N-number to ICAO hex converter.

What the Mode S code is

The FAA Registry lists a “Mode S Code” for every US-registered aircraft, written in base 8 (octal) and base 16 (hex). The six-digit hex form, for example A12239 for N172SP, is the 24-bit address people call the ICAO hex or ICAO24. The FAA publishes the code for each registration but, as far as we could find, not the rule that links an N-number to its code. This page therefore does two things: it applies the community-documented layout of the US block, and it shows the evidence that the layout is right.

How the conversion works

The US block holds 915,399 codes, from A00001 to ADF7C7, one for every valid N-number from N1 to N99999 including the letter forms. They are laid out in the order a person would count N-numbers. Each first digit from 1 to 9 owns a block of 101,711 codes. Inside it come 601 slots for the number itself and its letter forms (1 for the number, then each of 24 single letters followed by its 24 two-letter extensions: 1 + 24 × 25 = 601), then ten sub-blocks, one for each possible second digit, of 10,111 codes each. The pattern repeats one level down: a second-digit block is 601 letter slots plus ten third-digit blocks of 951, a third-digit block is 601 plus ten fourth-digit blocks of 35, and a fourth-digit block has just 35 slots, namely the number itself, 24 single letters (no two-letter forms fit at that length) and ten fifth digits. There are 24 letters because the FAA bars I and O (14 CFR 47.15(b)).

To convert, add up the offsets and add them to A00001. To reverse, divide the offset from A00001 by 101,711 to get the first digit, then peel off 601 and divide by 10,111, 951 and 35 in turn until the remainder is a letter slot. The page does exactly this, in your browser, with no lookup.

Worked example

Take N172SP. The 1 is the first digit and adds nothing (its block starts at A00001). The 7 skips the 601-slot letter area and seven blocks of 10,111: 601 + 70,777 = 71,378. The 2 skips 601 and two blocks of 951: 601 + 1,902 = 2,503. The letters SP are the 17th and 14th letters of the 24-letter alphabet, so they add 1 + 16 × 25 for S, then 1 + 13 for P: 415. The total offset is 71,378 + 2,503 + 415 = 74,296. In hex 74,296 is 12238, and A00001 + 12238 = A12239, the code the FAA Registry shows for N172SP, whose octal form is 50221071.

Checked against the FAA Registry

We looked up 34 N-numbers by hand in the FAA N-Number Inquiry on 20 October 2026, chosen to cover every shape: one to five digits, one letter, two letters, and numbers at both ends of the block. For 19 of them the registry shows both hex and octal; for the other 15 (including N1 and N99999) it shows only the octal Mode S code. The converter matched the registry on all 34, and converted each hex code back to the same N-number. Separately, a test converts all 915,399 possible N-numbers, checks that no two share a code, that the lowest is A00001 and the highest ADF7C7, and that every code converts back to its N-number. A sample:

Hex is as printed by the registry. Octal is the same number in base 8, which the registry also prints; the two agreed wherever we compared them.

Mistakes and edge cases

• Invalid shapes. N0, an I or O, a letter in the middle (N1A2), three letters (N12ABC) or a zero straight after N are all refused with a reason, following 14 CFR 47.15(b).
• Valid is not an aircraft. A well-formed N-number converts to a code whether or not an aircraft holds it. The registry lists N99999, for example, as a reserved number, not as an aircraft.
• Hex outside the block. A code below A00001 or above ADF7C7 is reported as not an N-number rather than forced into one.
• Squawk is not hex. The four-digit squawk code a controller assigns is unrelated to the 24-bit address. See the squawk codes reference.
• Case and spacing. “n 172-sp” and “N172SP” are the same input.

Who this is for

ADS-B and flight-tracking hobbyists who see a hex code and want the tail number, or have a tail number and want to filter a feed, developers cross-checking a data pipeline, and students curious how a registration number maps to a transponder address.

What this does not cover

• Whether a number is registered, who owns it, or what aircraft it is. Use the FAA Registry’s N-Number Inquiry.
• Any country other than the United States, and US military or other allocations outside the N-number block.
• What a given transponder is actually broadcasting. This page knows only the registry’s code, not what any aircraft’s equipment is set to send.
• How the FAA assigns codes when a registration changes. We found no FAA statement on that, so we make none.