Magnetic Declination & Bearing Conversion Calculator (True, Grid & Magnetic)
Convert compass bearings between magnetic, true and grid north using the local magnetic declination (east or west) and the map’s grid convergence or printed grid–magnetic angle. Enter degrees, mils, degrees-minutes-seconds or quadrant bearings (N45°30′E), update an old map’s declination with its annual change, and get back bearings, the compass setting and the off-course error if you skip the correction.
How it works
true = magnetic + declination (east positive, west negative) · grid = true − convergence (grid north east of true positive) · G–M angle = declination − convergence; magnetic = grid − G–M · current declination = map value + annual change × years · back bearing = bearing ± 180° · off-course distance = distance × sin(skipped correction) · 1° = 17.78 NATO mils = 1.111 gon
A compass points at magnetic north, a map is drawn on a grid whose north is parallel to its edges, and neither is exactly true north, so every bearing carried between map and ground must be adjusted by the angle between them. Declination — the magnetic-to-true angle — varies by place and slowly with time, printed in the map margin with its annual change; convergence — the grid-to-true angle — is small and fixed by where the map sits in its grid zone. Navigators combine the two into the single grid-to-magnetic (G–M) angle, and some maps print that angle directly, which you can enter instead. The calculator updates an old declination with its annual change, converts the bearing to all three references (in degrees, degrees-minutes-seconds, quadrant form, NATO mils and compass points), builds a conversion table for your map, and shows how far off course you would be after a given distance if you skipped the correction. It does not look declination up from your location: take a current value from your map, GPS or a declination calculator based on the World Magnetic Model.
Worked example
A grid bearing of 72° on a map printed in 2015 with 11.5° W declination changing 8′ a year westward: by 2026 the declination is 11.5° + 11 × 8′ = 12.97° W. With 1.2° of convergence the true bearing is 73.2° and the magnetic bearing 86.2° — a grid-to-magnetic angle of about 14.17° W, so add 14.2° to grid bearings before dialling them on the compass. The back bearing to check the leg is 266.2° magnetic. Skipping the correction would put you about 245 m (803 ft) off course after 1 km.
Sources: NOAA NCEI — magnetic field calculators and bearing help · MapTools — north references (true, magnetic, grid) and declination
Frequently asked questions
What does "east is least, west is best" mean?
When converting a map (true or grid) bearing to a compass bearing, subtract an easterly declination and add a westerly one. Going the other way — compass to map — reverse it. If you cannot remember, draw the two norths as arrows: the compass reads bigger when magnetic north lies to the west.
Can I ignore grid convergence?
Usually — it is under 1–2° in most of a UTM zone and many maps fold it into a single printed grid-to-magnetic angle. Choose “Grid–magnetic angle” if your map gives one; use declination alone if you are working with true bearings (GPS, celestial).
How do I find the current declination for my location?
Use a recent map, a GPS that shows declination, or an online calculator based on the World Magnetic Model (NOAA runs one; WMM2025 is valid from 2025 to 2030). If you only have an old map, enter its year and annual change and the calculator brings the value up to date.
How do I enter a bearing in degrees and minutes or mils?
Choose “DMS or quadrant” and type it as 245°30′, 245 30 15 or a quadrant bearing such as N 45°30′ E. For military mils (6,400 to a circle) or gon (400), use the unit menu next to the bearing.
How much does a small error matter?
Each degree puts you about 17 m off course per kilometre (about 92 ft per mile). A 10° declination ignored over 2 km is roughly 350 m — enough to miss a trail junction or a hut in poor visibility.
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Field rules of thumb for outdoor safety and navigation. Ice, terrain and weather conditions change quickly and locally — check conditions yourself, carry safety equipment and follow guidance from local authorities.