How squawk codes work
Transponder codes are octal: each of the four digits runs from 0 to 7, which gives 4,096 possible codes. Controllers assign a unique “discrete” code to each flight under their control. The code then ties the radar or ADS-B target on their screen to the flight plan. When a pilot is told to “squawk 4521”, they dial it in and the aircraft’s callsign appears next to its target.
A small set of codes means the same thing everywhere (7500, 7600 and 7700) or across a whole country, like 1200 for VFR flights in the US. Those fixed codes are what this section covers. To see which aircraft are squawking an emergency code at this moment, check emergencies now.
Learn more in how to read flight data and the glossary.