The number painted on the side of a US aircraft is its registration mark. Pilots call it the tail number. The FAA calls it the N-number, because every US mark begins with the letter N — the nationality mark assigned to the United States. The rest of it identifies one specific airframe.
It is worth knowing that the tail number is an administrative identifier, not a permanent one. It belongs to a registration, and a registration can be cancelled, transferred, or replaced with a different number on request. The airframe underneath keeps its manufacturer's serial number for life. That distinction turns out to matter a great deal if you are buying an aircraft, and we come back to it at the end.
The format rules
Under 14 CFR 47.15, a US registration number "may not exceed five symbols in addition to the prefix letter 'N'." Only three shapes are legal:
- All numbers — for example N10000.
- One to four numbers, then one letter — for example N1000A.
- One to three numbers, then two letters — for example N100AB.
Two constraints do most of the work of making marks readable over a radio:
- Letters come last, and only at the end. Digits then letters, never the other way around and never interleaved. There is no such thing as an N-number like NA100.
- The letters I and O are never used. The regulation is explicit: "The letters 'I' and 'O' may not be used." They are too easily confused with the digits 1 and 0.
The FAA also does not issue a number whose first symbol is a zero — the first character after the N is always 1 through 9. So there is no N0 anything, and a zero only ever appears after a digit from 1 to 9.
What the actual registry looks like
Those are the rules. Here is the reality, counted across every aircraft holding a current US registration in the FAA file as of 25 July 2026 — 315,010 aircraft:
- Digits plus two letters — 151,557 aircraft (48.1%). The most common shape by a clear margin. N123AB.
- Digits plus one letter — 118,670 (37.7%). N1234A.
- All digits — 44,783 (14.2%). N12345.
And by length, counting the symbols after the N:
- Five symbols — 268,613 aircraft (85.3%). The overwhelming majority.
- Four symbols — 41,215 (13.1%).
- Three symbols — 5,001 (1.6%).
- Two symbols — 181 (0.06%). Genuinely rare.
This is why short tail numbers are prized and traded: there are only so many of them, and 85% of the fleet is already carrying the maximum five. It also explains the shape of most numbers you will actually see — a five-symbol mark ending in one or two letters.
Both format rules hold across the entire file without a single exception, which is a useful thing to be able to state rather than assume. Of 315,010 current registrations, zero begin with a zero and zero contain an I or an O.
How big the letters have to be, and where
14 CFR 45.29 sets the size. Marks on a fixed-wing aircraft must be at least 12 inches high, and the same height, width, thickness and spacing on both sides. There are narrow exceptions: three-inch marks are allowed on a glider, and on an aircraft holding an experimental airworthiness certificate whose maximum cruising speed does not exceed 180 knots. Aircraft that were already displaying marks at least two inches high before 1 November 1981 were grandfathered, which is why you still see small marks on some older airframes.
The proportions are specified too. Characters must be two-thirds as wide as they are high — except the number 1, which is one-sixth as wide as it is high, and the letters M and W, which may be as wide as they are high. Strokes are one-sixth as thick as the character is tall. That is why N-numbers on wildly different aircraft all look like they came from the same typeface.
A tail number is not a permanent name
This is the part that catches people out. A registration number can be reserved for no more than one year at a time, renewable on application and payment. When a registration is cancelled, the number becomes unavailable for reassignment for five years — unless the former owner reserves it within a year of cancellation.
After that window, the number can go to a completely different aircraft. So one tail number may have belonged to several airframes across the decades, and one airframe may have worn several tail numbers. Both directions cause real confusion:
- An accident you find under a tail number may belong to a different aircraft that carried that number years earlier. See why a tail number isn't in the FAA registry.
- An airframe that was wrecked, repaired and re-registered under a new number reads completely clean on its current tail number. See how to tell if an aircraft was rebuilt after a crash.
The manufacturer's serial number is what does not change. It is stamped on the airframe, it is carried in FAA registration records, and it appears on most NTSB accident records. Searching on it follows the actual metal rather than the paperwork — which is how to look up an aircraft by serial number, and the reason a serial cross-check is the first thing our report runs.
Reading the number tells you nothing about the aircraft
Worth saying plainly, because it is a common misunderstanding: unlike a VIN on a car, an N-number is not encoded. The digits do not tell you the manufacturer, the model, the year, or where it was built. N172SP is not necessarily a Cessna 172. The number is an arbitrary label assigned to a registration, and the only way to get from the number to the aircraft is to look it up in the registry.
What a lookup gives you is the make, model and year, the registration status and expiry, the ownership chain, any documents filed against the tail, and any accident records matched to the airframe — including the ones filed under a previous tail number.