10-4 (also written 104, 10 4 or 10/4) is the police radio code for message received. Most published lists give it as an acknowledgment: “Message received” in the California Highway Patrol’s code card and APCO’s 1955 list, and “Affirmative (OK)” in APCO’s 1974 list and the Phoenix Police Department’s list.
One list uses it differently. The Metropolitan Police Department in Washington, DC, uses 10-4 for a two-member unit. Check the list for the agency you are listening to.
What 10-4 means
In 11 of the 12 lists in the table below, 10-4 is a form of acknowledgment or agreement. Five say “message received”: APCO’s 1955 list, the CHP, Folsom (“Copy; message received”), San Jose (“Message received and understood”) and Tucson (“Message received; OK; acknowledged”).
Others use shorter words. Phoenix and APCO’s 1974 list say “Affirmative (OK)”, Waynesville, Missouri says “Affirmative”, Chowchilla says “OK or acknowledgement”, the Lake County Sheriff’s Office in Florida says “O.K.”, and San Antonio says “Acknowledged”.
10-4 meaning by agency
Each published meaning we have is listed below, with its source.
| Agency or list | What 10-4 means | Source |
|---|---|---|
| APCO recommended list (1955) | Message received | APCO 1955 ten signals |
| APCO standard list (1974) | Affirmative (OK) | APCO 1974 brevity code |
| Phoenix Police Department (AZ) | Affirmative (OK) | Phoenix PD radio codes |
| Tucson Police Department (AZ) | Message received; OK; acknowledged | Tucson PD radio codes |
| California Highway Patrol (CA) | Message received | CHP aural brevity card |
| Chowchilla Police Department (CA) | OK or acknowledgement | Chowchilla PD dispatch manual |
| Folsom Police Department (CA) | Copy; message received | Folsom PD communications manual |
| San Jose Police Department (CA) | Message received and understood | San Jose PD 10-code guide |
| Metropolitan Police Department (DC) | Acknowledgment used for a two-member unit (you have a partner) | MPD radio handout |
| Lake County Sheriff's Office (FL) | OK | Lake County Sheriff 10-codes |
| Waynesville Police Department (MO) | Affirmative | Waynesville PD ten codes |
| San Antonio Police Department (TX) | Acknowledged | San Antonio PD General Manual 403 |
Eleven entries are an acknowledgment, from “Message received” to “Affirmative”. The twelfth, the Metropolitan Police Department in Washington, DC, uses 10-4 as an acknowledgment for a two-member unit.
The two APCO lists
The wording traces back to two lists from APCO, the public-safety communications association. The 1955 recommended list, published in the April 1957 APCO Bulletin, gives 10-4 as “Message received”. APCO’s 1974 aural brevity code gives “Affirmative (Ok)”.
The 1957 bulletin said that not all police departments use the standard ten signals and that the meaning of certain signals may vary slightly in some areas. Both wordings survive: the CHP, Folsom and Tucson follow the “message received” form, and Phoenix follows “affirmative (OK)”. Our guide on why police code meanings vary has more background.
10-4 in Washington, DC
The Metropolitan Police Academy’s handout “Use of Police Radios” lists 10-4 as “Acknowledgment used for a two-member unit (you have a partner)” and 10-99 as the acknowledgment for a one-member unit. It adds that 10-4 and 10-99 are used when going in service, to tell the dispatcher how many officers are in the unit and to acknowledge the dispatcher when they raise you.
The handout also says that, under the National Incident Management System, many jurisdictions including the department are moving to more plain language in place of ten signals. The public version redacts the full list of codes.
Is 10-4 still used?
Some manuals keep it even where they cut back on ten codes. San Antonio’s General Manual 403, effective December 21, 2016, says that in compliance with the National Incident Management System the use of ten codes is limited to twelve, and 10-4 “Acknowledged” is the first on the list. The California Highway Patrol’s Highway Patrol Manual 84.8 lists 10-4 “Message received” among the frequently used codes trainees should be familiar with.
See also do police still use 10 codes and the full list of ten codes.
How 10-4 is used on the radio
In the lists, 10-4 is the acknowledgment itself: it answers a message. The Washington, DC handout is the only document in the table that describes a procedure, using 10-4 or 10-99 when going in service and to acknowledge the dispatcher when raised.
None of the other lists describes what to say back to a 10-4. They define the code but do not give a reply procedure.
FAQ
What does 10-4 mean in police code?
It means message received. The California Highway Patrol and APCO’s 1955 list say “Message received”; APCO’s 1974 list and Phoenix say “Affirmative (OK)”; San Antonio says “Acknowledged”.
How do you respond to a 10-4?
The lists do not give a reply, because 10-4 is itself the acknowledgment. In the Washington, DC handout, units use 10-4 or 10-99 to acknowledge the dispatcher when they raise the unit.
Is 10 4 the same as 10-4?
Yes. 104, 10 4 and 10/4 are 10-4 written without the hyphen, with a space or with a slash. The meaning is the same in each list.
Does 10-4 mean the same thing everywhere?
Nearly. Eleven of the 12 lists in our table give a form of acknowledgment, but the Metropolitan Police Department in Washington, DC, uses 10-4 for a two-member unit.
Sources
- Standard "Ten Signals", The APCO Bulletin, April 1957, p. 7 (list prepared by the National Operating Procedure Committee in 1955), APCO (Associated Public-Safety Communications Officers). Updated April 1957. Standard list Checked October 2026
- Public Safety Communication Aural Brevity Code (APCO Project 14, Exhibit No. 7, p. 60), APCO (Associated Public-Safety Communications Officers). Updated 1974. Standard list Checked October 2026
- Phoenix Police Department – Radio Codes – Numerical Order, Phoenix Police Department. Updated 3/17/26. Official Checked October 2026
- Tucson Police Department Radio Codes, Tucson Police Department. Official Checked October 2026
- CHP 254 – Aural Brevity Card (01-2021 v2), California Highway Patrol. Official Checked October 2026
- Communications Training Manual (radio codes), Chowchilla Police Department. Official Checked October 2026
- Communications Manual (radio codes, p. 32), Folsom Police Department. Updated rev. 2019. Official Checked October 2026
- SJPD 10-Codes (study guide), San Jose Police Department. Updated 10/15/18. Official Checked October 2026
- 2.2 Use of Police Radios (training handout), Metropolitan Police Department. Official Checked October 2026
- 10 Codes and Signals, Lake County Sheriff's Office. Official Checked October 2026
- Ten Codes, Waynesville Police Department. Official Checked October 2026
- General Manual 403: Communications, San Antonio Police Department. Updated December 21, 2016. Official Checked October 2026
- Highway Patrol Manual 84.8, Chapter 3: Communications, California Highway Patrol. Official Checked October 2026