A jail number usually looks like a short string of digits, sometimes mixed with a letter or two, assigned by the facility holding the person. In the federal system it takes a strict eight-digit shape written as XXXXX-XXX. State prisons and county jails each set their own conventions, so the length and style can vary from four digits in a small county lockup to a letter-and-number combination in a state system. Whatever the format, this number is what mail, money deposits, visit scheduling, and phone accounts all key off of.
The Federal Format
Every federal inmate gets the same pattern. The U.S. Marshals Service and the Bureau of Prisons assign an eight-digit register number written as five digits, a hyphen, and three digits. The first five are unique to the individual. The last three identify the federal judicial district where the person was arrested or processed. A suffix of 054, for example, corresponds to the Southern District of New York. In districts that have processed more than 100,000 people, the leading zero in the suffix rolls over to a 1.
The BOP inmate locator expects this exact pattern when you search by number: five digits, hyphen, three digits. The register number stays with a federal inmate for the whole sentence, no matter how many times they’re transferred.
State and County Formats
Outside the federal system there is no single standard. Each state department of corrections and each county jail sets its own numbering convention.
Some state systems encode information in the number. A common pattern combines a two-digit year, a letter for the reception facility, and a sequence number, so someone admitted in 2024 at a particular intake center might carry a number like 24B1537. Other states use a plain run of six or seven digits with no embedded meaning.
County jails tend to use shorter numbers because they process fewer people. A small county may assign four- or five-digit numbers; a large urban jail may use six or more. Some local facilities prefix the number with a letter tied to the facility or to the first letter of the person’s last name. Every number is unique inside its own system, even though the formats across jurisdictions look nothing alike.
Booking Number vs. Inmate ID
These terms get used interchangeably, but they’re not the same, and the difference matters when you’re trying to send mail or look someone up.
A booking number is created at the moment of arrest, during intake. It is tied to that specific arrest event. If someone is arrested, released, and arrested again, they usually get a new booking number each time. It functions as a transaction number for one trip through the system.
An inmate ID number, sometimes called a department identification number or register number, is more permanent. It’s assigned when someone is formally processed into a correctional facility for a sentence, and it stays with them through transfers, housing changes, and program assignments. In many state systems the same number is reissued if the person returns to custody years later. When a facility asks for “the inmate’s number” for mail, visits, or deposits, it almost always means this permanent ID, not the booking number.
How to Find a Number You Don’t Have
If you don’t already have the number, the lookup depends on who holds the person.
Federal
The Bureau of Prisons runs a free online inmate locator at bop.gov. You can search by name, which requires at least a first and last name, or by register number if you already have it.1Federal Bureau of Prisons. Federal Inmates By Name – BOP
State and Local
Most state departments of corrections maintain their own online inmate search tools. Look on the state corrections department website for an “offender search” or “inmate lookup” page. For county jails, check the sheriff’s office website for that county; many offer online lookup where a name search returns the person’s jail number, housing location, and basic case information.2USAGov. How to Look Up Prisoners and Prison Records
If the online tools come up empty, call the facility. Front desk or records staff can usually confirm whether someone is in custody and give you the inmate number. Court documents and arrest paperwork also list these numbers, so check anything you already have before searching further.
What You’ll Do With It
Almost every interaction with a correctional facility requires the inmate’s identification number.
- Mail: every envelope must include the person’s full name and inmate number along with the facility address. Mail without the number is often returned or significantly delayed.
- Commissary deposits: adding money for food, hygiene items, or phone time runs through the facility’s approved deposit service, which asks for the inmate number. Most facilities use third-party vendors that charge a transaction fee, typically a few dollars per deposit.
- Visits: in-person and video visit scheduling systems ask for the number to pull up available time slots.
- Phone calls: the inmate’s phone account is tied to the identification number, and you may need it to set up a prepaid account or accept calls.
Does the Number Change?
In the federal system, the register number stays the same for the entire sentence, even across transfers. That’s part of why the format encodes the sentencing district rather than the current housing location.
State systems generally work the same way. The inmate ID assigned at intake follows the person through the corrections system, and some states reuse the same number if the person returns years later. Things get messier at the county level. Someone held in a county jail pretrial, transferred to state prison after sentencing, and later moved to another state facility could end up with a county booking number, a separate state inmate ID, and possibly that same state ID again on a later return. The state-level ID is the one that matters for most practical purposes like sending money or scheduling visits.
If you lose track of someone’s number after a transfer, the online lookup tools will show the current facility assignment. Search by name and the updated location comes up with the same identification number attached.