Reciprocal headings
A reciprocal heading points exactly 180 degrees opposite another heading, course, or bearing. Pilots use reciprocals constantly: runway directions, reverse courses, holding-pattern orientation, procedure turns, wind discussions, and quick situational-awareness checks.
The add-or-subtract-180 rule
If the original number is less than 180°, add 180°. If it is 180° or greater, subtract 180°. A heading of 070° has a reciprocal of 250°. A heading of 310° has a reciprocal of 130°. Keep the result in the aviation convention of 001° through 360°: 180° becomes 360°, and 360° becomes 180°.
The same rule works for whole degrees and decimals. For 003°, add 180° to get 183°. For 247.5°, subtract 180° to get 067.5°. A navigation display may round or format the result differently, so preserve the precision and direction reference of the value you started with.
Mental shortcut
Another quick technique is to add or subtract 200, then correct by 20. For example, 085 + 200 = 285, minus 20 = 265. The standard 180-degree method is usually simpler, but both are common mental-math approaches.
Runway designators and approach direction
In the United States, the runway number is the whole number nearest one-tenth of the runway centerline magnetic azimuth. A centerline near 184° is designated Runway 18; its opposite direction is near 004° and is designated Runway 36. That is why opposite runway numbers normally differ by 18. The designator is rounded and is not an exact heading.
Parallel runways use L and R, or L, C and R for three runways, as viewed from the approach direction for that runway. Do not infer left/right from a north-up airport diagram without checking which end is being approached. Use the current airport diagram or runway data when the precise direction matters.
The FAA Aeronautical Information Manual, section 2-3 defines runway designators. Magnetic variation changes over time, so a runway can be renumbered when its designator no longer corresponds to its magnetic azimuth.
Heading, course, and bearing are not the same thing
The reciprocal arithmetic is identical, but the meaning changes. Heading is where the aircraft nose points. Course describes an intended or actual path over the ground. Bearing describes a direction from one point to another. Keep the label attached to the number so a correct reciprocal does not become the wrong operational quantity.
Magnetic versus true
A reciprocal does not automatically convert between true and magnetic reference. If you begin with a magnetic heading, its reciprocal is also magnetic. Convert variation separately when needed.
Related PilotDesk tools
True / magnetic converter · Wind triangle · Holding leg guide
Source check
Reciprocal arithmetic is a direction relationship. Runway markings and magnetic directions follow FAA airport-marking guidance; current charted data controls the runway direction used for planning.