True Course to Magnetic Heading Converter

Enter your true course, magnetic variation, compass deviation, and distance to calculate your compass heading, magnetic heading, total error, and estimated transit time.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter the True Course (deg)
Input the intended ground track over Earth's surface, measured clockwise from true north. This is typically found on aeronautical charts.
2
Enter the Variation (deg)
Provide the angular difference between true north and magnetic north at your current location. This value, either east (+) or west (-), is also found on aeronautical charts.
3
Enter the Deviation (deg)
Input the magnetic error caused by the aircraft's own electrical systems and metal components. This value, specific to each aircraft, is found on a compass deviation card.
4
Enter the Distance (NM)
Specify the distance to a target or waypoint in nautical miles. This affects the calculated radar range.
5
Review your results
The calculator will display the Magnetic Heading, Compass Heading, and a Reference Radar Range based on your inputs.

Example Calculation

A pilot is planning a flight segment and needs to convert their true course into magnetic and compass headings for navigation, while also estimating a practical radar range for a target 100 nautical miles away.

True Course (deg)

270

Variation (deg)

-10

Deviation (deg)

Distance (NM)

100

Results

Magnetic Heading

280 deg, Compass Heading: 277 deg, Reference Radar Range: 12.3 NM

Tips

Prioritize Accurate Variation and Deviation

Always use the most current variation from up-to-date aeronautical charts and the specific deviation card for your aircraft. A 3-degree error in either can lead to a significant drift of 3 nautical miles over a 60-mile flight segment.

Understand Magnetic Compass Limitations

Magnetic compasses are susceptible to acceleration and turning errors. For instance, in the Northern Hemisphere, accelerating on an east or west heading causes a turn to north, while decelerating causes a turn to south.

Radar Range for Practical Use

The calculated reference radar range offers a quick estimate of maximum visual detection range at sea level, often used for initial target acquisition. For example, a target 100 NM away would have a practical radar range of approximately 12.3 NM, assuming standard atmospheric conditions and line of sight.

Navigational Accuracy: Converting True Course to Magnetic Heading

Precise navigation relies on accurately converting between true, magnetic, and compass headings. This True Course to Magnetic Heading Converter helps mariners and aviators account for magnetic variation and compass deviation, providing the exact compass heading to steer, along with total error and estimated transit time. For a 22-nautical-mile journey, a mere 5-degree error in heading due to uncorrected variation or deviation could result in being nearly two miles off course, a critical difference in challenging conditions in 2025.

Why Accurate Heading Conversion is Critical for Safe Navigation

For safe and efficient navigation, understanding the relationship between True North, Magnetic North, and your vessel's compass is paramount. Failing to account for magnetic variation (the difference between true and magnetic north) and compass deviation (local magnetic interference) can lead to significant navigational errors, causing a vessel to drift off course, miss a waypoint, or even become lost. This conversion ensures that the charted course, referenced to True North, translates accurately into the heading you steer by your onboard compass.

The Navigational Math: True to Magnetic to Compass

The conversion from True Course to Compass Heading involves two sequential corrections: first for Magnetic Variation, then for Compass Deviation. The process ensures that the intended path relative to True North is accurately translated to what your vessel's compass should read.

The core calculations are:

magnetic heading = normalize360(true course - magnetic variation)
compass heading = normalize360(magnetic heading - compass deviation)
total compass error = magnetic variation + compass deviation
back bearing = normalize360(true course + 180)

The normalize360 function ensures all angles remain within the 0-360 degree range. Magnetic variation is subtracted if East (positive value) and added if West (negative value). Compass deviation follows the same convention.

💡 Beyond magnetic corrections, understanding your precise geographic coordinates is fundamental for navigation. Our UTM to Latitude & Longitude Converter can help with coordinate system translations.

Plotting a Course: A Worked Example

Let's consider a mariner planning a journey with an intended true course of 120°.

Start with True Course: 120°.
Apply Magnetic Variation: The chart indicates a magnetic variation of -7° (7° West).
- Magnetic Heading = 120° - (-7°) = 127°.
Apply Compass Deviation: The deviation card for this vessel on a 127° magnetic heading shows a deviation of 2° (2° East).
- Compass Heading = 127° - 2° = 125°.
Calculate Total Error: -7° (variation) + 2° (deviation) = -5°.
Estimate Transit Time: For a 22 NM distance at an assumed speed of 6 knots, the transit time is 22 NM / 6 kt = 3.67 hours, or approximately 220 minutes.

The final Compass Heading to steer is 125.0°, which is a Southeast direction.

💡 For long-distance maritime travel, accurately planning for time zone changes at your destination port is as important as course corrections. Our UTC to Local Time Converter can assist.

The Imperative of Accurate Navigation in Marine Environments

Converting between true, magnetic, and compass headings is a non-negotiable aspect of safe and accurate navigation, particularly in dynamic marine environments. Over a 100-nautical-mile journey, an uncorrected 5-degree compass error can lead to being nearly 9 miles off target, a potentially dangerous situation near coastlines or in busy shipping lanes. Magnetic variation, which can range from +15° East in regions like the Gulf of Maine to -15° West off the coast of Oregon, constantly changes geographically. Compass deviation, unique to each vessel, requires a detailed deviation card, which must be updated if new electronics or significant metal objects are added.

Typical Deviations and Variations in Navigation

In practical navigation, magnetic variation, the angular difference between true and magnetic north, can range from near 0 degrees in certain "agonic lines" to over 20 degrees East or West in polar regions. For example, in parts of the United States, variation might be around 7-10 degrees West, while in Europe it could be 5-10 degrees East. This data is critical and explicitly printed on nautical charts or provided by electronic navigation systems. Compass deviation, caused by a vessel's own magnetic influences, is typically much smaller, usually within ±5 degrees for a well-compensated compass. This deviation is meticulously recorded on a "deviation card," which is a table showing the compass error for various magnetic headings, compiled by a certified compass adjuster.

Frequently Asked Questions

What is the difference between true north and magnetic north?

True north is a fixed geographical point at the Earth's rotational axis, while magnetic north is the wandering point on the Earth's surface towards which the north pole of a compass needle points. The difference between them, known as magnetic variation, can be up to 20 degrees in some regions.

Why does an aircraft's compass need a deviation card?

An aircraft's compass requires a deviation card because the metal components and electrical currents within the aircraft itself create localized magnetic fields that interfere with the Earth's natural magnetic field. This interference, called deviation, can cause the compass to read inaccurately by up to 10 degrees if not corrected.

How often should magnetic variation and deviation be checked?

Magnetic variation changes slowly over time due to the Earth's shifting magnetic poles, typically updated on aeronautical charts every 5 years. Deviation, however, should be checked more frequently, usually during annual aircraft inspections or after any significant electrical modifications, as it can change with aircraft configuration.