Windows
Hold Alt and type 236 on the numeric keypad.
Math
Copy the character, or use the shortcut that matches your keyboard and app.
Hold Alt and type 236 on the numeric keypad.
Option + 5
Type 221E then Alt + X
Insert → Special characters → infinity
Click the keycap, then paste with Ctrl+V or Cmd+V.
Num Lock on, hold Alt, type 236, release Alt.
Use the Unicode, HTML, CSS, or JS value below in documents and stylesheets.
U+221E∞∞\221E\u{221E}Alt + 236