Magnetic Deviation Calculator
Enter your known heading data to estimate compass deviation. This tool uses the common navigation convention that east is positive and west is negative.
Formula path used: True → Magnetic (via variation), then Magnetic → Compass (via deviation).
What is magnetic deviation?
Magnetic deviation is the error introduced by magnetic influences inside your vessel or aircraft. Metal structures, wiring, electronics, and even portable gear near the compass can pull the needle away from magnetic north. Unlike magnetic variation (which comes from Earth and changes by location), deviation is specific to your platform and often changes with heading.
Variation vs. deviation (and why both matter)
Magnetic variation
Variation is the angle between true north and magnetic north in a specific geographic area. It is published on charts and updated over time.
Magnetic deviation
Deviation is the angle between magnetic heading and what your compass actually indicates. It depends on your vessel/aircraft and can vary by heading, which is why navigators build a deviation card.
- Variation = environmental/location effect
- Deviation = platform/instrument effect
- Total compass error = variation + deviation (with signs)
Formulas used in this calculator
This page uses a signed-angle convention where east values are positive and west values are negative:
- Magnetic Heading = True Heading − Variation
- Deviation = Magnetic Heading − Compass Heading
The calculator automatically normalizes headings to 0–360° and reports deviation as an east/west value for practical use.
How to use this tool
- Input a true heading (from chart work, GPS course reference, or planned track).
- Enter local magnetic variation and choose E or W.
- Enter the compass heading observed on your instrument.
- Click Calculate Deviation to see magnetic heading and compass deviation.
Worked example
Suppose your true heading is 090°, local variation is 6°E, and your compass reads 081°:
- Magnetic heading = 090 − 6 = 084°
- Deviation = 084 − 081 = +3° (3°E)
That means your compass has an easterly deviation of 3° on that heading.
Best practices for reliable deviation values
1) Build and maintain a deviation card
Record deviation at regular heading intervals (for example every 15° or 30°). Interpolate between points as needed.
2) Keep magnetic clutter away
Portable speakers, tools, chargers, and even phones with magnetic mounts can alter compass readings.
3) Re-check after modifications
Any new electronics, wiring changes, or structural work can shift compass behavior and invalidate old correction data.
Common mistakes to avoid
- Mixing up variation and deviation signs (E/W).
- Forgetting to normalize headings around 000°/360°.
- Assuming one deviation value works for every heading.
- Using stale variation data from old charts.
Final note
This calculator is intended for planning, training, and quick checks. For operational navigation, always use current charts/publications, your official compass correction procedures, and professional judgment.