I don't know why, but every furnace seems to have different size filters. The designers must throw darts at a board to see what size it should be!My furnace needs 20x21x1 which is extremely odd. Other filter websites want to sell me a custom-made filter for a ton of money, but FilterBuy has them off-the-shelf for a great price!The filter is adequate. It's not as sturdy as a pricey filter from the big hardware stores, but then it's only half the price and it holds up long enough to be installed, last for 3 months, and be removed. They always come out dusty so I know they're working.Thanks for making a good, hard-to-find product at a reasonable price.