We get the question what is the difference between an exact and undercut furnace filter often. Usually when customers are having a hard time locating a specific furnace filter size. The industry as a whole regardless of manufacturer undercuts standard size filters. An example is the Pre-Pleat MERV 8 rated filter by AAF Flanders in the size of 16x20x1 which is one of our most popular filters has an actual size of 16-1/2x19-1/2x3/4. On the filter you will find the size printed of 16x20x1 but the actual measurements are slightly smaller, this is an "undercut" furnace filter.
This can create some confusion typically when a customer is using an odd size filter. Standard sizes are always undercut where custom filters can be "exact" or "undercut". This is the main reason we always recommend double checking your filters measurements. Depending on where your filters were purchased the size printed on your filter could be slightly smaller. This can create a bit of an issue if your custom/odd size filters don't fit.
We prefer to submit all orders as "exact" if you are using an odd/custom size filter. This way in the future when it's time to order more filters you won't question the size of your filters, or end up with a box of filters the wrong size.
If you have any questions about custom furnace filters please reach out to customer service who is always ready to help ensure you get the perfect fit for your HVAC system.