Design AtlasA solid icon fills its whole shape with color and has no outline. Because that fill holds up at small sizes and from a distance without blurring, it's the go-to for anywhere an icon needs to grab attention firmly — a selected tab in a bottom nav bar, a notification badge, an icon sitting inside a button.
It's often paired with a line icon: an unselected tab uses the line version, the selected one switches to solid, and that alone signals "you are here" without needing a separate color change. The catch is that if the line and solid versions don't share the same outer silhouette, the icon visibly shifts shape when it switches — so both versions need to be drawn against the same keyline.
Because the fill is large, small details — holes, gaps — are the first thing to disappear. Fill in a magnifying glass's lens as a solid disc, for instance, and it stops reading as a lens at all; it needs to stay a ring (a donut shape) to keep its meaning.
When to use
Use it wherever an icon needs to grab attention firmly — selected tab states, notification badges, icons inside buttons. When pairing it with a line version, draw both against the same keyline so the silhouette doesn’t shift on toggle.