Assessments of terrestrial carnivore diet are often required for ecological management and conservation purposes. Analysis and identification of food remains in scats is an important method often employed for these objectives. Proportions of consumed biomass estimated from scat samples may provide an accurate approximation of the actual diet when correction factors (CFs) are used. The red fox Vulpes vulpes is a widespread generalist carnivore that includes a wide range of foods in its diet, including livestock and plays a potential role as wildlife predator. We performed controlled feeding trials with red foxes to derive CFs for eight food categories, including both from animal and vegetal origin. CFs were calculated on a daily basis as the ratio between eaten fresh mass and total dry mass of the resulting scats. Red foxes consumed daily 615 ± 19 g (mean ± SE) of food and produced 6.4 ± 0.3 scats weighing 31 ± 1.2 g. The CFs varied significantly among food types, with the lowest values for partridges Alectoris rufa (CF = 12.5 ± 0.7) and the highest for red deer Cervus elaphus (CF = 47.3 ± 7.3), increasing linearly and significantly in relation to the individual body mass for the animal food types. CF precision was low for deer, fruit and hare Lepus granatensis, but more importantly, the CFs differed considerably among food types. We recommend considering intra-food variability when using CFs for estimating proportions of ingested biomass in studies of generalist carnivores such as red fox as a better support for management and conservation decisions.