See your title where YouTube cuts it.

100 characters. YouTube won’t take more, and it doesn’t allow < or > in a title.

Where its card runs out of lines. Up next on desktop shows three short lines, about 55 to 65 characters; a phone’s list shows two lines, about 75 to 90. The checker measures both on your own title.

Short enough that what matters shows before the cut. Put the point in the first 50 or so characters; the rest is for search.

No. It is counted and measured in your browser. Nothing you type is sent anywhere or stored, and the page sets no cookies.