When considering the term “proportionality,” most people intuitively assume that a quantitative value that can be attached to it. In this chapter, this assumption is questioned. First, the chapter presents empirical evidence that indicates that there is no agreement on a specific numerical formula among IHL experts or military officers. Second, the chapter critically evaluates attempts to create a formula for evaluating proportionality. Third, the chapter discusses the use of “rules of thumb” to reduce the uncertainties of the application of proportionality. We conclude that the principle is inherently vague, and intentionally so. No amount of knowledge or experience can lead to the same results, nor is it the goal of the principle of proportionality to achieve the same results across the board. In this regard, proportionality in IHL is not about numbers so much as it is about ensuring the undertaking of a process that weighs competing interests. The relative weighting to be used in any given case is intentionally left open, beyond the purview of the principle.