Compass Course Calculator
West is best going one way. Not the other.
Work out Compass Course. West is best going one way. Not the other. Names the misconception directly.
Compass course to steer
92°
True 90° · magnetic 92° · E
West is best, east is least — going from true to compass you add west errors and subtract east ones. Coming back the other way it reverses, which is where nearly every mistake in the whole subject comes from. Variation is a property of where you are: the angle between true and magnetic north at that place, printed on the chart and changing measurably year on year. It is an input here rather than a lookup, because a hard-coded table is wrong the day it ships. Deviation is a property of your own vessel — the magnetic field of the boat itself, which changes with heading and with anything metal you move. That is why a deviation card is per-boat and has to be redone when the electronics change.
How the Compass Course Calculator works
Converting between true, magnetic and compass courses in any direction, with the reciprocal, the cardinal point and the mnemonic that keeps the sign straight.
Also known as: true to magnetic course · west is best east is least · variation and deviation conversion · what compass course do i steer
Frequently asked questions
How do I convert true to magnetic?
Subtract east variation, add west. Going the other way it reverses — which is where nearly every mistake in the subject comes from.
What is the difference between variation and deviation?
Variation is where you are: the angle between true and magnetic north at that place. Deviation is your own vessel's magnetic field, and it changes with heading.
What is the mnemonic?
True Virgins Make Dull Companions going from true to compass, subtracting east errors; Can Dead Men Vote Twice coming back, adding them.
Why isn't variation built into this page?
Because it changes measurably year on year — the north magnetic pole has moved hundreds of kilometres since 1990. A hard-coded table is wrong the day it ships.
How do I find my deviation?
By swinging the compass — comparing it against a known bearing on several headings and building a deviation card. It has to be redone whenever anything magnetic on board moves.
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