Running Fix Calculator
A running fix uses two compass bearings of a single landmark taken at different times, with the distance run and course maintained between observations. The first bearing line is advanced along the course line by the distance run to create a transferred line of position (LOP). The intersection of the transferred first LOP and the second LOP gives your running fix position. Enter the bearing of the landmark at both times (true bearings), your course (true), and the distance run between observations. The calculator finds the distance off the landmark at the time of the second bearing.
Running fix formula (law of sines)
Relative bearing 1 (RB1) = Bearing1 - Course (adjusted to 0-360)
Relative bearing 2 (RB2) = Bearing2 - Course (adjusted to 0-360)
Interior angle A = RB2 - RB1 (angle at vessel position 1)
Interior angle B = 180 - RB2 (angle at landmark)
Interior angle C = 180 - A - B (angle at vessel position 2)
Distance off (nm) = Distance run * sin(A) / sin(B)
Uses the law of sines applied to the triangle formed by the two vessel positions and the landmark.
Running fix best practices
- For maximum accuracy, use bearings that change by 30-90 degrees between observations.
- The classic 45-90 degree pair (doubling the angle) gives distance-off equal to distance run.
- Advance the first LOP along the true course by the distance run, not the rhumb line on the chart.
- Always label the advanced LOP and the fix with their times for clear chart work.
Running fix: frequently asked questions
What is a running fix?
A running fix is a position obtained by taking two or more bearings of the same landmark at different times, then advancing the earlier bearing line along the course and distance run to create two simultaneous lines of position. Their intersection gives your position.
What are typical bearing pairs used for a running fix?
Classic pairs are bow and beam (0 and 90 degrees relative) or 45 and 90 degrees relative (bow-and-beam). These give convenient calculations: for example, in a 45-90 running fix, the distance off at the beam equals the distance run between the two bearings.
How accurate is a running fix?
Accuracy depends on the accuracy of your bearings, your course accuracy, and how well your speed log measures distance run. Current and leeway can introduce error because they affect the actual track. A running fix is less reliable than a simultaneous fix from two landmarks.
What is the doubling the angle on the bow rule?
When the relative bearing doubles (e.g., from 30 to 60 degrees), the distance run equals the distance off when the bearing doubled. This is a special case of the running fix and gives a quick mental calculation for the beam distance.
Can I use a running fix with GPS courses?
Yes. Use your GPS course over ground and speed over ground to determine the run between bearings. However, if there is significant current, you should use the actual over-ground track rather than the water track when advancing the bearing line.
Official sources
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator (Pub. 9).
- US Coast Guard Navigation Center: USCG Navigation Center.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.