Under 2 mm - prone to breaking or shattering, delicate. 2mm - Common for accessories such as quartz bangers and carb caps 3mm - This is a pretty standard glass thickness to borosilicate bongs and more. Works for most people. 4mm - Considered on the thicker side, and seen more on medium to large-sized pieces. 5mm - An industry standard for a thick glass bong or pipe. Over 5mm - Above and beyond in thickness, sometimes more common on larger pieces.