How to use the compass variation and deviation calculator
- Choose whether your heading is true, magnetic or compass, and enter it from 0 to 360 degrees.
- Enter the magnetic variation for your area and say whether it is east or west.
- Enter the compass deviation from your deviation card and say whether it is east or west. Use 0 if you do not have one.
- Read the other two headings and the steps that connect them.
Formula
Magnetic = True - East variation (or + West variation)
Compass = Magnetic - East deviation (or + West deviation)
Magnetic = Compass + East deviation (or - West deviation)
True = Magnetic + East variation (or - West variation); headings wrap at 360
The old rule "east is least, west is best" fits the direction from true to compass: east is subtracted and west is added. Going back from compass to true the signs reverse.
Heading 090 true and a reverse trip
A true heading of 090 with a variation of 5 degrees west gives a magnetic heading of 095, since west is added. With a deviation of 2 degrees east, the compass heading is 093.
Going back, a compass heading of 270 with 3 degrees of west deviation gives a magnetic heading of 267, and with 8 degrees of east variation the true heading is 275. A magnetic heading of 045 with 12 degrees of west variation is 033 true. A true heading of 005 with 10 degrees of east variation is 355 magnetic, because headings wrap.
True, magnetic and compass
True north points to the geographic pole. Magnetic north is where a compass needle would point without any local effects, and the angle between the two is the variation. A real compass in a boat or aircraft is also pulled by nearby metal and equipment, and that extra angle is the deviation. A chart or map gives the variation, and a deviation card from a compass swing gives the deviation for each heading.
To find a direction between two places see the bearing and azimuth converter, and for wind effects on a course the wind triangle calculator. The great circle distance calculator gives the distance and initial bearing between points.
Checking your numbers
Variation changes slowly with place and year, so use a recent chart. Deviation changes with heading, so use the value for the heading you are steering. This page is a calculation aid for learning and planning, and it should not be your only source of navigation.
Assumptions and limits
- Variation and deviation are each entered as a size from 0 to 90 degrees with an east or west direction.
- The deviation is the one for the heading you are using, taken from your own card.
- Headings are in whole circle degrees from 0 to 360, and 360 is shown as 000.
- Results are rounded to 0.1 degree.
Frequently asked questions
How do I convert a true heading to a compass heading?
Subtract east variation or add west variation to get magnetic, then subtract east deviation or add west deviation to get compass.
What does east is least, west is best mean?
Going from true to magnetic or compass, you subtract easterly errors and add westerly ones. Going the other way, the signs are reversed.
What is the difference between variation and deviation?
Variation is the angle between true north and magnetic north at a place. Deviation is the extra error from the metal and electrical equipment near the compass.
Where do I find the variation for my area?
Aeronautical and nautical charts print it, and national geomagnetic services publish models. Use a value for the current year.