The K prefix — contiguous US (CONUS)
ICAO assigns the letter K to the contiguous United States. To convert a 3-letter FAA identifier to ICAO, prepend K. To convert back, drop the K.
Airports that do NOT get a K prefix
Some US airports already have 4-letter FAA identifiers (typically smaller fields with numeric or mixed-character codes). These airports keep their existing identifier in the ICAO system — no K is added. Examples include many small GA airports with identifiers like 2G4 or N87.
Alaska — PA prefix
Alaska uses the PA prefix in the ICAO system (P = Pacific region, A = Alaska).
Hawaii — PH prefix
Hawaii uses the PH prefix (P = Pacific, H = Hawaii).
US territories
When to use which code
Flight plans filed domestically in the US use FAA identifiers (3-letter for most airports). International ICAO flight plans require the 4-letter ICAO code. METARs, TAFs, and NOTAMs always use ICAO identifiers.
If you file via Leidos Flight Service (1800wxbrief.com), the system accepts either format. Most EFB and flight planning apps search both. AeroCopilot automatically resolves FAA and ICAO identifiers interchangeably.