Magnetic Declination Calculator
Convert between true and magnetic bearings and estimate local magnetic declination for any location, free in your browser.
Location & date
Date defaults to today — declination changes over time, so the date matters. Runs entirely in your browser.
Result
Enter a valid position and date.
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Paste a position in any format (e.g. -31.9535, 115.8570) — it's parsed automatically.
- The date defaults to today; change it to any date in the model's range. Optionally add a height in km and your grid convergence.
- Read the magnetic declination (east or west of true north), its yearly drift, inclination, field strength, and the grid-magnetic angle if you entered convergence.
How it works
Magnetic declination is the angle between true north and magnetic north. A compass points to magnetic north, so true bearing = magnetic bearing + declination (east declination positive).
The value is computed from the World Magnetic Model (WMM2025) — a spherical-harmonic model of Earth's magnetic field published by NOAA/NGA. Because the magnetic pole drifts, the field has a yearly rate of change, so the model uses your chosen date. The coefficients are bundled, so the calculation runs entirely offline in your browser.
Grid-magnetic angle (grid north to magnetic north) = grid convergence − declination, where grid convergence is the fixed geometric angle between grid north and true north.
Worked example
Perth, WA today. Around −31.95°, 115.86° the declination is roughly 1–2° W and drifting slowly each year — enter the exact date for the current value.
Common mistakes
- Confusing declination with grid convergence — declination changes with date and is magnetic; convergence is fixed geometry.
- Using an old declination value. Because it drifts, a figure from years ago can be out by a degree or more — always use the current date.
- Mixing up the sign: east declination is positive, west is negative.
Frequently asked questions
What is magnetic declination (or variation)?
It's the angle between true north and the direction a compass needle points (magnetic north). It varies by location and slowly changes over time.
Why does the date matter?
Earth's magnetic field drifts, so declination changes by a few arc-minutes to tens of arc-minutes per year depending on where you are. Using the right date keeps the angle accurate.
How accurate is this?
It uses the official WMM2025 coefficients and matches the published reference values to within about a tenth of a degree, which is the model's stated accuracy for declination in most areas.
How do I convert a magnetic bearing to a true bearing?
Add the declination: true = magnetic + declination (east positive, west negative). To go the other way, subtract it.
What's the grid-magnetic angle?
It's the angle from grid north to magnetic north, used on topographic maps. It equals the grid convergence minus the declination. Enter your convergence to get it here.
Are my coordinates uploaded?
No. The model is bundled in the page, so the whole calculation runs locally in your browser — nothing is sent anywhere.
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- Basic Field Observation Log
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Cite this tool
Zerdly. (2026). Magnetic Declination Calculator. Retrieved from https://www.zerdly.com/tools/magnetic-declination-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.