In most ecosystems, ants are a dominant part of the arthropod community. However, understanding of their importance has been hampered by limited availability of data on ant abundance. We developed a model to estimate the size (biomass and number of workers) of Oecophylla smaragdina colonies in mango plantations in Darwin, Australia. The total nest volume of O. smaragdina colonies in a tree was related to the activity of the ants (R2=0.85), estimated as the density of ant trails in the tree. Subsequently, the relation between nest volume and ant biomass (R2=0.70) was added to enable a prediction of ant biomass directly from ant activity. With this combined regression the ant biomass in a tree equaled 244.5 g fresh mass*ant activity. Similarly, the number of workers in trees was estimated using the relationship between nest volume and worker numbers (R2=0.84). Based on the model, five O. smaragdina colonies assessed in a plantation in the dry season, ranged in size from 131,000-562,388 workers and from 0.8-3.3 kg total ant wet biomass for the smallest and largest colony, respectively. Correspondingly, the areal abundance of ants in the plantation was 353 workers/m2 and 2.1 g ant mass/m2. With this model, estimates on O. smaragdina abundance can be obtained with a minimum of workload and it provides a possibility to scale physiological experiments on per capita rates (e.g. food consumption) to an ecological context. Thus, colonies of O. smaragdina can serve as model organism providing valuable information on the impact of ants in ecosystems.