How to Calculate Road Gradient

Written by thomas james
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How to Calculate Road Gradient
Signs are used to warn approaching vehicles of steep gradients. (Jupiterimages/ Images)

The gradient of a road is a measure of how steep the road is. The gradient is sometimes referred to as the grade, incline, slope, rise or pitch. Automotive vehicles are rated in terms of the maximum grade they can ascend safely. Gradients can be expressed either in terms of a percentage or in terms of an angle, in degrees.

Skill level:
Moderately Easy

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Things you need

  • Meter or yard ruler
  • Spirit level
  • Plumb line
  • Calculator

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  1. 1

    Place the ruler flat on the surface of the road so that it is parallel with the direction of the slope. Place the spirit level on top of it.

  2. 2

    Raise the lower end of the ruler until the spirit level indicates that the ruler is horizontal.

  3. 3

    Hold the plumb line alongside the raised end of the ruler. Note the point on the plumb line corresponding with the height of the raised end of the ruler.

  4. 4

    Measure the length of the relevant section of the plumb line. This is the height of the horizontal ruler above the road surface. Make a note of this value.

  5. 5

    Divide the value found in Step 4 by the length of your ruler. Multiply this value by 100. This final value will be the slope expressed as a percentage.

Tips and warnings

  • In Step 3, ensure the plumb line is hanging straight down with the point of the plumb bob just touching the surface of the road. If your plumb line does not have markings on it, you can note the height of the raised end of the ruler by pinching the appropriate point on the plumb line with your free hand. Note that it doesn't matter whether you use inches or centimetres, as long as you consistently use one or the other. To find the average slope over a longer stretch of road repeat the outlined procedure at several points along the stretch of road and take the average of each result. To convert your percentage slope to degrees, use your calculator to divide it by 100, then find the arctangent of this value. Ensure the scientific calculator is set to use degrees, not radians.

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