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Compass Deviation Calculator

Convert headings with variation and deviation corrections

True Heading

233.0°

Magnetic

248.0°

Compass

245.0°

Find on nautical chart compass rose (inner ring annotation)

From your vessel’s deviation card (varies by heading)

All Three Headings

True

233.0°

Magnetic

248.0°

Compass

245.0°

Total Correction: -12.0°

TVMDC Conversion

TTrue Heading233.0°
+ Variation 15°W
MMagnetic Heading248.0°
− Deviation 3°E
CCompass Heading245.0°

TVMDC Memory Aids

Compass → True (ADD):

“True Virgins Make Dull Companions”

True → Compass (SUBTRACT):

“Can Dead Men Vote Twice”

Direction rule:

East is positive (+), West is negative (−). “East is least, West is best” — add West, subtract East when going True to Compass.

What You'll Need

Suunto A-30 Baseplate Compass with Clinometer

Suunto A-30 Baseplate Compass with Clinometer

$25-$404.5
View on Amazon
Gill Marine Sailing Gloves Deckhand 3/4 Finger

Gill Marine Sailing Gloves Deckhand 3/4 Finger

$25-$354.4
View on Amazon
Davis Instruments Quick Reference Navigation Rules Card

Davis Instruments Quick Reference Navigation Rules Card

$8-$124.6
View on Amazon

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon
Suunto A-30 Baseplate Compass with Clinometer

Suunto A-30 Baseplate Compass with Clinometer

$25-$404.5
View on Amazon
Gill Marine Sailing Gloves Deckhand 3/4 Finger

Gill Marine Sailing Gloves Deckhand 3/4 Finger

$25-$354.4
View on Amazon
Davis Instruments Quick Reference Navigation Rules Card

Davis Instruments Quick Reference Navigation Rules Card

$8-$124.6
View on Amazon

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

What is the TVMDC formula for compass correction?

TVMDC stands for True, Variation, Magnetic, Deviation, Compass. Going from Compass to True, you add corrections: True = Compass + Deviation + Variation. The mnemonic "True Virgins Make Dull Companions" helps remember the order. East corrections are positive, West corrections are negative.

  • T = True heading — referenced to geographic (true) north pole
  • V = Variation — angle between true north and magnetic north (from chart)
  • M = Magnetic heading — referenced to magnetic north
  • D = Deviation — error caused by metal on your specific vessel
  • C = Compass heading — what you actually read on the compass
DirectionMnemonicOperation
Compass → TrueTrue Virgins Make Dull CompanionsADD corrections
True → CompassCan Dead Men Vote TwiceSUBTRACT corrections
Q

What is the difference between variation and deviation?

Variation is the angle between true north and magnetic north at a given location, caused by Earth’s magnetic field. Deviation is the error in your specific compass caused by onboard metal, electronics, and wiring. Variation is found on nautical charts; deviation comes from your vessel’s deviation card.

  • Variation changes by geographic location — ranges from 0° to over 20° in the US
  • Variation changes slowly over time (~0.1°/year) — check chart date and annual change rate
  • Deviation is unique to each vessel and changes with heading
  • A deviation card lists correction values for every 15° or 30° of compass heading
  • Deviation should be under 5° after professional compass adjustment (swinging)
Q

How do I find magnetic variation for my location?

Magnetic variation is printed on every nautical chart near the compass rose. The inner ring of the compass rose shows magnetic bearings, and the annotation states the variation value and its annual rate of change. You can also use NOAA’s magnetic declination calculator online.

  • Nautical chart compass rose: look for the annotation (e.g., "Var 15°W (2024), annual decrease 0.1°")
  • US East Coast: typically 10°–20° West variation
  • US West Coast: typically 13°–18° East variation
  • Agonic line (0° variation) currently runs through central US from Wisconsin to Alabama
  • NOAA Magnetic Declination Calculator provides current values for any GPS coordinate
Q

How do I create a deviation card for my boat?

Swing the compass by motoring slowly in a circle and comparing compass readings against known bearings (ranges, GPS course, or a hand-bearing compass placed far from interference). Record the difference at every 15° or 30° of heading. A professional compass adjuster can reduce deviation with corrector magnets.

  • Best done on a calm day with minimal current in an open area
  • Use charted ranges (two fixed objects in line) as reference bearings
  • Compare compass reading vs. known magnetic bearing at each heading
  • Record deviation for every 15°–30° of compass heading (12–24 data points)
  • Professional compass adjustment typically costs $150–$300 and reduces deviation to under 3°
Q

Does East or West variation add or subtract?

When converting from Compass to True (adding): East is positive (+) and West is negative (−). The memory aid is "East is least, West is best" — meaning when going from True to Compass, subtract East and add West. The reverse applies when going Compass to True.

  • Compass to True: add East variation, subtract West variation
  • True to Compass: subtract East variation, add West variation
  • Same rule applies to deviation: East adds, West subtracts (Compass to True)
  • Example: Compass 245°, Dev 3°E, Var 15°W = Magnetic 248°, True 233°
  • Always double-check by reversing: True 233° − (−15°) = Mag 248° − (+3°) = Compass 245°

Example Calculations

1Compass to True (Var 15°W, Dev 3°E)

Inputs

Input TypeCompass
Compass Heading245°
Variation15° West
Deviation3° East

Result

True Heading233.0°
Magnetic Heading248.0°
Total Correction−12.0°

Deviation signed = +3° (East). Variation signed = −15° (West). Magnetic = 245 + 3 = 248°. True = 248 + (−15) = 233°. Total correction = −15 + 3 = −12°.

2True to Compass (Var 10°E, Dev 5°W)

Inputs

Input TypeTrue
True Heading090°
Variation10° East
Deviation5° West

Result

Compass Heading085.0°
Magnetic Heading080.0°
Total Correction+5.0°

Variation signed = +10° (East). Deviation signed = −5° (West). Magnetic = 90 − 10 = 80°. Compass = 80 − (−5) = 85°. Total correction = 10 + (−5) = +5°.

Formulas Used

Compass to True Conversion

True = Compass + Deviation + Variation

Converts a compass reading to true heading by applying deviation and variation corrections. East is positive, West is negative.

Where:

Compass= Reading from the vessel’s magnetic compass in degrees
Deviation= Compass error for that heading from the deviation card (+ for East, − for West)
Variation= Magnetic variation at your location from the nautical chart (+ for East, − for West)

True to Compass Conversion

Compass = True − Variation − Deviation

Converts a true heading (from chart) to the compass heading you need to steer. Reverse the sign convention.

Where:

True= True heading measured from the chart in degrees
Variation= Magnetic variation (+ for East, − for West)
Deviation= Compass deviation for the resulting magnetic heading

Total Correction

Total Correction = Variation + Deviation

The combined angular difference between compass heading and true heading.

Where:

Variation= Signed magnetic variation in degrees
Deviation= Signed compass deviation in degrees

Marine Compass Correction and the TVMDC Method

Accurate heading information is the foundation of safe marine navigation. Every mariner must understand the three "norths" — true north (geographic), magnetic north (where a compass needle points), and compass north (what your specific compass reads after being influenced by onboard metals).

The TVMDC system provides a structured method for converting between these three heading references. Variation accounts for the difference between true and magnetic north, which changes by location. Deviation accounts for the additional error your specific compass experiences due to nearby ferrous metals, speakers, wiring, and electronics.

Modern GPS chartplotters display true heading directly, but understanding compass corrections remains essential. GPS can fail, and magnetic compasses are still required safety equipment on all vessels. Practicing TVMDC conversions ensures you can navigate safely using basic tools when electronic systems are unavailable.

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Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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