Knitting
Too coarse and you break needles; too fine and the fabric goes transparent and slack.
— Ne
Gauge squared divided by the structure constant
This is a heuristic, not a law. Rib and interlock tolerate coarser yarn at the same gauge because each needle bed carries fewer loops per unit width.
Machine Gauge to Optimum Yarn Count Matcher — free, with the formula and a worked example, at Textile School.