Getting Filter Sizes

Getting proper filter sizes can be trickier than you'd think this is particularly difficult if your existing filter is either: improperly sized, missing, or in too poor a condition to determine what it was. The fact that filter manufacturers use “nominal” sizes can increase the frustration with this process. Nominal dimensions are different than the actual dimensions of the filter. In fact, nominal dimensions are almost always slight larger than the actual filter dimensions. This is done to allow for a little wiggle room inside the filter slot. This is fine for HVAC systems since air pressure inside the system presses the air filter frame a against the frame of the filter slot which creates a seal. In this way air won't leak around the edges. All you have to do is make sure that the filter sizes fits into the slot and is big enough to touch all four sides of the slit frame. The slot frame is usually fairly wide (half an inch or so) so that the filter size does not have to be exact. Again, this is fine for standard HVAC system air filters. The wiggle room created by using nominal dimensions makes sure that the filter will slide into the slot (i.e., won't be too big or too tight) while also properly seating against the filter slot frame (i.e, not too small).

1. Is your filter for a standard HVAC system or for a whole-house air cleaner? If the air filter simply slides into a slot in the air duct as it enters the air handler or at a return, you probably just need a standard HVAC system air filter. However, if you have an air cleaner installed in the system, a standard HVAC system air filter will probably not work. An air cleaner is a separate unit installed in front of the HVAC system air handler. It is designed to “scrub” the air using a variety of technologies, including ultraviolet light and high efficiency air filters. Air cleaners are made by a number of manufacturers such as Lennox, Air Bear, Honeywell, Carrier and many others. The manufacturer of your air cleaner should be pretty obvious…look for a logo, company name, trademark or something similar.

2a. Assuming you do not have an air cleaner, you have three ways to determine the correct filter size:

i) Look at the filter you are using now. The size is usually printed on the filter frame and will be something like 16 x 25 x 1 or 14 x 14 x 1. These dimensions refer to the nominal size of the filter. Nominal dimensions are not the actual dimensions of the filter. More on that below.

ii) If you can't find the dimensions printed on the frame, you can use a measuring tape to measure the outside dimensions of the filter. This is a simple process…just measure each dimension to the nearest 1/8″. An air filter has three dimensions, height, width and depth. To measure your filter, stand the filter on its side with the pleats running horizontally (parallel to the floor). Height is the vertical dimension from the floor to the top of the filter in this orientation, width is across from left to right, and depth is the thickness of the air filter from front to back. You can also determine these sides based on how the air filter is positioned in the system. Typically, the air filter is positioned in the system with the pleats running horizontally.

Filter depths are standardized but a little misleading. For example, a 1” filter is actually about 1/2” wide, while a 2” filter is about 1 1/2” wide. Measure yours and round that up to the nearest whole number. In other words, if the depth of your filter is 5/8”, you need a 1” filter.

Filter height and depth are the most critical measurements. Measure each dimension to the nearest 1/8”. In general, a filter height of 24″ corresponds to a true height of 23 and 5/8 to 23 and 7/8 inches.
iii) Some filters are just too flimsy to measure accurately. If this is the case, you need to measure the slot into which the filter is inserted.

2b. Assuming you have an air cleaner installed in your system, you can determine the correct filter size in three ways:

i) Every air cleaner manufacturer uses model numbers on their units. You can find the model number of your printed on the unit itself or stamped on a label. You may have to search a bit but it's there. Look up the manufacturer name and model number on our handy air cleaner reference page. We list many manufacturers and the appropriate filter size for each model.

ii) Can't find the model number? Pull out the filter that you are currently using and look for the manufacturer's name and part number. You can use the air cleaner reference page to verify the air cleaner manufacturer name and replacement air filter size.

iii) If you're still having no luck, you'll need to measure the filter dimensions to determine the correct size.

3. Once you know the size of your HVAC system air filter, or in the case of an air cleaner, the manufacturer name, unit model number and/or filter part number, go to Step 1 of our filter selection wizard and see if you can find your size in the drop down menu. Don't see your size? Contact us about ordering custom-sized filters.

Nominal Dimensions Versus Actual Dimensions
To make things really interesting and fun, the air filter industry labels filters with “nominal” dimensions.

Air cleaner filters are a different story. Air cleaners are designed to use filters that fit the unit exactly. While manufacturers often provide nominal sizes for air cleaner filters, you need to be very aware of the actual filter dimensions. For this reason, we highly recommend using a filter specifically designed to fit the air cleaner unit you have. In other words, don't just assume that a filter with nominal dimensions that match those listed by your unit's manufacturer will actually fit. Use the model number or filter replacement part number instead.