I recently purchased a furnace filter off of Amazon. Its dimensions were correct in the description but when I tried to install it I was immediately aware that it was the wrong size. On closer inspection I discovered that it was one-quarter inch smaller in all directions. A furnace filter has to fit correctly to be effective. Since air follows the path of least resistance a lot of the air would simply be shunted around the filter instead of through it. On closer inspection the filter said that it was a “slim fit” so caveat emptor-let the buyer beware when purchasing on line. Don’t make the mistake that I did, read the fine print in the product description!