In many cases counters are used to count special events within an
endless sequence of events. This paper discusses a specific calculation,
related to the probability of a counter overrun if the special event
occurs not in a predictable way but with a certain probability.
Different counter codes are compared with each other. A probability
formula is developed for special scenarios which are normally analyzed
by state diagrams and which can be numerically solved by the related
Markov chains. The target is to enable a non-numerical discussion of the
topic. It is shown how formulas can be found based on the rules of the
probability theory, and their correctness is verified by a comparison
with Markov chains.