Compass Declination Calculator for True and Magnetic Bearings

Compass Declination Calculator

Update declination from map epoch, convert true and magnetic bearings, and model drift risk from heading error over distance.

🧭Declination Scenario Presets

⚙Bearing And Declination Inputs

Input Bearing Reference

Input Bearing (deg)

Map Declination Magnitude (deg)

Map Declination Direction

Map Epoch Year

Current Year

Annual Declination Shift (min/yr)

Annual Shift Trend

Local Magnetic Anomaly (deg, East +) Optional local bias from geology or equipment.

Grid Convergence (deg, East +)

Route Distance For Drift Model (km)

Display Precision

Current Declination

0.0

deg

True Bearing

0.0

deg T

Magnetic Bearing

0.0

deg M

Drift If Declination Skipped

meters at route distance

📊Declination Control Grid

T - D = M

True To Magnetic

M + D = T

Magnetic To True

+180 deg

Reciprocal Check

tan(error)

Lateral Drift

📘Reference Tables

Declination Band	Magnitude	Field Risk	Recommended Cadence
Low	0-3 deg	Small route bias	Check per season
Moderate	4-10 deg	Visible drift	Check every trip
High	11-18 deg	Major route miss	Check each leg
Very High	19+ deg	Critical correction	Dual confirmation

Annual Shift Rate	5 Year Change	10 Year Change	15 Year Change
2 min/yr	0.17 deg	0.33 deg	0.50 deg
4 min/yr	0.33 deg	0.67 deg	1.00 deg
6 min/yr	0.50 deg	1.00 deg	1.50 deg
8 min/yr	0.67 deg	1.33 deg	2.00 deg

Input Type	Declination	Formula	Example Output
True 120	8E	M = T - D	112M
True 120	8W	M = T - (-D)	128M
Mag 250	6E	T = M + D	256T
Mag 250	6W	T = M - D	244T

Heading Error	Drift at 1 km	Drift at 3 km	Drift at 5 km
1 deg	17 m	52 m	87 m
2 deg	35 m	105 m	175 m
5 deg	87 m	262 m	437 m
10 deg	176 m	529 m	881 m

Tip: Store the corrected declination next to your route plan, not only on the map margin. Teams often copy bearing values but forget the updated declination context.

Tip: When using both map grid and magnetic bearings, convert from true in one direction only. Repeated cross conversion can stack sign mistakes over long routes.

Compass declination shows the difference between true north and magnetic north. It is the angle between the direction to geographic North Pole and the position that points the compass needle. Navigators suffered because of this magnetic problem for centuries

Currently magnetic north is in Canada, around 100 miles from the North Pole. Because it always moves, the declination adjusts according to place and time, following the magnetic changes in the Earth core.

What Is Compass Declination?

There is a line where geographic and magnetic north poles meet, so there is no declination here. Zero declination is present in central United States, north Africa, India, China and most part of Europe. East of that line the magnetic pole moves the compass needle west of true north, which gives west declination.

If magnetic north is east of true, the needle twists right, positive declination. West it twists left, negative declination.

When compass shows 60 degrees with -8 degrees west declination, real course is 52 degrees regarding true north. Mistakes in those calculations double the declination. To use field orientation on a map or vice versa, you must correct every diffrence.

Compasses for hiking, as those with base or protractor, have a rotating dial of 360 degrees, independent of the magnetic needle. For set declination to true north, you turn the dial until the right number of degrees are between north mark and magnetic indication of the needle tip. Compass with declination screw eases everything.

Join it to the map, and all courses will be true, matching the map, no need for declination.

Declination values appear on topo maps. In the right margin stands a little triangular diagram with average declination for the map center. In clear night, turn the dial to Polaris for true north, and the needle swings show magnetic declination.

Bad compasses get bubbles and become useless, so it is worth buying a quality compass.