Compass Deviation and Variation Calculator
Convert between true north, magnetic north, and compass heading.
Apply variation and deviation corrections for accurate navigation.
A compass does not point to True North. There are two corrections required for accurate navigation:
1. Magnetic Variation (Declination) The angle between True North and Magnetic North at your location. This varies by position on Earth and changes slowly over time. Values are found on nautical charts or aviation sectional charts.
- East variation: magnetic north is east of true north — subtract from true heading to get magnetic
- West variation: magnetic north is west of true north — add to true heading to get magnetic
2. Compass Deviation The error in your specific compass caused by nearby metal, electrical equipment, or the vessel’s own magnetic field. Deviation is measured during compass swinging and recorded on a deviation card.
Memory aid: TVMDC — True, Variation, Magnetic, Deviation, Compass “True Virgins Make Dull Companions” Working from True to Compass: add West, subtract East (for each correction). Working from Compass to True: reverse — add East, subtract West.
The formula (true to compass): Magnetic = True + (-Variation if East, +Variation if West) Compass = Magnetic + (-Deviation if East, +Deviation if West)
Worked example: True heading 045°, Variation 5°W, Deviation 2°E Magnetic = 045 + 5 = 050° Magnetic Compass = 050 − 2 = 048° Compass heading to steer
Finding your variation: Search for a local magnetic declination calculator using your latitude and longitude. Typical variation ranges are 0–25° depending on location.