Its a judgement call. The thickness is based on the view from the screener. I can tell you that shoes look thicker than they are. Expecially the tennis shoe. Soles wrap around the heel and the arch area. We cant put a ruler out there as this would just add to the mess. There are some screeners that prefer you take off all shoes just to reduce the arguments about the thickness. All shoes must be screened! This means that we look at them. If they meet the criteria, we ask that you remove them for X-ray exam. This is by far the quickest way for us to evaluate them. If you dont want to take them off, we still have to screen them. You get sent down for your shoes to be checked and end up with the full secondary screening. IF they dont meet the criteria for thickness, we try to figure if they have metal shanks in them and advise you of that. however, TSA looking at them fulfills the screening requirement IF THEY ARENT OVER THE THICKNESS.