The official Colorado abbreviation for mail is CO, written as a pair of capitals and no dot. Place it after the city in any address, as in Boulder, CO 80302, and in the state field of online forms and shipping labels.
The older written form is Colo., with a period. It still appears in news datelines, some reference lists and older documents. In ordinary sentences, the full name Colorado usually reads best.
Quick facts
Postal code
CO (capitals, no period)Traditional
Colo.AP style
Colo. in datelines and lists; spell out in story textCapital city
DenverZIP codes start with
80 or 81Used in
Mailing addresses, forms, maps, data tablesNot to be confused with
CT (Connecticut), CA (California)
Colorado abbreviation chart
| Form | What it is | Use it for |
|---|---|---|
| CO | USPS two-letter code | Envelopes, packages, address forms, spreadsheets |
| Colo. | Traditional abbreviation | Datelines, footnotes, lists, older style guides |
| Colorado | Full name | Sentences, letters, essays, most published writing |
A quick rule: if a machine or mail carrier reads it, use CO. If a person reads it in a sentence, write Colorado.
Writing CO on mail and forms
The Postal Service introduced two-letter state codes so addresses would fit cleanly and sort easily. For Colorado, that code is CO. Keep it on one line with the city and ZIP code: city, comma, CO, then the five digits.
A return address:
J. Ortiz
410 Pine St.
Pueblo, CO 81003
Sorting machines look for the state code in exactly that spot, just before the ZIP.
The same code works on job applications, tax forms, online checkout pages and driver's license records. Colorado ZIP codes begin with 80 or 81, which is a quick way to double check a Colorado address.
When to use Colo.
Before the two-letter codes, writers used longer shortened forms like Colo. They still have a place:
- News datelines: many American papers write "ASPEN, Colo." at the start of a story.
- Lists and captions: where the full name would crowd the line.
- Older citations and records: historical books and court papers often use Colo.
Avoid Col., which many readers will take as the short form for colonel, and avoid Cola, which is not a standard form at all.
Do not mix up CO with CT or CA
Several state codes start with C, and they are easy to swap on a busy form.
- CO: Colorado
- CT: Connecticut (see the Connecticut abbreviation)
- CA: California (see the California abbreviation)
One slip of a letter can route a parcel to the other side of the country, so it helps to read the state field one more time before you click submit. The letters CO also have other meanings, such as carbon monoxide and company. Our CO meanings page covers those.
Using the abbreviation for Colorado in sentences
In everyday writing, keep the state name whole. A few examples show the difference:
- Good: "We moved to Colorado for the mountains."
- Good: "Denver, Colorado, is about a mile above sea level."
- Avoid in prose: "We moved to CO last spring."
- Fine in a text message: "Back in CO tonight."
Notice the Denver line: when a city and state sit mid-sentence, the state gets a comma on both sides.
Quick facts and tips
Colorado is often called the Centennial State, because it joined the Union in 1876, a hundred years after the Declaration of Independence. Most of the state runs on Mountain Time.
For tips on every state code, including look-alikes such as MA, ME, MD, MI, MN, MO, MS and MT, our state abbreviations hub covers all fifty.
Questions people ask
Which two letters stand for Colorado?
CO. Those are the letters the Postal Service assigns, always capitalized and never followed by a dot.
Is Colorado abbreviated CO or CA?
Colorado is CO. CA is California, and mixing them up is one of the most common address errors in the West.
What is the AP style abbreviation for Colorado?
Reporters use Colo. at the top of a dateline and in tight lists. Once the story itself begins, they write Colorado.
Does CO need a period?
No. The postal code is always plain letters. Colo., the traditional form, is the one that carries a dot.
Can I write Colo. on an envelope?
Your letter would probably get there anyway, yet CO is what the Postal Service prefers. It is shorter and easier for sorting machines to read.