Downtown Chicago answers to 312
Area code 312 covers central Chicago — the Loop, the Near North Side, the Near West Side and the downtown ZIP codes 60601 through 60607. It has done so since 1947, when Illinois entered the North American Numbering Plan with just two codes. This guide maps every code in the Chicago region and gives the exact dialling strings that reach a 312 number from a US mobile, from abroad and from a VoIP line.
| Area code | Where it covers | How it came about | How to dial it |
|---|---|---|---|
| 312 | The Loop, Near North Side, Near West Side; downtown ZIPs 60601–60607 | Original 1947 code | 312 + 7 digits (10 digits total) |
| 773 | Chicago outside the central 312 zone | Split from 312 in 1996 | 773 + 7 digits (10 digits total) |
| 630 / 331 | DuPage County suburbs | Overlay pair | 10 digits only — 7-digit dialling ended with the overlay |
| 847 / 224 | North and northwest suburbs | Overlay pair | 10 digits only — 7-digit dialling ended with the overlay |
| 815 / 779 | Will, Kankakee and Grundy counties | Overlay pair | 10 digits only — 7-digit dialling ended with the overlay |
| 618 | The rest of Illinois beyond Chicago | Original 1947 code | 618 + 7 digits (10 digits total) |
Area code 312 — and only 312 — covers the central city.
Illinois entered the North American Numbering Plan in 1947 with just two area codes: 312 for Chicago and 618 for everything else. Both remain in service, which makes 312 one of the plan's original codes.
In 1996 the 773 area code split away from 312. The split left 312 on top of central Chicago: the Loop, the Near North Side, the Near West Side and the downtown ZIP codes 60601, 60602, 60603, 60604, 60605, 60606 and 60607.
Everything in Chicago outside that central zone moved to 773. So a downtown address takes 312, a city address beyond the core takes 773, and the suburbs use their own paired codes.
The suburbs pair every area code with an overlay twin, and overlays changed local dialling for good.
Three overlay pairs serve the Chicago suburbs. In DuPage County, 331 overlays 630. In the north and northwest suburbs, 224 overlays 847. In Will, Kankakee and Grundy counties, 779 overlays 815.
An overlay means two codes serve the same streets. The moment one goes live, seven-digit local dialling stops in that region: every local call must be dialled as ten digits, area code included.
The practical rule for a caller: downtown Chicago is 312, the rest of the city is 773, and a suburban number can carry either code of its local pair — so never guess the area code from the town name alone.
The number itself is always ten digits; what you add before them depends on where you dial from.
From a US mobile, dial the ten digits straight: 312 plus the seven-digit subscriber number. Do not add 1 — a mobile routes on the full national number, and the 1 is the long-distance prefix used from landlines.
From a US landline at long distance, dial 1, then 312, then the seven-digit number. Inside an overlay region, remember that seven-digit local dialling no longer exists: the call needs all ten digits even if the number is across the street.
From outside the United States, dial your exit code, then country code 1, then 312 and the seven-digit number — from the UK the full string is 00 1 312 followed by the seven digits. On a VoIP line, which is not tied to a physical place, always dial the full ten-digit national number, or the international form beginning with country code 1 if your provider asks for it.
Two dates anchor the public history of 312: its birth in the 1947 plan and the 1996 split that created 773.
Each figure on this page is drawn from the plan's own rules — code boundaries, dialling strings and the anatomy of a ten-digit number.
The reference list collects the public numbering-plan rules and dialling conventions cited across this page.