Sapwood area is an important parameter for estimating canopy transpiration in the forest water cycle. However, sapwood area highly varies across species and forest ecosystems and is difficult to measure directly. Therefore, species- and site-specific allometric equations are needed to estimate the sapwood area of all trees in a forest. Here, we conducted a comprehensive campaign to measure sapwood thickness and to estimate the sapwood area of 14 common tree species in a successional forest in Thailand. These data represent the first comprehensive measurements of sapwood area in south-east Asian successional forests growing under diverse environmental conditions in terms of soil moisture and canopy density. The results show that a power function can significantly explain the relationship between sapwood area and stem size, represented by diameter at breast height (DBH), in all species in both primary and secondary forests. Interestingly, a single equation could describe the sapwood area~DBH relationship in all species and forest stages, except for Dipterocarpus gracilis, an emergent, dominant species in the primary forest. The latter showed slower growth in sapwood area once the trees reached a DBH of ~30 cm. Overall, our results can benefit future studies that estimate canopy transpiration of tropical forests with similar conditions as in our study sites.