Black soldier fly larvae (BSFL; Hermetia illucens) can quickly convert organic substrates into body proteins and lipids. Due to its high conversion efficiency, this specie has gained significant attention in the field of insects as feed and food. However, there is a lack of understanding of the digestive processes underlying this potential. The main challenges in assessing digestibility in BSFL are accurate feed intake measurement and proper faeces collection, as larvae feed and excrete in the same moist substrate. In this study, a new indicator called Estimated Digestibility (ED) was introduced, which measures the difference between distributed feed and frass macronutrient weight, divided by macronutrient weight in distributed feed. ED was evaluated at increasing larval densities to ensure complete feed ingestion and frass free from refused feed. Initial diet and frass samples were analyzed for dry mater (DM), starch, nitrogen, ether extract (EE), neutral detergent fibre, acid detergent fibre, acid detergent lignin, ash and energy. The results showed a sigmoidal patern for ED of all fractions except fibres, with asymptotic ED of 80.3±1.3% for DM, 99.0±2.3% for starch, 78.6±1.1% for nitrogen, 95.3±1.5% for EE, 58.4±1.0% for ash and 80.6±1.2% for energy. This new indicator is a way of measuring digestibility in insects living in their feeding substrate. It offers insight into the digestive efficiency of BSFL.