Many forests have a long history of human land use, which shapes species communities and ecosystem processes, making robust and quantitative measures of land-use intensity in forests desirable. We here introduce the ForMIX (Forest Management IndeX), a compound index combining altered tree species composition, tree removal, deadwood availability and stand maturity, which are each calculated as the deviation from expectations in an unmanaged old-growth forest reference. The index and its components allow for mechanistic inference on the consequences of land use in forests as they are based on biotic resources and niches directly affected by forest land use. Using basic forest inventory data from 150 sites distributed over three regions of Germany, we demonstrate the properties of ForMIX, which differentiates well among forest types and silvicultural systems and is robust to decisions regarding reference values and components. Reference values used in ForMIX are dynamic, could be adapted to ongoing climate change and may require refinement for different geographic regions. ForMIX advances the quantification of land-use intensity in forests by being biologically meaningful, usable and comparable across forest types, derivable from standard forest inventory data, and easy to apply, understand and interpret.