Heading & Course Converter
Convert between true, magnetic, and compass headings using magnetic variation and compass deviation. The TVMDC chain made easy.
About this calculator
This calculator walks the classic TVMDC chain -- True, Variation, Magnetic, Deviation, Compass -- that every student pilot learns for converting between the three reference headings a flight plan touches. True Course / Heading is measured from true north, the reference charts and winds-aloft forecasts use; Magnetic Heading corrects for Magnetic Variation, the local angle between true north and magnetic north (which changes with geographic location and drifts slowly over years, so it must be read from a current sectional chart's isogonic lines, not assumed); Compass Heading further corrects for Compass Deviation, the aircraft-specific error from onboard magnetic interference (radios, metal structure), found on that specific aircraft's compass correction card. Both corrections follow the same sign convention -- "East is least, West is best" -- meaning an EAST variation or deviation is SUBTRACTED going from true toward compass (and ADDED going the other direction), while a WEST value is added going toward compass.
Convert Direction lets you run the chain either way: True → Magnetic → Compass when you're planning a flight from charted true courses, or Compass → Magnetic → True when you're working backward from what the aircraft's compass shows. Reciprocal Heading is simply the entered course plus or minus 180°, useful for planning a return leg or entering a traffic pattern's downwind leg without re-deriving the whole TVMDC chain in the opposite direction.
Inputs
US: +15°E to +17°E (Pacific NW) to −14°W to −20°W (New England)
Typically ±2° to ±5°
Results
True Course / Heading
90°
Magnetic Heading
80°
Compass Heading
78°
How to Use This Calculator
- Select conversion direction: True→Compass or Compass→True.
- Enter the starting heading value.
- Enter magnetic variation from sectional chart isogonic lines (East = positive, West = negative).
- Enter compass deviation from the correction card in your aircraft.
- Remember: TVMDC — True Virgins Make Dull Companions.
How the result changes across these scenarios
| Scenario | True Course / Heading | Magnetic Heading | Compass Heading |
|---|---|---|---|
| West Variation (New England) | 180° | 195° | 193° |
| East Variation (Pacific NW) | 270° | 254° | 257° |
| No Variation (agonic line) | 0° | 0° | 359° |
What each input means
- Convert Direction
- Choose which direction to convert.
- True Course / Heading
- Heading or course referenced to true north (from charts).
- Compass Heading
- Heading shown on the magnetic compass in the cockpit.
- Magnetic Variation
- Positive = East variation, Negative = West variation. Found on sectional chart isogonic lines.
- Compass Deviation
- Deviation for the current heading from your aircraft's compass correction card.
What each result means
- Reciprocal (True)
- Opposite direction — useful for return trips.
How this is calculated
Worked example, using the default values
- True Course/HeadingGiven90° = 90° True
- Magnetic HeadingMagnetic = True − East Variation (or + West Variation)90° − (+10°) = 80° Magnetic
- Compass HeadingCompass = Magnetic − East Deviation (or + West Deviation)80° − (+2°) = 78° Compass
- Reciprocal HeadingReciprocal = True ± 180°90° + 180° = 270° True
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What does "East is least, West is best" actually mean when applying variation or deviation?
It's a memory aid for the sign of the correction when converting FROM true TOWARD compass: an East variation or deviation is SUBTRACTED (making the resulting heading numerically "least," or smaller), while a West value is ADDED (making it "best," or larger). Converting the other direction -- Compass → Magnetic → True -- simply reverses both operations: East values are added and West values are subtracted going back toward true.
Why does Magnetic Variation change with location, while Compass Deviation doesn't?
Magnetic Variation is the angle between true north and magnetic north AT A GIVEN GEOGRAPHIC LOCATION, which shifts as you fly across the country (and drifts slowly year to year as Earth's magnetic field itself changes) -- that's why it's read off a current sectional chart's isogonic lines for the area you're flying over. Compass Deviation instead comes from magnetic interference specific to ONE aircraft's instruments and structure, recorded on that aircraft's own compass correction card, and doesn't change with where the aircraft happens to be.
If I enter a Compass Heading, do I still need to know Magnetic Variation?
Yes -- Compass → Magnetic → True first removes Compass Deviation to get Magnetic Heading, then still needs Magnetic Variation to finish the conversion to True Course / Heading. Both corrections apply regardless of which direction you're converting; only the starting input (True Course / Heading vs. Compass Heading) and the order the corrections are applied change.
Why is Reciprocal Heading useful beyond just "the opposite direction"?
Pilots use it constantly in practice: planning the return leg of a flight without recomputing the whole TVMDC chain from scratch, or entering a traffic pattern's downwind leg (flown opposite the runway heading). Because it's simply True Course / Heading plus or minus 180°, Reciprocal Heading is always exactly 180° from True Course / Heading no matter what Magnetic Variation or Compass Deviation apply to the original course.
Does a large Magnetic Variation always mean my compass reading will be very different from true north?
Largely yes for the Magnetic Heading step, since Magnetic Variation is applied directly between True Course / Heading and Magnetic Heading -- a location with a large variation (parts of Alaska exceed 20°) shifts Magnetic Heading noticeably from True Course / Heading. Compass Heading then applies the separate, typically much smaller Compass Deviation (often just a few degrees) on top of that, so Compass Heading usually tracks close to Magnetic Heading even where variation itself is large.
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