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How do I measure my air filter, or furnace filter ? How do I measure my air filter, or furnace filter ? Most air filters have their dimensions stated in their nominal state which simply means the actual dimensions of the filter are rounded up or down. Below is an example of the nominal dimensions, and the actual size of the air filter compared to each other. Why air filter manufacturers began this practice is beyond me. What I do know is that this practice has made buying an air filter very confusing for some folks. Take the popular 16x25x5. Without even thinking about it I can come up with three 16x25x5 filters that are all different in their actual sizes depending on the brand of the air filter. The table below shows two 16x25x1 that have different actual dimensions. There are more. The take-away of this story is that you should be very careful when ordering an air filter because the dimensions you see on the filter frame or filter housing are almost never the true dimensions of the filter. These items are very expensive to ship back and forth, not to mention the inconvenience to you. So, double check, double check. Don't assume that if you see a 16x25 by whatever on our site, that this is the filter to order. You need the model number, or the "ACTUAL" dimensions. I refer to the 16x25x5 air filter here, but these discrepancies in sizes apply to any filter, including 20x20 - 20x25, etc. See below for more ideas.
 HOW TO DETERMINE THE SIZE AIR FILTER TO ORDER: Method 1: If you are currently using a throw-away fiberglass air filter, take the size written on your existing air filter and deduct 1/4 inch from both sides. For example, if your throw away filter is: 20 X 20, then you would order 19 3/4 X 19 3/4 actual dimension.
______ Method 2: Actually measure your existing air filter, but be sure that your existing filter fits well.
______ Method 3: Measure the opening provided for the air filter in your furnace, ceiling filter grill, or wall filter grill and deduct 1/4 inch from both sides. For example, if the opening provided for the filter in your furnace measures 18 X 20, then you
would order 17 3/4 X 19 3/4
Method 4: Look up the model number of the filter housing, or the part number of the existing filter
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