Online Tool Store
Compass Declination Calculator
Convert between magnetic and true bearings using local declination, with the sign convention that stops the correction being applied backwards.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Math & Science
Compass Declination Calculator
Frontend preview — no upload or external service.
Converted bearing
A magnetic bearing of 127° with 4.2° East declination is a true bearing of 131.2°. Going the other way, a true bearing of 127° is 122.8° magnetic.
How the Compass Declination Calculator works
- Find the declination for your location and the current year — it drifts, so an old map's figure may be out of date.
- Note whether it is east or west; the sign determines which way the correction goes.
- Convert in the right direction, and check the result against a known feature before trusting it over distance.
The method
Declination is the angle between magnetic north and true north, and it changes with both location and time.
true = magnetic + east declination; true = magnetic − west declination
With 4.2° East: a magnetic 127° is a true 131.2°. Getting the sign backwards produces an error of twice the declination, which is 8.4° here — enough to miss a target completely over a few kilometres.
FAQ
Which way does the correction go?
East declination is added when converting magnetic to true, and subtracted going the other way. West is the reverse. Applying it backwards doubles the error, which is why the mnemonic matters more than the arithmetic.
Does declination change over time?
Yes, and measurably. The magnetic poles move, so declination drifts by a fraction of a degree a year in many places. A map printed a decade ago may state a figure that is noticeably out.
What is grid north?
A third north — the vertical grid lines on a projected map, which differ slightly from true north away from the projection's central meridian. Maps usually print all three and the angles between them.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Both conversion directions | ✓ | Error-prone | ✓ |
| Explains the sign convention | ✓ | ✗ | Sometimes |
| No account | ✓ | ✓ | ✗ |
| Current declination data | ✗ | Map print | Yes |
Compass Declination Calculator makes the sign convention explicit, because applying the correction backwards doubles the error rather than removing it.