Colorado · Mountain Time
303 Area Code
Area code 303 is in Colorado, serving Denver, Boulder and the surrounding Front Range metro area in north-central Colorado. It is in the Mountain Time zone. It shares its territory with overlay codes 720 and 983, so 10-digit dialing is required.
Last updated October 1, 2026 · Source: NANPA
Where is the 303 area code?
303 is a Colorado area code covering Denver, Boulder and the surrounding Front Range metro area in north-central Colorado. In the wider directory it sits in the West region.
Cities in the 303 area code
- Denver
- Aurora
- Lakewood
- Thornton
- Arvada
- Westminster
- Centennial
- Boulder
- Highlands Ranch
- Littleton
- Broomfield
- Longmont
- Castle Rock
- Parker
- Commerce City
- Brighton
Counties served by 303
Denver County, Arapahoe County, Jefferson County, Adams County, Douglas County, Boulder County and Broomfield County.
303 overlay codes and dialing
The 303 territory is covered by an overlay of 3 area codes: 303, 720, 983. All of them serve the same geographic area, and the original code is 303. 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 303 area code
303 is one of the original area codes assigned in 1947, when AT&T and Bell Laboratories created the North American Numbering Plan and divided the country into the first 86 numbering plan areas. It has served the Denver area ever since, with 720 and 983 later added as overlays to supply more numbers.
Is 303 a scam or spam risk?
303 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 303 number on your screen is no guarantee the caller is in Denver, CO or trustworthy. See spam & scam-risk area codes for how spoofing works and how to respond.
Other Colorado area codes
Explore more
See every code in Colorado, compare codes across the West region, or view all Mountain 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
303 area code FAQ
Where is the 303 area code?
The 303 area code is in Colorado, covering Denver, Boulder and the surrounding Front Range metro area in north-central Colorado, in the West region.
What cities are in the 303 area code?
Cities in the 303 area code include Denver, Aurora, Lakewood, Thornton, Arvada, Westminster, Centennial, Boulder, Highlands Ranch and Littleton.
What time zone is 303 in?
303 is in the Mountain Time zone.
What is the overlay for the 303 area code?
303 shares its territory with 720 and 983. All 3 codes (303, 720, 983) serve the same area, so 10-digit dialing is required for every call.
When did the 303 area code start?
303 went into service on January 1, 1947 as one of the original 1947 North American area codes.
Do I have to dial 10 digits for 303?
Yes. Because 303 is an overlay area, you must dial the full 10-digit number (area code + 7 digits) for every call.
Is 303 a toll-free or premium code?
No. 303 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.