How to Get the Right Air Conditioner Filter Size
You can tell the actual air conditioner filter size by reading the label on the filter itself. By removing the filter out from the bottom of the air handler or out of the filter back grill you will be able to determine what size filter that needs to be purchased.
The difference between the actual air conditioner filter size and the nominal size of the air filter is about minus one half inch on both sides. When you go out to purchase a replacement for your air filter, it makes good sense to bring the label from your old filter with you and get a filter with an identical label. At other times, you may not have the benefit of the label because it was worn off or the filter never had a label. In such a case, measure sides of the filter and write these figures down on a piece of paper; also write them on the system itself so that you will always know the filter size to be replaced.
If the old filter is unavailable. Take your measurements from the sides of the area where the filter was supposed to have been. Then deduct one half inch from both sides to get the correct size filter for easy removing and replacing of your filter.