Area Code 331

About Area Code 331

Area code 331 is a geographic telephone area code assigned to Naperville, Illinois. It operates in the Central time zone and was first introduced in 2007. Overlay for 630. Western suburbs of Chicago.

Within Illinois, area code 331 shares numbering capacity with 12 other area codes. The state's 12,549,689-person population (2023 Census Bureau estimate) is served by a combined 13 active codes, with 331 primarily responsible for the Naperville region. Illinois's state capital is Springfield, and its largest city is Chicago.

When you receive a call from a 331 number on your Caller ID, the call originated from a phone number assigned to the 331 area code — but be aware that Local Number Portability (in effect since 2003) means the caller could be physically located anywhere in the US or Canada. Area code 331 only confirms where the number was originally assigned.

Where is the 331 area code located?

Area code 331 is assigned to Naperville, Illinois, in the Central time zone. The state of Illinois uses 13 active area codes, and 331 is one of them. Calls placed from the 331 area code originate from numbers assigned by NANPA — the North American Numbering Plan Administrator — and the 331 prefix appears on Caller ID for both landline and wireless calls placed from this region.

The Central time zone observes Daylight Saving Time, shifting from UTC-6 to UTC-5 in summer. The local time shown above is always current, so you can quickly check whether it's a reasonable hour to call back.

What cities and counties use the 331 area code?

The 331 area code serves 11 notable cities, listed below in order of population. The largest city in the 331 territory is Naperville (152,189 residents in DuPage County).

• Naperville, IL — 152,189 residents (3.9% of 331 numbers)
• Chicago, IL — 2,696,555 residents (68.7% of 331 numbers)
• Aurora, IL — 180,542 residents (4.6% of 331 numbers)
• Joliet, IL — 150,363 residents (3.8% of 331 numbers)
• Rockford, IL — 146,713 residents (3.7% of 331 numbers)
• Springfield, IL — 113,393 residents (2.9% of 331 numbers)
• Elgin, IL — 111,287 residents (2.8% of 331 numbers)
• Peoria, IL — 113,150 residents (2.9% of 331 numbers)
• Champaign, IL — 88,029 residents (2.2% of 331 numbers)
• Waukegan, IL — 89,321 residents (2.3% of 331 numbers)

Phone number format in area code 331

All phone numbers within area code 331 follow the standard North American 10-digit format:(331) NXX-XXXX. The NXX is the central office prefix (also called an "exchange") and XXXX is the subscriber line number. For long-distance calls within the NANP, dial 1 + 331 + NXX + XXXX. From outside North America, dial your country's international access code, then 1 (the NANP country code), then the 10-digit number.

If you need to verify whether a specific 331 number is wireless, landline, or VoIP, you can use our reverse phone lookup tool. Be aware that number portability means the area code no longer guarantees the caller's physical location — only where the number was first assigned.

History of area code 331

Overlay for 630. Western suburbs of Chicago.

The original 86 area codes were assigned by AT&T and Bell Labs in 1947 as part of the North American Numbering Plan. At the time, rotary-dial phones meant codes with lower digits (like 212 for NYC, 213 for LA, 312 for Chicago) were given to the largest cities because they took less time to dial. Today's area code assignments still reflect those early choices, even though touch-tone and cell phones made the original reason obsolete.

Is the 331 area code safe to answer?

Area code 331 itself is a normal geographic code with no elevated spam risk. That said, scammers can spoof any area code — including 331 — so if you don't recognize the full number, let it go to voicemail and verify the caller's identity before calling back.