Premise of research. Understanding the relationship between field-measured operating stomatal conductance (g(op)) and theoretical maximum stomatal conductance (g(max)), calculated from stomatal density and geometry, provides an important framework that can be used to infer leaf-level gas exchange of historical, herbarium, and fossil plants. To date, however, investigation of the nature of the relationship between g(op) and theoretical g(max) remains limited to a small number of experiments on relatively few taxa and is virtually undefined for plants in natural ecosystems. Methodology. We used the g(op) measurements of 74 species and 35 families across four biomes from a published contemporary data set of field-measured leaf-level stomatal conductance in woody angiosperms and calculated the theoretical g(max) from the same leaves to investigate the relationship between g(op) and g(max) across multiple species and biomes and determine whether such relationships are widely conserved. Pivotal results. We observed significant relationships between g(op) and g(max), with consistency in the g(op): g(max) ratio across biomes, growth habits (shrubs and trees), and habitats (open canopy and understory subcanopy). An overall mean g(op) : g(max) ratio of 0.26 +/- 0.11 (mean +/- SD) was observed. The consistently observed g(op) : g(max) ratio in this study strongly agrees with previous hypotheses that an ideal g(op) : g(max) ratio exists. Conclusions. These results build substantially on previous studies by presenting a new reference for a consistent g(op) : g(max) ratio across many levels and offer great potential to enhance paleoclimate proxies and vegetation-climate models alike.