Black soldier fly larvae (BSFL) are highly efficient at converting organic substrates into insect 25 biomass suitable for livestock nutrition. BSFL conversion efficiency depends on the type and 26 profile of their feed, and formulation approaches use macronutrient contents to reach a targeted 27 zootechnical performance. However, digestibility varies between ingredients and BSFL 28 digestibility data is lacking due to methods unsuitable for insects living in their feed. This study 29 defined Estimated Digestibility (ED) as the difference between distributed feed and frass 30 macronutrient weight, divided by macronutrient weight in distributed feed, and explored its 31 variation with larval density. Using the same amount of chicken feed, same feeding time and 0 32 to 29 larvae/cm², ED of dry matter (DM), starch, nitrogen, ether extract (EE), neutral detergent 33 fibre, acid detergent fibre, acid detergent lignin, ash and energy were evaluated, along with 34 survival rate, feed conversion ratio (FCR) and larval individual weight gain on a fresh and dry 35 basis. ED of DM, starch, nitrogen, EE and energy increased with larval density following an 36 asymptotic trend. ED of ash increased between 0 and 11.4 larvae/cm² but decreased for higher 37 densities. Using a logistic model, asymptotic ED of DM (80.3±1.3%), starch (99.0±2.3%), 38 nitrogen (78.6±1.1%), EE (95.3±1.5%), ash (58.4±1.0%) and energy (80.6±1.2%) were 39 determined. No effect of areal density on ED of fibre fractions was detected, presumably due 40 to an inappropriate dosage method. Survival rate and individual weight gain peaked at densities 41 below 5 larvae/cm², FCR (dry to fresh) was improved with increasing density, while FCR (dry 42 to dry) was optimised at 11.4 larvae/cm². Asymptotic ED is a new indicator corresponding to 43 the maximal fraction of macronutrients potentially digestible by BSFL. It should be used to 44 develop a diet formulation approach based on digestible rather than crude macronutrient 45 contents. Further studies should focus on evaluating asymptotic ED in different ingredients.