The fundamental rule for capitalization after a colon is: capitalize the first word after a colon if what follows is a complete, independent sentence. Lowercase it if what follows is a phrase, list, or continuation of the sentence before the colon.

Complete sentence after colon — capitalize:

• ▸'She had one rule: Never arrive late to a meeting.'
• ▸'The answer was obvious: The project had run out of funding.'
• ▸'He made his position clear: This was not negotiable.'

Phrase or list after colon — lowercase:

• ▸'She needed three things: a pen, a notebook, and silence.'
• ▸'The package includes: installation, configuration, and support.'
• ▸'He had two concerns: timing and budget.'

The logic is straightforward. A complete sentence after a colon can stand on its own — it has a subject and a verb. A phrase or list cannot stand alone. When the material after the colon could function as its own sentence, capitalize it. When it is a continuation or elaboration of the sentence before the colon, lowercase it.

The rule above represents the mainstream consensus — but the two most widely used editorial style guides handle one edge case differently.

AP Style (Associated Press): Capitalize the first word after a colon only when it is followed by a complete sentence — and the sentence is a direct, formal statement or quote. AP applies this fairly narrowly and prefers lowercase in most cases where the material after the colon is clearly explanatory.

Chicago Manual of Style: More permissive about capitalizing after a colon. Chicago says to capitalize when a complete sentence follows the colon, with no additional conditions. If the independent clause after the colon begins with a proper noun, it is always capitalized under both guides.

MLA (Modern Language Association): Follows the same general principle — capitalize after a colon when a full independent clause follows, lowercase otherwise.

For practical purposes: if you follow AP, be conservative and prefer lowercase when in doubt. If you follow Chicago, capitalize whenever a full sentence follows. If you follow no particular guide, use the core rule — complete sentence after the colon means capitalize.

When a colon introduces two or more complete sentences, capitalize the first word of each sentence. This applies under both AP and Chicago:

• ▸'The review found three problems: The data was incomplete. The analysis was flawed. The conclusions were unsupported.'
• ▸'There are two reasons this matters: First, the timeline is fixed. Second, there is no budget for revisions.'

In these cases, each sentence after the colon functions as a separate independent clause, so each gets a capital letter at its start.

Titles and headings follow different rules from running prose. In a title that uses a colon — a book subtitle, article headline, or blog post title — the word immediately after the colon is always capitalized, regardless of whether it begins a complete sentence.

• ▸'Naming Conventions: A Complete Developer's Guide' — 'A' is capitalized even though it begins a phrase, not a sentence
• ▸'Title Case Rules Explained: Which Words Get Capitalized?' — 'Which' is capitalized
• ▸'Text Case for SEO: URL Slugs, Title Tags, and Headings' — 'URL' is capitalized

This applies consistently in title case and sentence case titles alike. The colon in a title acts as a hard break — whatever follows starts fresh with a capital letter.

Two specific colon uses have their own conventions that differ from the general rule.

Business letter salutations: 'Dear Ms. Patel:' or 'To Whom It May Concern:' — the word after the colon is on a new line and begins the letter body. Capitalize the first word of the body regardless of what it is. This is not the same as mid-sentence colon usage.

Labels and prefixes in documents: 'Note: All submissions must be in PDF format.' — 'Note' is a label, not a sentence. The word after the colon here ('All') is capitalized because it begins a complete sentence, following the standard rule.

Warning and notice labels — 'Warning:', 'Important:', 'Caution:' — typically written in all caps or bold, and what follows is capitalized because it is usually a complete instruction or statement.

Use this as a checklist when you are unsure:

• ▸Complete sentence follows → capitalize: 'One fact was clear: The deadline could not move.'
• ▸Phrase or list follows → lowercase: 'She brought three things: coffee, notes, and patience.'
• ▸Title or heading colon → always capitalize what follows
• ▸Multiple sentences follow → capitalize each one
• ▸Proper noun follows → always capitalize (it would be capitalized anywhere)
• ▸Quotation follows → capitalize if the quote is a complete sentence