One-step KOH activation enables the direct conversion of cellulose-based biomass into activated carbon (AC), yet reproducible control over pore structure and carbon yield remains challenging. While many studies emphasize the KOH-to-precursor mass ratio, the water volume used for dissolving a fixed amount of KOH is rarely regarded as an independent parameter. Here, we demonstrate that the amount of water introduced with the KOH solution and its spatial distribution within the cellulose precursor after impregnation, either absorbed into the internal fiber network or retained as excess liquid, govern the thermal transformation pathway and the resulting AC properties. Using paper sheets with a defined absorption capacity as a model precursor, ACs were prepared at constant KOH loading (1:1) while systematically varying the added water volume. This approach establishes distinct initial conditions, ranging from complete solution uptake to excess external solution. Combined thermal analysis (TGA-MS), ATR-FTIR, XRD, XPS, electron microscopy, N₂ adsorption, and SAXS reveal that exceeding the precursor's absorption capacity promotes early carbonate formation, enhances alkaline degradation, delays char formation, reduces carbon yield from ∼9% for C35 to ∼1% for C20, and strengthens CO₂-assisted physical activation, leading to larger pores. In contrast, reduced water addition shifts the pathway toward earlier charring, limits the conversion of hydroxide into carbonate, increases yield, and promotes micropore-dominated structures. These findings identify water volume at constant KOH loading as a decisive variable in the investigated solution-impregnation system and demonstrate that it should be controlled alongside the activator-to-precursor mass ratio when targeting reproducible AC properties.