Navigation
Compass error
Convert between compass, magnetic and true, or find the compass error and deviation (or the gyro error) from a bearing whose true value you know.
Fill in the fields to see the result.
How it works
Compass + deviation = magnetic; magnetic + variation = true. Errors are signed east positive, west negative. Their sum is the compass error. Going from true to compass, the signs reverse.
Memory aid: error east, compass least; error west, compass best — with an easterly error the compass reads less than true.
Example: compass course 090°, deviation 3° E, variation 5° W. Magnetic 093°, true 088°, compass error 2° W.
From an observed bearing (a transit, or a celestial azimuth): compass error = true bearing − compass bearing; deviation = compass error − variation. A gyro that reads less than true is reading low: add the error to gyro readings.
For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.
More tools
All tools- Port to port distances
- Great circle and rhumb line
- Speed, distance, time and ETA
- Current triangle
- Compass error by the sun
- Sunrise, sunset and twilight
- Dead reckoning position
- Distance off
- Sextant altitude and noon latitude
- Required speed for an ETA
- CPA and TCPA
- True wind
- Horizon and light range
- MLC hours of rest
- Ship’s clocks and time zones
- IMO and MMSI number check
- Dock water allowance and loading
- Height of tide
- Mean draught and trim
- GM from rolling period
- Squat and under-keel clearance
- Anchoring
- Search patterns (IAMSAR)
- Fuel consumption and endurance
- Speed and consumption
- Propeller slip
- Indicated power
- Bunker quantity
- Fuel heating temperature
- Cylinder oil feed rate
- Electrical power
- Pay and contract dates
- Unit and position converter
- Signal flags and phonetic alphabet
- COLREGs lights and shapes
- Beaufort and sea state scales
- Marine VHF channels