The slope of a line is the angle at which it rises or falls, and a ratio is a comparison of values. Based on this, the slope can be expressed as a ratio. In the case of a line's slope, the ratio is the "rise" of the line expressed in relation to the "run" of the line. You may have to work with slope ratios in an algebra class in high school or college. You also may need to have an understanding of this type of calculation if you work in a career that involves math.
Locate two points on a graph. These points should each be expressed by a set of coordinates. The first coordinate is the "x" coordinate and the second coordinate is the "y" coordinate. For example, if you have (2,3), then there is a point at 2 on the x axis and 3 on the y axis.
Subtract the second y coordinate from the first one. For instance, if you have (4,6) and (3,2), then you would subtract 2 from 6 to get 4. This is the rise.
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Subtract the second x coordinate from the first one. In this example, you would subtract 3 from 4 to get 1. This is the run.
Express rise to run as a ratio. In this example, you would write 4:1. This means that for every 4 units that the line rises, it runs 1 unit. Another way of stating this is as the fraction 4/1, which can be simplified to 4. This means that the slope of the line is 4 or 4:1.