Compass Deviation Calculator
Converting between true, magnetic, and compass bearings is a fundamental navigation skill. The TVMDC rule (True, Variation, Magnetic, Deviation, Compass) guides the process. Variation is the local angle between true and magnetic north from the chart; deviation is the vessel-specific error in your compass on the current heading from your deviation card. Use positive values for easterly (E) variation and deviation, and negative for westerly (W). Enter a true course to find the compass course to steer, or enter a compass course to find the true course.
TVMDC conversion formula
True to Compass (T to C):
Magnetic = True - Variation (E positive)
Compass = Magnetic - Deviation (E positive)
Compass to True (C to T):
Magnetic = Compass + Deviation (E positive)
True = Magnetic + Variation (E positive)
Memory aid: "Cadets And Midshipmen Very Tired" (C to T, Add; T to C, Subtract)
Easterly variation and deviation are subtracted when converting T to C (they make compass larger than true).
Variation and deviation sources
- Magnetic variation is printed on NOAA nautical charts in the compass rose, with the annual rate of change.
- NOAA publishes current magnetic declination at ngdc.noaa.gov/geomag-web for any US location.
- Deviation is vessel-specific and heading-dependent. Obtain or create a deviation card by swinging the compass.
- GPS displays true course over ground; subtract variation to get magnetic, then subtract deviation to get compass.
Compass deviation: frequently asked questions
What is the difference between true, magnetic, and compass north?
True north points toward the geographic North Pole. Magnetic north is where the magnetic compass needle points, which differs from true north by the magnetic variation (declination). Compass north is what your specific compass reads, which may differ from magnetic north by deviation caused by magnetic influences on the vessel.
What is magnetic variation?
Magnetic variation (or declination) is the angle between true north and magnetic north at a specific location. It varies by location and changes slowly over time. Variation is published on nautical charts as the amount and direction (east or west) and an annual rate of change. NOAA publishes magnetic declination data for US locations.
What is compass deviation?
Compass deviation is the error in your specific compass caused by local magnetic influences on the vessel, such as the engine, electrical wiring, and steel fittings. Deviation varies with the vessel's heading and is determined by swinging the compass (comparing compass readings to known magnetic bearings). It is recorded on a deviation card.
What is the TVMDC rule?
TVMDC stands for True, Variation, Magnetic, Deviation, Compass. Starting from true course: subtract easterly variation (or add westerly) to get magnetic course; subtract easterly deviation (or add westerly) to get compass course. To reverse from compass to true, add easterly values or subtract westerly.
How often should I update my deviation card?
Deviation should be checked whenever significant changes are made to the vessel (new equipment, moved batteries, etc.) and at least annually. Deviation can change if the vessel is struck by lightning or if magnetic equipment is added or removed.
Official sources
- NOAA National Centers for Environmental Information: Magnetic Declination Calculator.
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.