Enter the percentage of gravel, sand, and fines into the calculator to determine the group index. This calculator can also evaluate any of the variables given the others are known.

Group Index Formula

The following formula is used to calculate the group index.

GI = (0.2 * a) + (0.005 * b) + (0.01 * c)


  • GI is the group index
  • a is the percentage of gravel (%)
  • b is the percentage of sand (%)
  • c is the percentage of fines (%)

To calculate the group index, multiply the percentage of gravel by 0.2, the percentage of sand by 0.005, and the percentage of fines by 0.01. Add all these results together to get the group index.

What is a Group Index?

A Group Index is a numerical value used in soil classification to estimate the suitability of a soil sample for use in road and runway construction. It is calculated based on the soil’s particle size distribution and the liquid limit and plasticity index. The higher the group index, the less suitable the soil is for construction purposes.

How to Calculate Group Index?

The following steps outline how to calculate the Group Index (GI).

  1. First, determine the percentage of gravel (a).
  2. Next, determine the percentage of sand (b).
  3. Next, determine the percentage of fines (c).
  4. Use the formula GI = (0.2 * a) + (0.005 * b) + (0.01 * c) to calculate the Group Index.
  5. After inserting the values of a, b, and c into the formula, calculate the result.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Percentage of gravel (a) = 40%

Percentage of sand (b) = 25%

Percentage of fines (c) = 10%