Magnetic variation & deviation calculator. With the work shown.

It depends which way you are going. Correcting — working up from what the compass card reads toward a true direction you can plot — you add easterly errors and subtract westerly ones ('correcting, add east'). Uncorrecting — working down from a true course to the course to steer — you reverse it and add westerly errors, subtract easterly ('uncorrecting, add west'). A quick sanity check: with westerly variation the magnetic number is always the larger one, which is what the old saying 'east is least, west is best' is really telling you.

Variation is a property of the earth — the angle between true north and magnetic north where you are. You read it from the compass rose on the chart, and it is the same for every vessel in that area. Deviation is a property of your vessel — magnetic interference from the hull, engine and electronics pulling the card away from magnetic north. You read it from that vessel's own deviation card, and unlike variation it changes with the heading you are on, so the same boat can show 2° E on one course and 4° W on another.

Compass error is variation and deviation combined, carrying their signs — the total angle between what your compass reads and true north. Once you have it, true heading = compass heading + compass error, with easterly error positive. The calculator shows it under the worked steps so you can check a problem two ways.

The earth's magnetic field drifts, so a compass rose states the variation together with the year it was correct for and an annual increase or decrease in minutes of arc — for example 'VAR 14°30′W (2020) ANNUAL INCREASE 6′'. Six years on, that is 36′ more westerly variation, or 15°06′W. Exam problems routinely hand you a rose from an earlier year and expect the correction; open the 'Chart rose printed for an earlier year?' panel above to apply it.

No, and assuming it is causes a lot of lost marks. A deviation card lists a separate value for each heading around the card, typically every 15°. Read the deviation for the heading you are actually on — and note that most cards are entered with the vessel's compass heading, so when you are uncorrecting from a true course you may have to work to the nearest heading and check your answer.

It does the arithmetic correctly and shows its work, which makes it a useful check on a plot or a study aid for the exam. It is not a substitute for the charted variation for your actual position, a current deviation card swung for your vessel, or your own plot. For live magnetic declination anywhere on earth, use the NOAA NCEI magnetic field calculator linked in the sources below.