How do I make Vim automatically reformat paragraphs as I type so lines stay within the textwidth?

Explanation

Vim's formatoptions setting controls how automatic text formatting works. Adding the a flag enables automatic paragraph reformatting: as you insert or delete text, Vim continuously rewraps the paragraph so no line exceeds textwidth. This turns Vim into a word-processor-like editor for prose and documentation writing.

How it works

• :set textwidth=80 — sets the target line width (required for a to have any effect)
• :set formatoptions+=a — enables auto-format mode; paragraphs reformat on every insert
• A paragraph is delimited by blank lines; only the current paragraph reflows as you type

The a flag is most useful combined with other formatoptions flags:

set textwidth=80
set formatoptions+=a  " auto-reformat
set formatoptions+=w  " trailing whitespace = paragraph continuation

Example

With textwidth=40 and formatoptions+=a, typing in a paragraph:

The quick brown fox jumps over the lazy
dog and then runs back again

After inserting "very " before "quick":

The very quick brown fox jumps over
the lazy dog and then runs back again

Vim automatically rewrapped the line to stay within 40 characters.

Tips

• Disable temporarily with :set formatoptions-=a (useful when pasting)
• The q flag in formatoptions lets gq reformat manually even without a
• For code, add a only via a filetype-specific autocmd (e.g., for markdown/text files)
• 'formatexpr' can be set to a custom function for language-aware reformatting
• Check current flags with :set formatoptions?