The grade of highway represents the inclination of highway surface to horizontal plane. You may also use the words slope or gradient instead of grade.

The gradient is evaluated as tangent of the angle of inclination or as a ratio of y to x, where y represents the "rise" or vertical distance and x represents the "run" or horizontal distance.

You may use the following notation for slope or grade and the following equation to evaluate the slope, such that:

`m = tan alpha = (Delta y)/(Delta x) => alpha = tan^(-1)((Delta y)/(Delta x))`

If you need percentages to express the grade, you may multiply by 100 the ratio `(Delta y)/(Delta x).`

**Hence, by definition, the grade of highway expresses the inclination of highway surface to horizontal plane and it is evaluated using the formula **`tan alpha = (Delta y)/(Delta x).`