Slope Ratio (H:V) to Percent Grade

Earthwork drawings almost never label a cut face in percent. They call it 3:1 or 2:1, and the convention in North American civil practice is horizontal first — three metres out for every one up. Read it backwards and a 4:1 lawn slope becomes a 400 % cliff, which is how the argument usually starts on site. The conversion is a plain reciprocal: G = 100/n, so 2:1 is 50 %, 3:1 is 33.3 %, and 4:1 is 25 %. These are not arbitrary numbers — 3:1 is the flattest slope a ride-on mower handles safely, 2:1 is about the steepest that will hold topsoil and vegetation without armouring, and anything steeper generally wants riprap, geogrid or a wall.

A worked example: a detention pond is specified with 4:1 side slopes and the pond is 1.8 m deep, so each bank eats 4 × 1.8 = 7.2 m of horizontal room and the equivalent grade is 100/4 = 25 %. Note that geotechnical reports sometimes quote the same slope V:H instead — always check which number carries the "1".