Magnetic variation and compass correction

What this calculator shows

This calculator applies magnetic variation and compass deviation in sequence to convert an entered true heading into magnetic and compass headings. Variation is the angular difference between true north and magnetic north at a location. Deviation is the error introduced by magnetic influences in a particular aircraft.

The values are entered by the user because variation depends on location and date, while deviation depends on the aircraft and compass installation. The calculator does not look up a current magnetic model or aircraft compass card.

How to use the calculator

Enter a true heading from 0 to 360 degrees. Enter the magnitude and east or west direction of magnetic variation, then the magnitude and direction of compass deviation. The URL retains all values for review.

Use the variation shown by a current chart or other appropriate navigation source for the location. Use the aircraft's current compass correction card for deviation. The correction card may list deviations by compass heading rather than one single value. This tool accepts one entered deviation and cannot interpolate a card.

How the correction works

Apply variation to true heading to obtain magnetic heading, then apply deviation to magnetic heading to obtain compass heading. The mnemonic "east is least, west is best" means subtract an east correction and add a west correction when moving from true toward magnetic and from magnetic toward compass.

magnetic = true - east variation + west variation

compass = magnetic - east deviation + west deviation

Headings wrap around north. For example, subtracting 8 degrees from a true heading of 4 degrees gives a magnetic heading of 356 degrees, not a negative heading.

Example calculation

Suppose the entered true heading is 100 degrees, variation is 10 degrees east, and deviation is 2 degrees west.

1. Apply east variation: 100° - 10° = 90° magnetic.
2. Apply west deviation: 90° + 2° = 92° compass.

The result is 90 degrees magnetic and 92 degrees by compass. This example illustrates the arithmetic only; it is not a value for a particular location or aircraft.

When to apply the conversion

The conversion is useful when a task combines directions from sources that use different north references. Sectional chart courses and runway numbers are generally magnetic, while some weather and mapping products report directions relative to true north. The reference should be identified before entering the values; this tool cannot infer whether a source is true or magnetic.

Compass deviation is aircraft-specific. A compass correction card may show that an indicated compass heading differs by a small amount from the desired magnetic heading. The correction can change around the compass, so an entered single value is an approximation when the card lists several heading-dependent corrections.

Common mistakes

Do not apply the same correction twice. If an input is already magnetic, applying local variation again changes it incorrectly. Likewise, compass deviation should not be added to a true heading before variation has been applied.

East and west describe the direction of the offset, not the direction the airplane turns. The calculator applies the signed arithmetic and wraps results at north, but the pilot still needs to confirm that the direction and magnitude came from the correct chart or aircraft reference.

Assumptions and limitations

The tool assumes each correction is a signed angular offset and applies the corrections in sequence. It does not calculate ground track, wind correction, course, bearing, or an aircraft-specific compass error curve. The direction labels and values must match the navigation source and aircraft card.

True, magnetic, and compass headings are not interchangeable. Mixing a true-north weather observation with a magnetic runway direction without accounting for variation can lead to a mismatched comparison. Confirm the reference used by each source before using the values in a separate calculation.

Sources and methodology

The correction sequence follows the heading and compass concepts presented in the FAA Pilot's Handbook of Aeronautical Knowledge. Use current aeronautical charts for local variation and the aircraft's compass correction card for deviation.

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