Area Code 312
Illinois
Area code 312 serves Illinois, primarily Chicago, in the Central Time (CT) time zone. It has served the region since 01-Jan-1947.
Published: Last reviewed: Verified against NANPA and FCC records
Overlay Area Code
This area code shares its geographic footprint with an overlay code, so the same cities are served by two area codes. Because overlays leave existing numbers untouched, this region's relief planning favors adding codes over redrawing boundaries.
The codes serving this same region: area code 872 (Chicago).
With prefix utilization already at 98.1%, numbering resources here are under sustained pressure — context that explains why overlays were introduced.
In service since: 01-Jan-1947
Area Code Splits
2 area codes have been carved out of this code's original territory through geographic splits. Each split was a relief action: as demand grew, part of the region was reassigned to a new code rather than overlaying the whole territory.
Split into: area code 708 (Cicero), area code 773 (Chicago).
Demographics & Economics
- Total Population
- 293,470
- Population Density
- 5,484 people/sq miabove U.S. average (U.S.: 95 people/sq mi)
- Median Household Income
- $83,498near U.S. median (U.S.: $80,734)
- Poverty Rate
- 13.5%near U.S. average (U.S.: 12.5%)
- Unemployment Rate
- 7.0%above U.S. average (U.S.: 5.2%)
- Education Attainment
- Bachelor's Degree+42.7%above U.S. average (U.S.: 35.7%)High School or Lower28.2%
Statistics are population-weighted aggregations from Census ACS 5-year estimates. Population figures are approximate — see Sources & Methodology for details on how area code populations are derived.
See how this area code compares to all US area codes in the income ranking and density ranking.
Time Zone
- Central Time (CT)
NANPA Relief Status
According to the latest NANPA analysis (July 2026), this area code is not currently listed as exhausting within the next 36 months. No relief planning is in progress for this NPA.
Source: NANPA NPA Relief Planning Reports · See all relief planning statuses
Current Time
Prefix Utilization
Geographic Coverage
State
IL
Major Cities
- Chicago
- West Chicago
- Norridge
- Riverdale
- Harwood Heights
Historical Timeline of Area Code 312
Area code 312 was established January 1, 1947, as one of the original 77 US NANP area codes. Assigned to serve the entire Illinois side of the Chicago metropolitan area, 312 required only 5 rotary dial pulses (3+1+2), the minimum for any multi-state code, reflecting Chicago's status as the third-largest US city. Illinois received four original codes: 312 (Chicago), 217 (central), 815 (northern), and 618 (southern). Source: NANPA
Key milestones:
- November 11, 1989: Area code 708 split from 312, creating a separate code for Chicago's surrounding suburbs and reducing 312 to the city proper. Permissive dialing ended February 9, 1990. Source: TelecomChicago
- April 17, 1995: The FCC issued Declaratory Ruling FCC 95-19 (IAD File No. 94-102), rejecting Ameritech's proposed overlay-and-segregation plan for 708 relief. The ruling established that numbering relief must be technology-neutral under 47 U.S.C. §202(a) and §201(b), ordering a three-way geographic split of 708 into 630, 847, and retained 708. Source: Federal Register
- October 12, 1996: Area code 773 split from 312 to serve Chicago neighborhoods outside the downtown core, reducing 312 to an enclave code of approximately 4-5 square miles. Chicago became one of only three US cities split between multiple area codes. This was the last geographic split in Illinois. Source: NANPA
- June 30, 1999: The Illinois Commerce Commission approved an 872 overlay for the 312 and 773 NPAs in Docket No. 98-0847. Implementation was delayed nearly a decade by number conservation measures including thousands-block number assignment. Source: ICC
- November 7, 2009: The 872 overlay activated with mandatory 1+10-digit dialing, completing the 312/773/872 overlay complex serving all of Chicago. Source: NANPA PL-390
The combined 312/773/872 overlay complex is projected to provide numbering relief until Q3 2043, approximately 18 years from current projections.
Regulatory History
Area code 312 has been shaped by several significant federal and state regulatory actions spanning over 60 years, establishing precedents that influenced numbering policy nationwide. Source: Federal Register Source: ICC
| Year | Event | Docket | Regulatory Action & Impact |
|---|---|---|---|
| 1947 | NANP Inception | AT&T / Bell System | Area code 312 established as one of the original 77 US NANP area codes, assigned to IL. |
| 1988 | 708 Split Authorization | IL-88-04-015 / IL-88-06-012 | NANPA authorized the split of 312, creating area code 708 for Chicago suburbs. Illinois Bell set the November 11, 1989 effective date. Source: TelecomChicago |
| 1995 | FCC Technology-Neutrality Ruling | IAD File No. 94-102 | FCC 95-19 rejected Ameritech's overlay-and-segregation plan, ruling that numbering relief must be technology-neutral under 47 U.S.C. §202(a) and §201(b). Ordered a three-way split of 708 into 630, 847, and retained 708. Established a landmark precedent cited in subsequent FCC orders nationwide. Source: Federal Register |
| 1996 | 773 Split Authorization | IL-95-07-015 | NANPA authorized splitting 312 into downtown (retains 312) and rest of Chicago (new 773). 312 reduced to an enclave code of approximately 4-5 square miles. Source: NANPA |
| 1999 | 872 Overlay Approval | ICC Docket 98-0847 | Illinois Commerce Commission approved an all-services distributed overlay with NPA 872 for the 312 and 773 NPAs. Implementation was delayed nearly a decade by number conservation measures. Source: ICC |
| 2002 | Number Conservation Measures | ICC Docket P2002-0093 | ICC implemented number pooling and thousands-block assignment, allowing carriers to share blocks of 1,000 numbers instead of requiring full 10,000-number NXX assignments. This delayed 872 activation by nearly a decade. Source: ICC |
| 2009 | 872 Overlay Activation | NANPA PL-390 | NANPA Planning Letter PL-390 set the implementation timeline. Permissive 1+10-digit dialing began August 7, 2009; mandatory 1+10-digit dialing took effect November 7, 2009. The 312/773/872 overlay complex was completed. Source: NANPA PL-390 |
The combined overlay complex is projected to provide numbering relief until Q3 2043.
County Coverage
Population-weighted county share based on aggregated ZCTA population (100% = total mapped population for area code 312).
| County | Population | Share |
|---|---|---|
| Cook, IL | 293,470 | 100.0% |
Area Code 312 Family Tree
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All Descendants
Area code 312 was originally assigned in 01-Jan-1947 to serve IL. Over the years, it has generated 8 descendant area codes through a combination of geographic splits and overlays as demand for telephone numbers grew.
Call Context
This area code serves a major metropolitan region with high population density in IL. The primary city is Chicago.
Nearby Area Codes
Area codes geographically closest to 312.
Other Area Codes in Illinois
Area codes that also serve Illinois, linked for easy comparison.