compass variation calculator

What is compass variation?

Compass variation (also called magnetic declination) is the angle between true north (geographic north) and magnetic north (the direction your magnetic compass points). Because Earth’s magnetic field is not perfectly aligned with its rotational axis, the two north directions are not the same in most places.

If you navigate using a chart, route plan, or bearing referenced to true north, variation is the correction that helps you convert to magnetic values you can steer with a compass. In marine and aviation navigation, getting this correction right is essential for accuracy and safety.

Variation vs. deviation: what’s the difference?

Variation (declination)

• Caused by Earth’s magnetic field.
• Depends on your geographic location.
• Changes slowly over time (annual drift).
• Published on charts and in navigation databases.

Deviation

• Caused by local magnetic influences on your vessel or aircraft.
• Depends on your own equipment and heading.
• Can change when electronics or metal gear are moved.
• Typically taken from a deviation card or calibration data.

In practical terms: variation gets you from true to magnetic, and deviation gets you from magnetic to compass.

How this compass variation calculator works

This calculator follows a signed approach:

• East = positive
• West = negative

Then it applies:

• T = M + Var
• M = C + Dev

With those two relationships, the tool can compute all three heading references from any one known heading. It also normalizes values to a standard 0–360° range so you never get confusing negative headings or values above 360°.

Step-by-step usage

1. Select which heading you already know: True, Magnetic, or Compass.
2. Enter that heading in degrees.
3. Enter local variation and choose East or West.
4. Enter deviation (or leave at 0 if not needed).
5. Click Calculate to view all converted headings.

Worked examples

Example 1: True to Magnetic

Suppose your true course is 100°, and variation is 7°E. Using the formula, magnetic heading is 100° − 7° = 93°M.

Example 2: Magnetic to True

If magnetic heading is 250° and variation is 4°W, convert west to negative and apply T = M + Var. So T = 250 + (−4) = 246°T.

Example 3: Compass to True with deviation

You steer 180°C, deviation is 2°E, and variation is 9°W. First find magnetic: M = 180 + 2 = 182°. Then true: T = 182 + (−9) = 173°T.

Common mistakes to avoid

• Mixing up East and West signs.
• Using outdated chart variation values.
• Ignoring deviation when steering by compass.
• Failing to normalize angles near 0°/360°.
• Assuming deviation is constant at all headings.

Quick practical tips

• Write your heading workflow in one place: T ↔ M ↔ C.
• Keep a current deviation card onboard.
• Recheck magnetic data when traveling long distances.
• Use cross-checks: GPS track, landmarks, and charted bearings.

Final note

A compass variation calculator is a simple but powerful navigation aid. Use it to reduce mental math errors, speed up planning, and keep your true, magnetic, and compass headings consistent. For real-world operations, always confirm with official chart data, local procedures, and your organization’s safety standards.