These Air filter Sponges are used for Monotub set ups to create filtered fresh air exchange to prevent contaminates from going into your tub. These sponges are 1.77" wide and 1.18" thick so they are perfect for tubs that has holes drilled out 1.75" and under. Holes that are 1.75" are perfect fit and not tight. You will need to stuff these into the hole if your hole is 1.5". It was be snugged in there and wont easily fall out. I personally have ran three to four 1.5" holes on the bottom and two on top of my bins for even fresh air exchange.