string-width

string-width is a built-in function in `C source code'.

(string-width STRING)

Return width of STRING when displayed in the current buffer.
Width is measured by how many columns it occupies on the screen.
When calculating width of a multibyte character in STRING,
only the base leading-code is considered; the validity of
the following bytes is not checked. Tabs in STRING are always
taken to occupy `tab-width' columns.