Bootstrap is the more common of the two in our index: it runs on 9,657 websites against 521 for shadcn/ui, and 8 sites use both. Counts come from real scans, not vendor claims.
Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.
shadcn/ui is a component system built with Radix UI and Tailwind CSS.
8 sites run both Bootstrap and shadcn/ui. That is 2% of the sites using shadcn/ui, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.