Black soldier fly larvae (BSFL; Hermetia illucens) are increasingly studied for their ability to convert organic substrates into body proteins and lipids that can be used for animal nutrition. Although many studies have used BSFL high weight gain to highlight their strong feed conversion efficiency, little is known about the inherent efficiency of each of the four feed conversion stages: ingestion, digestion, absorption, and metabolic utilisation. Assessing digestibility requires quantifying the amount of feed ingested and the associated faeces produced. However, this is challenging in BSFL because they feed and release excreta in the same substrate, which also hosts complex microbiota participating in digestion. This study introduced a new indicator called Estimated Digestibility (ED), defined as the difference between distributed feed and frass macronutrient weight, divided by macronutrient weight in distributed feed. The evolution of ED was assessed with increasing larval density in order to ensure complete feed ingestion and frass free from refused feed. ED was measured on a standard diet with densities from 0 to 29 larvae/cm2 for dry matter (DM), starch, nitrogen, ether extract (EE), neutral detergent fibre, acid detergent fibre, acid detergent lignin, ash and energy. The results showed a sigmoidal pattern for ED of all fractions except fibres. Asymptotic ED was 80.3±1.3% (mean ± standard error) 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. Asymptotic ED is the closest estimation of digestibility as defined in other species. It offers perspective on the understanding of BSFL digestive efficiency and could be used for diet formulation.