Question: I measured the metal frame of my furnace filter at 19 1/2 x 24 1/2 x 3/4 inches but after looking closer at it I saw a measurement of 20 x 25 x 1 on the frame. Now I’m not sure what size filter to replace it with.
Shopping for a new air filter for residential use, usually only requires that you remove the old one and check to see what size is written on the frame, then purchase a new one online or pick one up at your local hardware store based on that information.
Sometimes, for many different reasons, you can’t determine furnace filter size by looking at it. Trying to determine the proper size replacement for your HVAC (Heating, Ventilation and Air Conditioning) equipment can be a bit of a puzzle.
How Sizes Listed on the Filters Label Compare to Actual Filter Sizes
Typically, manufacturers label their air filters in inches. For example 20 x 20 x 1 or 20 x 25 x 2 and so on. What often causes confusion is that the sizes listed on the filter and the actual size vary by as much as 1/2 of an inch. You measure the filter because there is no label on it telling you what size it is and you get odd sizes. This can make it difficult to order the proper size with confidence.
So Here’s How it Usually Works for Standard Filter Sizes
If you measure your old filter and it is 15 1/2 x 19 1/2 x 3/4 inches, the size you want to order is 16 x 20 x 1. The reason filters are actually smaller than the size listed on them is to allow for a margin of error. They are actually sized to fit into a filter rack that is 16 x 20 x 1. If you ordered a filter with dimensions the same size as the filter rack it would obviously not fit.
The same rule applies to the thickness of the filter. A filter that is labelled 1 inch thick will actually measure closer to 3/4 of an inch thick. A 2 inch filter will measure closer to 1 3/4 inches thick.
This rule applies to all filter types including Pleated, Media Pads, Electrostatic, HEPA, Fiberglas Panel filters and most other types available.
Something to Check Before Applying the Above Information
Before ordering a new filter based on the information above make sure the filter rack is not undersized. This is easily done by removing the filter and measuring the filter rack size.
If the filter rack is a bit smaller than standard filter measurements simply contact the filter supplier and ask them for the exact measurements of the filter that you think you need. This should eliminate and doubt when sizing a filter.
Special Order Filter Sizes Cost More
Often people order special sized filters online when they only require standard sizes. Custom order filters are more expensive than standard size filters because they have to be custom made. This takes more time and they cannot be mass produced. Therefore manufacturing costs increase and they have to charge more so they can still make a profit.
When Should You Order a Custom Size Filter
For most people, standard filter sizes will do just fine. If however, you are an allergy sufferer and require the absolute cleanest air possible in your home, a custom order filter is not a bad idea.
The tighter a filter fits into the filter rack the better job it is able to do. This is because a loosely fitting filter allows more air to bypass the filter and return to the living space, making it less effective. For example if you go the expense of installing a HEPA Pure filter and it fits poorly, you are not getting the full benefit of the filters higher efficiency.
This doesn’t mean that the air conditioner filter should fit tightly, that would make it difficult to change. It simply means that it should not be a poor fit either.
Don Munn – has written 72 posts on this site.
Don has been working in the HVAC industry for more than 25 years and shares his experience and knowledge on this site. You can also follow him on: