The micro sign.
More from Tech Symbols.
The quickest way is to copy Āµ from this page and paste it anywhere. You can also insert it with its Unicode, HTML, or alt code listed above.
Āµ is U+00B5 (decimal 181).
Use µ in HTML to render Āµ on a web page.
On Windows you can type it with Alt+181 (hold Alt and enter the number on the numeric keypad).
Yes ā copying is completely free and unlimited, and works right in your browser with no sign-up.