When I added an "A4" preset to a browser-based collage tool, I thought the hard part was the layout maths. It wasn't. The hard part was that "A4" on a canvas is a shape, not a size — and if you don't say that out loud, your users find out at the print shop.
Source: [Dev.to](https://dev.to/bellsal_b44bf6d/your-canvas-a4-export-is-not-a4-dpi-print-size-and-the-number-nobody-checks-5akh)