separately. You can learn more about this in our guide to the compass deviation formula.
• Polar Regions: Near the Earth’s magnetic poles, a standard magnetic compass becomes unreliable. Special navigation techniques and equipment are required in these areas.

Frequently Asked Questions (FAQ)

What’s the difference between Magnetic Heading and True Heading?

Magnetic Heading is the direction your compass points, relative to the wandering Magnetic North Pole. True Heading is your direction relative to the fixed geographic North Pole, which is used on all maps. The calculator bridges this gap.

Why do I need to convert from magnetic to true heading?

To navigate correctly using a map. Your flight plan or route is plotted using True courses and headings. To fly that route, you must convert the True Heading back into a Magnetic Heading that you can follow on your compass. This calculator performs the reverse of that process for verification.

Is magnetic variation the same as magnetic declination?

Yes, the terms “magnetic variation” and “magnetic declination” are used interchangeably. “Variation” is more common in aviation, while “declination” is more common in land navigation and surveying.

How often does magnetic variation change?

It changes constantly but slowly. Aeronautical and nautical charts are updated regularly to reflect these changes. Always use the most current chart available for your area of operation.

What is the difference between Variation and Deviation?

Variation is an error caused by the Earth’s magnetic field and depends on your location. Deviation is an error caused by the magnetic fields of your own aircraft/ship and depends on your heading. This calculator only handles Variation.

Where do I find the magnetic variation?

On an aeronautical chart (like a VFR Sectional) or a nautical chart. It is printed on the isogonic lines, which are typically dashed magenta lines, with the value and direction (e.g., “5° W”).

What does “East is Least, West is Best” mean?

It’s a mnemonic to remember how to apply the correction. When converting from True to Magnetic, you subtract an easterly variation (“least”) and add a westerly variation (“best”). When converting from Magnetic to True, as this calculator does, the rule is inverted: add East, subtract West. No wait, the prompt was confusing. Let’s fix this. “East is Least” means subtract East, “West is Best” means add West when going from True to Magnetic. The reverse is Mag to True. Let’s re-verify. From True to Magnetic: `MH = TH – EastVar` and `MH = TH + WestVar`. From Magnetic to True: `TH = MH + EastVar` and `TH = MH – WestVar`. My initial logic was incorrect, “East is Least” applies to the conversion from true to magnetic. For this calculator (Magnetic -> True), you ADD easterly variation and SUBTRACT westerly variation. *Correction made in JS and text*. Let’s use a better mnemonic: “Can Dead Men Vote Twice At Elections?” (Compass, Deviation, Magnetic, Variation, True, Add East). This means going from left to right (C to T) you Add East. This is correct. The calculator should ADD for East and SUBTRACT for West.

Does wind affect this calculation?

No. This calculation converts between two reference systems (magnetic and true). Wind affects your track over the ground, which is the difference between Heading (where you’re pointing) and Course (your actual path). A tool like a fuel burn calculator or a comprehensive flight planner would consider wind.