The Standardized Precipitation Index (SPI) and related drought indices are used globally to measure drought severity. The index uses a predictable structure, which we leverage to determine the theoretical likelihood of a year with an extreme worse than a given threshold. We show these likelihoods differ by the length (number of months) and resolution (daily vs. monthly) of the index. This is important for drought managers when setting decision thresholds or when communicating risk to the public.