Lot sizing is a substantial factor of economic production. There are various lot sizing approaches considering the same cost factors in different ways. Their results are usually quite similar. A common weakness of all lot sizing approaches is the limited consideration of relevant cost factors. The costs, which are neglected, usually rise with increasing lot sizes. This leads to illusory 'exact' results because the lot sizes calculated are generally too large. This paper compares functions and results of different lot sizing approaches and discusses neglected cost factors. In conclusion, an estimation of real economical lot sizes is presented.