How do I do math calculations without leaving insert mode?

Explanation

The expression register (<C-r>=) lets you evaluate Vimscript expressions on the fly and insert the result directly into your text. Press <C-r>= in insert mode, type any expression, and hit <CR> — the result appears at your cursor position.

How it works

• While in insert mode, press <C-r>= to open the expression prompt at the bottom of the screen
• Type any valid Vimscript expression (arithmetic, string manipulation, function calls)
• Press <CR> to insert the evaluated result at the cursor

Example

You're writing a comment and need to calculate a value:

// Timeout is set to | milliseconds

With the cursor at | in insert mode, press <C-r>= then type 60 * 60 * 1000<CR>. The result is inserted:

// Timeout is set to 3600000 milliseconds

You can also use it for string operations:

<C-r>=toupper('hello')<CR>

Inserts HELLO at the cursor.

Tips

• Use <C-r>=system('date')<CR> to insert the output of a shell command
• Access Vim variables: <C-r>=&shiftwidth<CR> inserts the current shiftwidth value
• Use <C-r>=line('.')<CR> to insert the current line number
• In normal mode, "=2+2<CR>p uses the expression register to paste the result
• Combine with printf() for formatted output: <C-r>=printf('0x%04X', 255)<CR> inserts 0x00FF
• The expression register is also available in command-line mode with <C-r>=