8 point grid

Pick one number, make everything a multiple of it, and most spacing arguments stop. The number is usually eight: paddings, gaps, icon sizes, control heights, and block dimensions are all 8, 16, 24, 32 and up, and anything that lands on 13 or 27 is wrong by definition rather than by opinion. The rule is blunt on purpose, because its value is that it can be checked without taste.

Eight earns the job by dividing well. Screens ship at a range of densities, and a layout written in points or density-independent pixels gets multiplied by 1.5, 2, or 3 on the way to physical pixels: eight survives all of them as a whole number, where five becomes 7.5 at 1.5x and lands on a half pixel that the renderer has to guess at. Eight also halves twice before it runs out, which is where the companion 4 point step comes from. Most systems are honest about being on a 4 point grid with a strong preference for even multiples, and Carbon calls its 8px base the mini unit for exactly this reason.

The convention and the shortlist built on it are different things, which is the line between this term and spacing scale: the 8 point grid says what every value must be a multiple of, and a spacing scale says which of those multiples a team is actually allowed to spend. Adopting the unit without publishing the ladder still leaves 8, 16, 24, 32, 40, 48, 56 and 64 available for the same job, which is how a system on a strict grid ends up looking inconsistent anyway.

Two things sit outside the grid and pretending otherwise causes damage. Type is one: font sizes come from a type scale and line boxes land where the metrics put them, so forcing a heading to a multiple of eight is a way of making text worse to protect an invisible rule. Optical adjustment is the other: a circular glyph, a piece of punctuation, or a border that reads as heavy needs a pixel or two that the grid does not offer. The useful discipline is to snap boxes and spacing hard, let type and optics be argued separately, and keep the exceptions few enough to name.