Black soldier fly larvae (BSFL) efficiency at converting organic by-products into insect biomass suitable for livestock nutrition depends on the nature and macronutrient profile of their feed. Digestibility is a key parameter to the development of a diet formulation approach, but methods to assess digestibility in most insects are unsuitable for BSFL as ingesta is not measurable and pure faeces can not be obtained and analysed. This work defined estimated digestibility (ED) as the difference between feed and frass nutrient weight, divided by the weight of nutrient in feed, and explored its variation with larval density. ED of dry matter (DM), starch, nitrogen, ether extract (EE), neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL), ash and energy were evaluated using the same amount of chicken feed, same feeding time and densities from 0 to 29 larvae/cm2. ED of DM, starch, nitrogen, EE and energy increased with larval density following an asymptotic trend. ED of ash also increased between 0 and 11.4 larvae/cm2, but decreased for higher densities. For NDF, ADF and ADL, no clear results were obtained presumably because the fibre dosing procedure used was not specific enough and also accounted for chitin in the frass. ED of all nutrients but fibre fractions were modelled using a logistic equation. This method was able to propose asymptotic ED values for some major nutrients. This new indicator corresponds to the maximal fraction of each nutrient that BSFL can digest and shows potential for the development of a BSFL diet formulation approach based on digestible fractions rather than crude nutrient content. Future studies should compare asymptotic ED of various substrates to provide insight on BSFL ability to digest different ingredients.