Expandable infinitely, but effectiveness depends on your shop vac. I installed under the bottom of a plywood top workbench and drilled 1/8th inch holes, 3/4 inches apart across the whole span and seems to work well. I used a 1/4inch wide foam gasket to seal the brim and drilled holes and used screws to secure it to the bottom side of the work bench. Made to fit a 1-3/4 inch (outside dimension)shop vacuum hose.