Summary The sapwood area supporting a given leaf area (Huber value, v H ) reflects the coupling between carbon uptake and water transport and loss at a whole‐plant level. Geographic variation in v H presumably reflects plant strategic adaptations, but the lack of a general explanation for such variation hinders its representation in vegetation models and assessment of its impact on the global carbon and water cycles. Here we develop a simple hydraulic trait model to predict optimal v H by matching stem water supply and leaf water loss, and test its performance against two extensive plant hydraulic datasets. We show that our eco‐evolutionary optimality‐based model explains nearly 60% of global v H variation in response to light, vapour pressure deficit, temperature and sapwood conductivity. Enhanced hydraulic efficiency with warmer temperatures reduces the sapwood area required to support a given leaf area, whereas high irradiance (supporting increased photosynthetic capacity) and drier air increase it. This study thus provides a route to modelling variation in functional traits through the coordination of carbon uptake and water transport processes.