This section explains how the saprobic index of a water body is computed according to the Zelinka & Marvan method; without adjusting for several confounding factors.
In a first iteration, the abundance A of each indicator species is counted and converted to categories ranging from 1 to 7. An abundance of 1 means that only one or two animals was found, while the class 7 means more than 1000 individuals during a survey. There are different abundance classes — for example, some methods use classes where the next-bigger class contains roughly double the number of individuals. The following table follows the DIN 38410-1 (2008) standard used in Germany, where the next-bigger class is about three times larger than the previous one.
The saprobic value s denotes how much organic matter must be present for an aquatic species to thrive. An animal with a saprobic value 1 can only survive in water with little organic matter present, while one with a value of 4 requires water bodies with a large amount of organic matter. The aforementioned example, the Lymnaea stagnalis snail, has a saprobic value of 2.0. The annelid worm Tubifex tubifex needs a lot of organic matter and has an s value of 3.6.
The weighting factor g has a value of either 1, 2, 4, 8 or 16, and denotes a tolerance range. If a species can survive in both unpolluted and heavily polluted water, g is very small because finding the species in a survey has little predictive value. In practice, only indicator species with a weighting factor g ≥ 4 are used. For example, a caddisfly, Agapetus fuscipes, has a g value of 16, while the zebra mussel's value is g = 4.
The saprobic index of a water body - the water quality - is finally computed with the following formula:
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The water body's quality, in Roman numerals, is the rounded value of S.