Colorado · Mountain Time
Boulder, CO Area Codes
Boulder is served by 3 area codes (303, 720, 983). Here's which is original, which are overlays, and how to pick one for a new local number.
Last updated October 3, 2026 · Source: NANPA
Area codes serving Boulder
Boulder sits in Colorado, part of the West region and the Mountain Time zone. Its numbers are spread across an overlay of 3 codes that all cover the same ground: 303, 720, 983.
The original code is 303 (in service since 1947). As the area ran low on numbers, regulators added overlay codes on top instead of splitting the city, so longtime residents kept their numbers while new lines received a newer code.
| Code | Area served | In service |
|---|---|---|
| 303 | Denver, Boulder and the surrounding Front Range metro area in north-central Colorado | January 1, 1947 |
| 720 | Denver, Boulder and the surrounding Front Range metro area in north-central Colorado | June 1, 1998 |
| 983 | Denver, Boulder and the surrounding Front Range metro area in north-central Colorado | June 17, 2022 |
Nearby cities sharing these area codes
- Denver
- Aurora
- Lakewood
- Thornton
- Arvada
- Westminster
- Centennial
- Highlands Ranch
- Littleton
- Broomfield
- Longmont
- Castle Rock
- Parker
- Commerce City
- Brighton
How to pick an area code for a new number
When you activate a new line in Boulder, the carrier assigns whichever code has numbers available — you usually can't choose. If a specific code matters to you (for example the prestige of 303), ask the carrier whether any blocks remain, or request a number transfer. All codes here dial identically and cost the same.
Dialing in Boulder
Because Boulder is an overlay area, 10-digit dialing is mandatory: always dial the area code plus the seven-digit number, even for a neighbor. From outside the US, dial +1, then the area code and number. See 10-digit dialing and overlay area codes for the background. For every code in Colorado, visit the Colorado area codes page.
Quick answers
Boulder area code FAQ
What area codes does Boulder use?
Boulder, CO is served by 3 area codes: 303, 720, 983.
Which is the original Boulder area code?
303 is the original code for Boulder, in service since 1947; the others were added later for more numbers.
What time zone is Boulder in?
Boulder is in the Mountain Time zone.
Do I need to dial the area code in Boulder?
Yes. With multiple overlapping codes, 10-digit dialing is required for every call in this area.