Four inch furnace filters measure 3-3/4" wide as is standard in Canada.

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Available in many sizes listed below (16x25x4, 20x25x4, 20x20x4 and more)

Different qualities for more or less effectiveness as per the MERV rating system:

Throwaway, MERV 8, MERV 10, MERV 11, MERV 13 and Carbon odour control.