Calculating the arithmetic mean in particle (grain) size, am, with the phi scale (Folk, 1964: 81), is quite complicated. Its calculation, therefore, should be done in the numeric, or arithmetic way, where the sizes for the percentiles are directly given:
- - For the sizes, with the greatest frequencies, the values of 25; 50 and of 75 weight %, are included in the calculation.
- - 5 and 95 wt. % are suggested as the ends from the graph. With this, 90 % of the total particle-size data go into the calculation.