Unfortunately, you can not use a 5" backup pad on a 6" tool, nor can you use a 6" backup pad on a 5" tool. This is because the weight of the backup pad is specifically calibrated for the size of the tool, therefore the weight balance will be off and cause sanding mishaps. A solution to this problem is the Mirka backup pad 915GV28-130G. This is a 5" backup pad that allows for the use of 5" abrasives on a 6" tool.