Why do squawk codes only use digits 0 through 7?

A Mode A squawk is written as four octal digits. Each position has eight possible values, 0 through 7, so there are 8^4 = 4,096 possible values from 0000 through 7777. Digits 8 and 9 are not valid octal digits.

What are the primary emergency squawk codes?

ICAO and FAA publications identify 7500 for unlawful interference, 7600 for radio communication failure, and 7700 for an emergency. Consult the current publication and follow ATC instructions for your jurisdiction.

What is squawk 7400 used for?

FAA Order JO 7110.65 states that code 7400 may be transmitted by an unmanned aircraft system when the control link between the aircraft and pilot is lost. This statement is FAA-specific and is not presented as a universal allocation.

What does an unlisted valid octal code mean?

It means only that the value has valid Mode A syntax and is not identified in this limited reference. It does not establish that the code is available or assignable in a particular country, region, or facility.

Does this reference replace ATC instructions or flight manuals?

No. Transponder code allocation varies by country, flight information region (FIR), and facility letter of agreement. Always follow current ATC instructions and applicable national publications.

Aviation Transponder Code Reference

Identify verified transponder squawk codes, check 4-octal-digit Mode A syntax, and review governing authorities.

Aviation Operational Notice: This reference is for informational and flight-briefing study only. Code allocations, block assignments, and procedures vary by Flight Information Region (FIR) and national civil aviation authority. Always comply with active Air Traffic Control instructions, operational regulations, and current aeronautical publications. All validation runs client-side in your browser; no queries are transmitted.

Squawk Code Decoder

Enter a 4-digit octal code (Mode A/3A: digits 0–7 only).

Quick Presets

Enter any 4-digit code above or select a preset to evaluate.

Mode A Technical Framing

Total Combinations:: 4,096 (8^4)
Register Layout:: 4 octal digits
Valid Octal Range:: 0000 – 7777
Primary Emergency Codes:: 7500, 7600, 7700

Verified Transponder Code Reference

Primary source verified: FAA AIM, JO 7110.65, and cited ICAO publications.

Code	Type	Designation & Operational Meaning	Authority / Region

Verified Primary Authorities Verified: September 2026

Summary

Identify verified transponder squawk codes, check 4-octal-digit Mode A syntax, and review governing authorities.

How it works

Enter any 4-digit octal squawk code (digits 0–7) or select a quick preset.
View the verified designation, authority source, and operational classification.
Check Mode A syntax: four positions using digits 0–7 give 4,096 possible values from 0000 to 7777.
Filter the reference table by code, category, description, or regulatory region.

Use cases

Quick cross-referencing of emergency and special-purpose squawk codes against FAA and ICAO publications.
Flight crew and dispatcher preflight briefing review.
Aviation students and flight simulator users verifying Mode A octal numbering mechanics.
ADS-B receiver operators decoding standard transponder returns.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu