How long can a YouTube title be?
YouTube allows up to 100 characters. That doesn’t mean people see 100: most placements show two lines and cut the rest with “…”. Where the cut falls depends on the screen and the letters you use — W takes far more room than i.
That’s why this tool draws the title at each placement’s real width instead of counting characters.
Put the hook in the first 40 characters
The sidebar and some phone layouts show the least. If the subject and the reason to click are in the first 40 or so characters, the title still works when the end is cut.
Title and thumbnail work as a pair
The thumbnail is seen first, the title second. They shouldn’t repeat each other: let the image show the moment and the title add what the image can’t say.