Air filter sizes come in a variety of dimensions. Most standard filters print the dimensions on the filter frame. You can use those filter dimensions to select the appropriate size replacement filter.
Unfortunately, some filters are not marked. In this case you will need to measure the dimensions of your existing filter.
Please also be aware that many filters may have actual dimensions that are not exact whole numbers—for example 17½" x 23¾" x 1". If this is the case, you should round the fractional dimensions up to the nearest whole number. In the example, this would require a filter with nominal dimensions of 18" x 24" x 1".
If you still aren't sure, please contact us and we'll do our best to help.