Area Code 331
Illinois
Area code 331 serves Illinois, primarily Aurora, in the Central Time (CT) time zone. It is an overlay area code sharing territory with area code 630.
Overlay Area Code
This area code is part of an overlay region, meaning the same geographic area is served by multiple area codes. Overlay codes were introduced to meet growing demand for phone numbers without requiring existing customers to change their numbers.
This area code shares its geographic region with: area code 630 (Aurora).
In service since: 07-Oct-2007
Demographics & Economics
- Total Population
- 293,470
- Population Density
- 5,488 people/sq miabove U.S. average (U.S.: 93 people/sq mi)
- Median Household Income
- $81,797near U.S. median (U.S.: $75,144)
- Poverty Rate
- 13.3%above U.S. average (U.S.: 11.5%)
- Unemployment Rate
- 6.9%above U.S. average (U.S.: 3.6%)
- Education Attainment
- Bachelor's Degree+41.9%above U.S. average (U.S.: 33.7%)High School or Lower28.4%
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
- Aurora
- Naperville
- Bolingbrook
- Wheaton
- Downers Grove
- Plainfield
- Elmhurst
- Lombard
- Romeoville
- Bartlett
- Carol Stream
- Streamwood
- Hanover Park
- Addison
- Oswego
- Woodridge
- Glendale Heights
- St. Charles
- Glen Ellyn
- Batavia
- West Chicago
- Westmont
- South Elgin
- Lisle
and 37 more cities
Historical Timeline of Area Code 331
Area code 331 was assigned by NANPA on July 23, 1999, and went into service on October 7, 2007, as an all-services overlay on area code 630 covering Chicago's western suburbs. The more-than-eight-year gap between assignment and activation was driven by mandatory number-pooling conservation that extended 630's capacity. 331 was part of a simultaneous overlay batch that also assigned area code 464 (overlaying 708) and area code 872 (overlaying 312/773), all under Illinois Commerce Commission Docket 98-0847. Source: NANPA PL-195
Key milestones:
- 1947: Area code 312 created as one of the original 86 North American area codes, serving Chicago and northeastern Illinois. According to NANPA records.
- 1989: Area code 708 split from 630 on November 11, 1989, taking Chicago's inner suburbs. According to NANPA records.
- 1996: Area code 630 entered service on August 3, 1996, split from 630 to cover DuPage and most of Kane counties. Mandatory 1+10 digit dialing took effect November 30, 1996. Source: Wikipedia
- 1999: NANPA assigned 331 on July 23, 1999, under PL-195, based on ICC Docket 98-0847 approval. The assignment required mandatory thousand-block number pooling to be implemented in 630 before 331 activations could begin. Source: NANPA PL-195
- 2007: Area code 331 entered service on October 7, 2007, as an all-services overlay on 630. Mandatory 1+10 digit dialing was required across the 630/331 region. Source: Wikipedia
The 630/331 overlay complex serves Chicago's western suburbs across DuPage, Kane, and parts of surrounding counties, covering communities including Aurora, Naperville, Wheaton, and Downers Grove. NANPA projects no exhaustion of the complex until Q2 2066. Source: NANPA
Regulatory History
Area code 331 was governed by three key regulatory actions: Illinois Commerce Commission Docket 98-0847 approved the overlay in 1999, NANPA PL-195 formally assigned the code, and FCC 00-429 established the numbering framework for its implementation. Source: NANPA PL-195
| Year | Event | Docket | Regulatory Action & Impact |
|---|---|---|---|
| 1999 | Overlay Approval | ICC 98-0847 | Illinois Commerce Commission approved all-services overlay for 630 (331), along with overlays for 312/773 (872) and 708 (464). Mandated number pooling implementation before new code activation. Source: ICC |
| 1999 | Code Assignment | PL-195 | NANPA assigned NPA 331 to overlay NPA 630 based on ICC Docket 98-0847 approval. Start dates to be determined based on number pooling activation and its impact on extending existing area code life. Source: NANPA PL-195 |
| 2000 | Numbering Framework | FCC 00-429 | FCC established 60% utilization threshold for carriers, mandatory 10-digit dialing in overlay regions, and comprehensive audit program. This order governed the numbering framework under which 331 was implemented. Source: FCC 00-429 |
County Coverage
Population-weighted county share based on aggregated ZCTA population (100% = total mapped population for area code 331).
| County | Population | Share |
|---|---|---|
| DuPage, IL | 1,049,109 | 49.3% |
| Kane, IL | 437,352 | 20.6% |
| Will, IL | 276,330 | 13.0% |
| Kendall, IL | 232,928 | 11.0% |
| Cook, IL | 129,147 | 6.1% |
| DeKalb, IL | 1,588 | 0.1% |
Area Code 331 Family Tree
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Area code 331 was originally assigned in 07-Oct-2007 to serve IL.
Call Context
This area code serves a major metropolitan region with high population density in IL. The primary city is Aurora.
Nearby Area Codes
Area codes geographically closest to 331.
Other Area Codes in Illinois
Area codes that also serve Illinois, linked for easy comparison.