How to Find the Percentage of a Grade

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Calculation of the grade of a slope in a percentage form is needed in many engineering and drainage applications. For example, warning signs are used in hilly areas where downward grades exceed 4 percent for an extended distance. The warning signs tell motorists and truck drivers to slow down or use a low gear to control speed. The grade of slope is also needed when perfroming grading work or when laying pipe for drainage. Minimum slope grades of 0.5% to 1.0% are needed to promote positive drainage. Finding the percentage of grade is actually an easy calculation.

Calculating Percent Grade from Coordinates

Using coordinates is often preferred by surveyors and those who lay out construction grading for a project. Coordinates are typically provided in a state plane system using northing and easting values with an elevation value. To show an example of calculating percent grade from coordinates, simple x,y, and z values are used here:

Point 1: 200, 50, 87

Point 2: 295, 75, 90

  • First the horizontal distance between the two points is determined using the x and y values of the coordinates.

The following formula is used, based on the point coordinates given.

SQRT((295-200)2+(75-50)2)= 98.23

  • Next, determine the elevation difference from the coordinates. It is the difference in the z values so 90-87= 3
  • The grade is calculated by dividing the elevation difference by the horizontal distance.

grade = 3/98.23 = .0305

  • To get the grade in percent form multiply the grade by 100.

.0305 x 100 = 3.05 percent grade

Determining Percent Grade Slope wth a Level

A level is a valuable tool in determining percent grade of a slope. Here is the way it works.

  • Place either a standard level or a sting line level on a slope.
  • Get the level exactly horizontal (level)
  • Measure the distance from each end of the bottom of the level to the ground
  • Measure the length of the level
  • The percent grade is the difference in distance from the bottom of each end of the level divided by the length of the level multiplied by 100..

A string line level is used to measure a percent slope grade over a longer distance. Smart Levels are also available which have a digital display which shows the percent slope grade when the smart level is placed on the ground. This provides a direct reading with no calculation of grade required.

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