At least on Windows 10, functions QFontMetrics::width(), QFontMetrics::boundingRect() and QFontMetrics::tightBoundingRect() return rectangle that is too small or way too large for the font.

On Linux boundingRect() and tightBoundingRect() return a size that is smaller than the text that would be drawn.

In Qt 5.6 on Windows with native font engine, tightBoundingRect() is even worse than in Qt 5.5.1 where even height is unreasonably huge.