Percent Grade from Rise and Run

Grade is the trade's way of saying slope: rise over run, multiplied by a hundred so it reads as a percentage. The Romans understood the idea long before the notation existed — the aqueducts feeding the capital fell at grades near 0.02 %, and the Pont du Gard section drops about 2.5 cm per kilometre, a precision that still impresses. The trap is the denominator. Grade uses the horizontal run, not the sloped distance you would tape along the ground, and not the hypotenuse. On flat work the difference is invisible; on a 30 % haul road the sloped length is 4.4 % longer than the run, and quantities computed off the wrong one come back to bite at pay time.

A worked example: a parking lot drains 8 in over a 100 ft run. Convert first — 8 in is 0.667 ft — so G = 100 × 0.667 / 100 = 0.67 %, comfortably above the 0.5 % minimum most agencies allow for asphalt and below the 5 % that makes accessible routes non-compliant. Read backwards, the same formula sizes a ramp: to hold 5 % over a 3 ft rise you need 100 × 3 / 5 = 60 ft of run.