While the principle of magnetic flux leakage (MFL) is relatively simple, its application in inline inspection (ILI) of carbon steel pipe is far more complex. MFL system design and analysis encompass complex interactions between the magnetic field and flux leakage produced by metal loss defects in the pipe wall, making signal identification & interpretation difficult. Thus, the need for tool tolerances. This paper discusses the cause and effect of wide-ranging factors which influence the reported depths & dimensions of MFL In-line Inspection (ILI) data.