Route Variation Change Calculator
Crossing isogonic lines on a long leg: how much the magnetic heading must drift en route while the true course never moves — and when to re-trim the bug.
Flying coast-to-coast across the US crosses ~20° of variation: a constant true course requires a continuously drifting magnetic heading. GPS navigators re-derive it silently — paper planners break long legs at variation midpoints.
Formula
⚠️ For flight planning and education only — verify with official sources and certified equipment. Not for primary navigation.
Crossing isogonic lines on a long leg: how much the magnetic heading must drift en route while the true course never moves — and when to re-trim the bug.
About Route Variation Change Calculator
On a long westbound leg the GPS course needle stays planted while the heading bug needs nudging every hundred miles — not wind, but geography: you're crossing isogonic lines, and the translation between the unchanging true course and the magnetic heading drifts beneath you. This calculator quantifies it for any leg — both endpoint headings, the total en-route change and the per-100-nm drift rate — the long-cross-country detail that separates tidy navlogs from mysterious heading creep.
How to use Route Variation Change Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula MH = TC + var(W)/−(E) at each point; the true course holds while magnetic drifts substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Route Variation Change Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula MH = TC + var(W)/−(E) at each point; the true course holds while magnetic drifts with sources cited on the page
- ✓Flying coast-to-coast across the US crosses ~20° of variation: a constant true course requires a continuously drifting magnetic heading. GPS navigators re-derive it silently — paper planners break long legs at variation midpoints.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How fast does variation change across a route?+
In the continental US, roughly 1° per 60–100 nm of east-west travel (isogonics run loosely north-south), totaling ~20° coast to coast — from 16°E in Seattle to 14°W in Maine. North-south routes mostly parallel the lines and see little change. Northern Canada and the polar regions compress the lines dramatically; mid-ocean they relax.
How do paper navlogs handle a changing variation?+
Segment it: apply each leg's mid-point variation (read off the nearest isogonic), which keeps the per-leg error under half a degree for typical leg lengths. For one long leg crossing several lines, break it at the navlog level even if you won't turn — the headings differ, and that's precisely the information the log exists to carry.
Why does my GPS course needle not show any of this?+
GPS navigation is computed in true and converted to magnetic for display using a built-in magnetic model (WMM) evaluated at your present position — continuously. The desired track readout therefore drifts numerically along the route exactly as this tool predicts, while the geometry never changes. Database currency matters: a decade-old unit's magnetic model has drifted measurably.
Did variation drift really break VOR airways?+
Continuously: VOR radials are calibrated to the station's variation at its last alignment, which ages as the field drifts — some stations' declared variation differs several degrees from today's actual. That's why airway courses occasionally disagree with your computed magnetic course, and why the published value (calibrated to the station) wins when flying VOR airways.
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