Dynamic Type

Dynamic Type is the arrangement where the reader sets the text size once, in system settings, and every app that plays along redraws itself at that size. The app never asks for a number. It asks for a role (body, headline, caption, footnote) and the platform hands back whatever that role measures at the setting in force, which on iOS runs from twelve steps below the default up to the accessibility sizes near the top of the range. Android says the same thing with the sp unit: a size written in sp is multiplied by the system font scale, while the same number written in dp is not. On the web the equivalent is honouring the browser’s own font size, which is what setting type in rem rather than px buys.

The split with point size is worth saying plainly. A point size is what the designer specifies. Dynamic Type is what the reader chooses. A layout that only holds together at the designer’s number is not a layout with a rendering bug, it is a layout that has quietly made the reader’s setting unavailable, and for someone who needs the largest steps that is the whole interface gone.

What breaks is almost never the type. It is the box around it: a row given a fixed height, a label set to one line, a title and a timestamp sharing a line that only just fits at the default. The rule that keeps a screen honest is that text may take whatever room it needs and the layout reflows around it, so a row that no longer fits becomes a stack rather than a truncation. The same goes for the space the letters sit in: paddings, tap targets and gaps written against the type scale rather than against fixed pixels, which is the sibling requirement text spacing makes.

A type scale is what makes this tractable. Roles that are already defined as steps of one ratio can all be multiplied together and keep their relationships, so the heading stays a heading at every setting instead of colliding with the body. And the payoff is readability rather than novelty: most people who turn the setting up are not testing an edge case, they are reading.