Stair Comfort Rule (2R + T)

Twice the riser plus one tread should land near 25 inches. The rule dates to François Blondel, who in the 1670s reasoned that a person climbing lifts a foot twice as far as they advance it, so the two dimensions ought to trade off at a fixed rate. Metric practice uses 630 mm, close enough to 25 in that the two conventions rarely disagree in the field. A 7 in riser wants an 11 in tread; a 175 mm riser wants a 280 mm going.

Treat it as a comfort check rather than a law. Codes govern the riser and the tread separately, with minimum tread depths around 10 in or 250 mm, and a stair can satisfy every code number while still feeling wrong underfoot. If your 2R + T lands under about 24 in the stair feels cramped and mincing; over about 26 in and you find yourself stretching for each step. Both are the sort of thing you notice on the third trip carrying drywall.