Summary Carbonyl sulfide (COS) has emerged as a multi‐scale tracer for terrestrial photosynthesis. To infer ecosystem‐scale photosynthesis from COS fluxes often requires knowledge of leaf relative uptake (LRU), the concentration‐normalized ratio between leaf COS uptake and photosynthesis. However, current mechanistic understanding of LRU variability remains inadequate for deriving robust COS‐based estimates of photosynthesis. We derive a set of closed‐form equations to describe LRU responses to light, humidity and CO₂ based on the Ball–Berry stomatal conductance model and the biochemical model of photosynthesis. This framework reproduces observed LRU responses: decreasing LRU with increasing light or decreasing humidity; it also predicts that LRU increases with ambient CO₂. By fitting the LRU equations to flux measurements on a C₃ reed (Typha latifolia), we obtain physiological parameters that control LRU variability, including an estimate of the Ball–Berry slope of 7.1 without using transpiration measurements. Sensitivity tests reveal that LRU is more sensitive to photosynthetic capacity than to the Ball–Berry slope, indicating stomatal response to photosynthesis. This study presents a simple framework for interpreting observed LRU variability and upscaling LRU. The stoma‐regulated LRU response to CO₂ suggests that COS may offer a unique window into long‐term stomatal acclimation to elevated CO₂.