Using the picture below or the sample in class, it's time to determine the percentage of sand, silt, and clay relative to the total soil level. To calculate the percentage, divide the depth of each layer of soil by the total soil depth in the jar, and multiply by 100. (Use millimeters). Because sand is the biggest particle, it falls out first, silt is the second layer because its size is between sand and clay, and the top layer is clay because it is the smallest particle.