Illinois · Central Time
331 Area Code
Area code 331 is in Illinois, serving western Chicago suburbs in DuPage, Kane, Kendall and northern Will counties. It is in the Central Time zone. It shares its territory with overlay code 630, so 10-digit dialing is required.
Last updated October 1, 2026 · Source: NANPA
Where is the 331 area code?
331 is a Illinois area code covering western Chicago suburbs in DuPage, Kane, Kendall and northern Will counties. In the wider directory it sits in the Midwest region.
Cities in the 331 area code
- Aurora
- Naperville
- Bolingbrook
- Wheaton
- Downers Grove
- Elmhurst
- Lombard
- Carol Stream
- St. Charles
- Glendale Heights
- Oswego
- Batavia
- Woodridge
- Addison
Counties served by 331
DuPage County, Kane County, Kendall County, Will County and Cook County.
331 overlay codes and dialing
The 331 territory is covered by an overlay of 2 area codes: 331, 630. All of them serve the same geographic area, and the original code is 630. Because more than one code covers the same ground, 10-digit dialing is mandatory — you must dial the area code for every local call. New phone lines may be assigned any of these codes.
History of the 331 area code
331 went into service on October 7, 2007 as an overlay of 630, relieving number exhaustion in the Aurora area. Rather than splitting the region, the overlay lets existing customers keep their numbers while new lines draw from the added code. Learn more in our overlay guide.
Is 331 a scam or spam risk?
331 is a legitimate geographic area code, not a premium or toll line, so a call from it costs you nothing extra. That said, scammers frequently spoof real local area codes to look familiar on caller ID. A genuine 331 number on your screen is no guarantee the caller is in Aurora, IL or trustworthy. See spam & scam-risk area codes for how spoofing works and how to respond.
Other Illinois area codes
Explore more
See every code in Illinois, compare codes across the Midwest region, or view all Central Time codes. You can also browse the 3XX block or the complete numeric list of US area codes.
New to how this all works? Start with how area codes work, then read up on overlays, 10-digit dialing, and spam & scam-risk codes. To check another number, use the area code lookup tool.
Quick answers
331 area code FAQ
Where is the 331 area code?
The 331 area code is in Illinois, covering western Chicago suburbs in DuPage, Kane, Kendall and northern Will counties, in the Midwest region.
What cities are in the 331 area code?
Cities in the 331 area code include Aurora, Naperville, Bolingbrook, Wheaton, Downers Grove, Elmhurst, Lombard, Carol Stream, St. Charles and Glendale Heights.
What time zone is 331 in?
331 is in the Central Time zone.
What is the overlay for the 331 area code?
331 shares its territory with 630. Both codes (331, 630) serve the same area, so 10-digit dialing is required for every call.
When did the 331 area code start?
331 went into service on October 7, 2007 as an overlay of 630.
Do I have to dial 10 digits for 331?
Yes. Because 331 is an overlay area, you must dial the full 10-digit number (area code + 7 digits) for every call.
Is 331 a toll-free or premium code?
No. 331 is a standard geographic area code, so calling it costs the same as any local or long-distance call on your plan. Toll-free calls use 800, 888, 877, 866, 855, 844 or 833.