Getting the envelope right matters as much as what's inside it. One missing line — an ID number, a unit designation, a PO box instead of the physical address — and a facility's mailroom sends the whole thing back. Here's the exact format to use, with real examples for county jail, state prison, and federal mail.
The general format
Almost every facility expects the same basic structure, top to bottom:
- Line 1: Inmate's full committed name + ID number
- Line 2: Facility name (and housing unit, if the facility uses one)
- Line 3: Facility's mailing address or PO box
- Line 4: City, State, ZIP
Always write your own return address in the top-left corner of the envelope — mail without a return address is rejected outright at most facilities.
Example: county jail
County jails typically use the facility's street address directly, sometimes with a housing pod or unit number:
John A. Smith #24-013456 Example County Jail, Pod 3B 100 Justice Center Drive Springfield, ST 62701
Confirm the pod or unit designation with a fresh inmate search — it changes if someone is moved within the facility, and mail addressed to an outdated unit can be delayed or returned.
Example: state prison
State department-of-corrections facilities usually assign a DOC number and may route all mail through a central processing address rather than the physical facility:
Maria L. Torres #A123456 State Correctional Institution 1 Prison Road Facility City, ST 45678
Some states process incoming mail at a centralized scanning facility rather than the prison itself — if that's the case, the DOC's official website will list the correct processing address, which can look different from the prison's physical location. Always use the state's current mailing instructions, not just the facility's street address.
Example: federal (Bureau of Prisons)
Federal inmates are identified by a register number rather than a state DOC number or county booking number:
David R. Nguyen #98765-012 Federal Correctional Institution P.O. Box 1000 Facility City, ST 12345
The register number format is distinctive — a five-digit number, a dash, then a three-digit number identifying the sentencing district. Get it exactly right; the BOP's mail system relies on it to route mail to the correct person.
Finding the right ID number
The single most common reason mail bounces is a missing or incorrect inmate ID number. The number goes by different names depending on the system:
- Booking number — county jail, assigned at intake, can change if transferred.
- DOC number — state prison, stays with the person for the length of their state sentence.
- Register number — federal BOP, follows the person across every federal facility they're held at.
Confirm the current number and facility with an inmate search before you address anything — numbers and facilities can both change, especially soon after an arrest or a transfer.
Formatting rules that affect delivery
- Use a plain white envelope — no colored envelopes, stickers, or decorations on the outside.
- Write clearly in blue or black ink, or type the address if allowed.
- Don't abbreviate the inmate's committed (legal) name — use it exactly as the facility has it on file, even if they go by a nickname.
- Include your full return address — first and last name, street address, city, state, and ZIP.
- Check whether the facility wants "Attn:" language, a housing unit, or a specific line order — some do, and getting the order wrong can delay processing even if all the information is present.
PO box vs. street address — how to know which to use
Some facilities want mail sent to the physical building; others route everything through a PO box or a centralized scanning center, especially larger state systems that scan mail digitally before delivering a copy to the inmate. Sending to the wrong one can mean delay rather than outright rejection, but it's still worth confirming — the facility's official website or a quick call to the mailroom will tell you which address is currently correct.
What to do if you're not sure your address is right
If a letter comes back, it's almost always a formatting issue rather than a personal rejection. Re-check, in order: the inmate's current facility (people are transferred more often than families expect), the ID number, and the exact address format the facility currently uses — DOC and BOP mailing instructions do change. A fresh inmate search is the fastest way to confirm all three before you resend.
Quick checklist
- Full committed name, spelled exactly as on file
- Correct, current ID number (booking / DOC / register)
- Correct address format for this specific facility (street vs. PO box)
- Your full return address in the top-left corner
- Plain white envelope, legible ink, no decorations
Let us handle the formatting
If keeping every facility's exact format straight feels like a lot to track, our send letters service does it for you — you write the letter or upload photos, and we format the address correctly for that specific facility, print, add postage, and mail it. For tips on what to actually write once the envelope is squared away, see our guide on how to write a letter to an inmate.