Density should match the user's task — consumer apps breathe, pro tools pack.
Pro tools: 8–12px gaps, 12–13px body, ≥4.5:1 contrast. Add a 'comfortable' toggle.
A trading terminal, Linear, or a spreadsheet deliberately packs rows tight: the expert wants more on screen and scans fast, so generous padding just means more scrolling and less context. A meditation app or a marketing page does the opposite, giving each element room so a first-time visitor feels calm and unhurried. Same components, opposite density, because the task and the user are different. Matching density to context is what makes each one feel right.
Density is a spectrum set by the task, so there is no single right value to copy between products. Accessibility also puts a floor under it: even the densest pro tool still needs adequate tap targets, legible type, and 4.5:1 contrast, so “pack it tighter” stops where usability breaks.
A trading dashboard is used for hours. The team debates 'breathable' vs 'dense'.