Accurate values for the thickness of electron-transparent specimens in electron microscopy are of general interest, e.g. as a parameter for quantitative simulations and calculations in the field of transmission electron microscopy (TEM). Several thickness-determination techniques exist, e.g. based on plasmon losses in electron energy loss spectra, convergent-beam electron diffraction, or exploitation of thickness contours in images acquired under two-beam diffraction conditions. However, the accuracy, precision, and time consumption differs significantly and often yields thickness values only for a small sample region. We will show in this work that scanning transmission electron microscopy (STEM) in a scanning electron microscope (STEM-in-SEM) is well suited for thickness determination with rather satisfying accuracy (error within a few percent). This technique has been further elaborated by us after previous initial work. We will give an in-depth instruction and discussion of the technique so that users can avoid pitfalls.