Well I can answer my own question, and leave it for the next researcher. The answer is 2mm. However, you measure only the pad, not the backer. Since this is a impossibility, to measure with accuracy the thickness of the pad with out the backer other than laying a ruler along the side, or using good ole eyeball guesstimation, its not a very satisfactory answer. I buy nice shiny accurate tools to be able to measure stuff and feel good and warm and fuzzy all over.