The number of plots in an experiment is determined solely by the number of treatments that are being tested, and the number of times that each of those treatments is replicated. The relationship is simple: the total number of plots is the number of treatments multiplied by the number of replications.
However, despite this apparent simplicity, there is an underlying mathematical relationship (Hathaway, 1963) connecting the number of replications, the plot size (area), with the ability to detect statistical differences. The derivation of this relationship relies on using direct observations from previous experiments, such as yield, to calculate an index of variability unique to each field for a fixed plot size. The index of variability ranges from 0 to 1, with 0 being a completely heterogeneous (varying) field, and 1 a completely homogeneous field (uniform).