Use the free tool below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
What this tool does
A magnetic compass on a boat does not point at magnetic north. The boat itself bends the earth’s field: engines, wiring, speakers, tools, and even the keel all pull the needle a little off true magnetic north, and the size of that pull changes with the direction the boat is heading. That error is called deviation, and every boat has its own pattern of it. This compass deviation correction calculator converts between compass headings and magnetic headings using a deviation table that you enter yourself, from your own boat’s deviation card. It never invents deviation values, because a made-up table would be worse than useless.
The tool starts with a table of zeros, which means no deviation at all. Those zeros are placeholders, not measurements, and the first thing you should do is replace them with the real figures from your boat’s deviation card. Once your table is in, the tool interpolates between your entries, so a heading halfway between two card entries gets a deviation halfway between their values, and it wraps correctly past north. It also applies variation, the difference between magnetic north and true north for your area, to give a true heading as well.
How to use it
Get your deviation card first. It is made by swinging the compass: turning the boat through known headings and recording what the compass reads at each one. Many boats carry a card near the compass, and a compass adjuster can make one for you. Type the card into the box, one entry per line, as compass heading, deviation, for example 90,5 for a deviation of 5 degrees east on an east heading. East deviation is positive, west deviation is negative, so a west deviation of 3 degrees is written as minus 3.
Then enter the heading you want to convert and pick the direction. Compass to magnetic answers the question: my compass reads this, what is the magnetic heading? Magnetic to compass answers the reverse: I want to steer this magnetic course, what should my compass read? Enter the local variation, east positive, from your chart, if you want the true heading too. Press the button and read the deviation used, the corrected heading, and the true heading.
The rules behind the conversion
Two old sayings carry the whole subject, and they are worth memorising. Compass to magnetic: add east deviation, subtract west deviation, or in short, add east. Magnetic to compass: subtract east deviation, add west deviation. Variation follows the same pattern with true and magnetic: true equals magnetic plus variation east. The tool applies those rules with signs, so a positive deviation is east and a negative one is west, and the arithmetic stays consistent in both directions.
Interpolation is where the tool earns its keep. A deviation card usually lists deviation every 15 or 30 degrees, but you steer every heading in between. The tool draws a straight line between neighbouring card entries, which is the standard way to estimate deviation between measured points. It also handles the wrap past north: a heading of 350 degrees sits between the 345 and 0 entries, and the tool treats the 0 entry as 360 for that calculation, so the interpolation stays smooth across the north point instead of jumping.
A worked example
Suppose your card reads 0 degrees of deviation on north, 5 degrees east on east, 0 on south, and 5 degrees west, written as minus 5, on west. You are steering a compass course of 045 degrees. The tool interpolates halfway between north at 0 and east at 5, giving a deviation of 2.5 degrees east. Your magnetic heading is therefore 47.5 degrees. With a variation of 4 degrees west, written as minus 4, your true heading is 43.5 degrees. Three numbers, three different norths, and each step is a small addition you can check by hand.
Limits and safety
This tool is only as honest as the table you type in. The starting zeros are not a deviation card for any real boat, and using them for navigation would mean steering with uncorrected compass error, which can be several degrees. Deviation also changes over time: new equipment, moved gear, repairs, and even a toolbox left near the compass can change it. Re-swing your compass after any change near it, and whenever you suspect the card is out of date.
This tool is an educational estimator only. It is not professional navigation advice, and it does not replace a proper compass adjustment, official charts, variation data for your area, or the judgement of the person in charge. Never navigate by compass alone in poor visibility or near hazards without confirming your position by other means. Use the tool to learn the conversion, to check a card, and to practise the method, and keep your real deviation card on the boat.
Common questions
What is the difference between deviation and variation? Deviation is caused by your boat and changes with heading. Variation is caused by the earth’s field and changes with position, not heading. Both must be corrected to go from a compass reading to a true course, and the tool handles them separately so you can see each step. Why does my compass read differently on different headings? Because the boat’s magnetic effect on the needle depends on the angle between the boat and the earth’s field, which changes as you turn. That is exactly why a deviation card has an entry for every heading, not a single number.
Versión en español: Calculadora de Corrección de Desvío de Compás