Area code 312 was assigned in 1947 to the Chicago metropolitan area. Since the October 1996 split it has covered only part of Chicago, and area code 872 has overlaid it and 773 since November 7, 2009.
Quick answer
- 312 was assigned in 1947 to the Chicago metropolitan area.
- It covers part of Chicago.
- The rest of the city has used 773 since October 1996, and the suburbs use other codes.
- Overlay code 872 shares the same territory since November 7, 2009.
The essentials
312 area code: quick facts
| Area code | 312 |
| Coverage | Part of Chicago; the rest of the city uses 773 and 872 |
| State | Illinois |
| Standard time | UTC-6 (time.gov) |
| Assigned | 1947, to the Chicago metropolitan area |
| Overlay on the same territory | 872 (in service November 7, 2009) |
| Code split off the rest of Chicago | 773 (October 1996) |
| Country code | +1 (United States) |
How 312 became Chicago's code
- 1947: 312 is assigned to the Chicago metropolitan area.
- 1989: The suburbs are split off to area code 708, and 312 is reduced to Chicago.
- January and August 1996: 708 is split. The northern suburbs get 847 in January 1996, and the western suburbs get 630 in August 1996.
- October 1996: The 312/773 split is implemented in Chicago, and 773 takes the rest of the city. The Illinois Commerce Commission chronology gives only the month, so this page does too.
- November 7, 2009: Area code 872 goes into service as an overlay on 312 and 773.
Where area code 312 is and is not used
312 covers part of Chicago. Since the October 1996 split, the rest of the city has used 773, and 872 now overlays both codes.
The suburbs were moved out of 312 in 1989. The southern suburbs kept 708, the northern suburbs received 847, and the western suburbs received 630. Suburban areas therefore use the codes listed in the neighbors table below.
Splits that moved territory out of 312
Each split took a part of the old 312 territory and gave it a new code. The sequence explains why neighboring areas have different codes.
- 1989: the suburbs moved to 708, leaving 312 with Chicago.
- January and August 1996: 708 split, giving 847 to the northern suburbs in January and 630 to the western suburbs in August.
- October 1996: the rest of Chicago moved to 773, leaving 312 with part of the city.
- November 7, 2009: 872 went into service as an overlay on 312 and 773.
The lesson for businesses is that a number's area code shows where it was first assigned. Your customers may still have older numbers from any of these codes, so keep routing rules for all of them.
Neighboring area codes in the Chicago area
312 sits beside several codes. The table shows which code covers which part of the Chicago area, so you can tell at a glance where a number was assigned.
| Code | Where it applies | Relation to 312 |
|---|---|---|
| 312 | Part of Chicago | Assigned in 1947 for the Chicago area, reduced in 1989 and 1996 |
| 872 | The same territory as 312 and 773 | Overlay since November 7, 2009 |
| 773 | The rest of Chicago outside 312 | Split off from 312 in October 1996 |
| 708 | Suburbs not moved to 847 or 630 | Created in 1989 when the suburbs split from 312 |
| 847 | Northern suburbs | Created in January 1996 from the 708 split |
| 630 | Western suburbs | Created in August 1996 from the 708 split |
The 872 overlay on the 312 territory
An overlay adds a new area code to the same geographic area, so existing numbers do not change. The Illinois Commerce Commission records one overlay on the 312 territory:
- 872 (in service November 7, 2009): overlays 312 and 773. The Illinois Commerce Commission's chronology records the overlay designation on August 16, 1999, and its October 30, 2009 notice confirms the November 7, 2009 start.
Because the codes overlap, numbers in the same area can have different area codes. The area code does not show which building or floor a number belongs to.
| Code | Added | Where it applies |
|---|---|---|
| 312 | 1947 | Part of Chicago |
| 773 | October 1996 | The rest of Chicago outside 312 |
| 872 | November 7, 2009 | Overlay on 312 and 773 |
Time zone for area code 312
Standard time is UTC-6, and daylight time is one hour ahead, at UTC-5. NIST's time.gov lists the standard-time offset as UTC-6, and 15 U.S.C. 261 sets the third zone at UTC-6.
In the United States, daylight saving time starts on the second Sunday in March and ends on the first Sunday in November. Clocks change at 2:00 a.m. local time. This rule has applied since 2007, under the Energy Policy Act of 2005.
How to dial a 312 number
A 312 number has a three-digit area code and a seven-digit local number. Since November 7, 2009, calls to 312 numbers have required the area code, including for local calls. The Illinois Commerce Commission's October 30, 2009 notice says that from that date all calls must be dialed 1 + area code + the 7-digit number, which is 11 digits in total.
AT&T's carrier guidance lists 312 plus the 7-digit number for local calls, 1 + 312 + the 7-digit number for long distance within the U.S., and +1 312 + the 7-digit number for international calls, For local calls, AT&T's format differs from the ICC's 1 + area code rule, so confirm the local format with your carrier before you set dialing rules.
The 1995 exhaust notice and the 1996 split
The Illinois Commerce Commission's chronology records an August 1995 notice of impending 312 exhaust, and a November 1995 ICC approval of a 312/773 split for Chicago.
The split moved the rest of Chicago to 773 in October 1996. The 2009 overlay added 872 to the same territory without moving existing numbers, so an existing number keeps its area code.
Using a 312 number for business
A 312 number shows that it is in the 312 area code, which serves part of Chicago. It does not show where your team sits, and a phone system can route 312 calls to staff in any location.
Because 872 overlays 312 and 773, a Chicago business can hold numbers from all three codes. Numbers in the 312 territory can be 312 or 872, and numbers for the rest of the city can be 773 or 872. Whether a number can be assigned to your business depends on your phone provider and its current inventory.
Important: Confirm availability with your provider before you plan a campaign or print a number on materials.
Before you use a 312 number
- Confirm the number is available from your provider, and get its terms in writing.
- Ask who owns the number and whether it can be moved to another provider if you switch.
- Route 312 and 872 calls for the 312 territory, and 773 and 872 calls for the rest of the city, by location and not by area code alone.
- Set dialing rules for 1 + area code + the 7-digit number for 312 and 872 before you go live, and check the local format with your carrier.
- Plan for caller ID. A local number does not prove who is calling, and caller ID can be faked.
- Keep routing for older 708, 847, and 630 numbers from the suburbs.
Frequently asked questions
Which area is the 312 area code for?
Area code 312 serves part of Chicago. The Illinois Commerce Commission's chronology records the October 1996 split that created area code 773 for the rest of the city.
Is area code 312 being split or replaced?
The last split of 312 in the Illinois Commerce Commission chronology is the October 1996 split that created 773. Area code 872 has overlaid 312 and 773 since November 7, 2009, so the codes share the same territory.
Is 312 a scam area code?
No. 312 is a real Chicago area code, and AT&T describes it as legitimate. However, the FTC warns that scammers fake caller ID to look local, so a 312 number is not proof of who is calling.
Do I need to dial the area code for a 312 number?
Yes. Since November 7, 2009, calls to 312 numbers have required the area code, even for local calls. The Illinois Commerce Commission's October 30, 2009 notice says all calls must be dialed 1 + area code + the 7-digit number. AT&T's carrier guidance lists the local format as 312 plus the 7-digit number, so confirm the local format with your carrier.
Sources · reviewed October 2026
Chronological history of area code relief in Illinois, Illinois Commerce Commission
Used for the 1947 assignment of 312 to Metropolitan Chicago, the 1989 312/708 split, the August and November 1995 exhaust notice and split approval, the 1996 dates for 847 (January) and 630 (August), the October 1996 split of 773 (month only), and the August 16, 1999 overlay designation of 872.
Illinois Commerce Commission release, October 30, 2009
Used for the November 7, 2009 start of 872 and the 1 + area code + number dialing rule that began that day, including for local calls.
Used for the standard-time offset of UTC-6.
15 U.S.C. 260a, advancement of time (Cornell LII copy of the U.S. Code)
Used for the second-Sunday-in-March and first-Sunday-in-November rule, the 2:00 a.m. change time, and the March 1, 2007 effective date set by Pub. L. 109-58, the Energy Policy Act of 2005.
15 U.S.C. 261, zones for standard time (Cornell LII copy)
Used for the third zone being UTC-6.
312 Area Code: Chicago, IL, AT&T
Used for the statement that 312 is a legitimate Chicago code, the local, long-distance, and international formats, and the neighbor spoofing warning. This is carrier guidance, not an official numbering source.
Scammers can fake caller ID info, FTC consumer alert, May 4, 2016
Used for the warning that scammers fake caller ID to look local, that a familiar or local-looking number does not confirm who is calling, and that callers should not be trusted because of caller ID alone.