Calculating the Slope Percentage Convert the rise and run to the same units and then divide the rise by the run. Multiply this number by 100 and you have the percentage slope. For instance, 3″ rise divided by 36″ run = . 083 x 100 = an 8.3% slope.Click to see full answer. Hereof, how do you calculate fall ratio? FALL = GRADIENT X DISTANCE For example, calculate the fall in a 50 metre section of foul water pipework if the gradient is to be 1 in 80. A gradient of 1 in 80 is converted to a number instead of a ratio.Furthermore, what is the minimum slope for water flow? Generally, minimum slope for water to run off is 1% (1/8″ per 1′). However, minimum slope for a flat roof by building code here is 2%. (1/4″ per 1′). You have more than enough slope for your roof, however you will likely need some sort of membrane roofing since it is very low slope. Keeping this in view, how is the slope of a line defined as a ratio? The steepness of a hill is called a slope. The same goes for the steepness of a line. The slope is defined as the ratio of the vertical change between two points, the rise, to the horizontal change between the same two points, the run.What is a 1 in 20 slope?as a ratio of one part rise to so many parts run. For example, a slope that has a rise of 5 feet for every 100 feet of run would have a slope ratio of 1 in 20. This means that for every 4 units (feet or metres) of horizontal distance there is a 1-unit (foot or metre) vertical change either up or down.”