The standard in the manufacture of air filters is that a 20x20x1 air filter actually measures 19-¾ x 19-¾ x ¾. This is known as undercut, and it includes custom sizes. For example, a 27-3/8 x 23-5/8 x 2 filter is actually 27-1/8 x 23-3/8 x 1-¾. This means that in 1 and 2 air filters, the nominal size does not include the bottom cut and the actual size does include the bottom cut.
The pleats of the filter running horizontally or vertically will not affect its effectiveness, so the number of unique air filter sizes on the market can be reduced by always listing the smallest measurement first. If you measure your ventilation and find that you need a filter with a nominal size of 30x20, you will discover that there is no such thing as a 30x20. It's best to have some wiggle room in the slot of the HVAC unit, so you can easily slide the filter in and out without it bending or breaking. Different manufacturers cut their whole house filters into different sizes and then round them to standardized measurements, meaning that two 20x25x5 whole house filters made by two different manufacturers are probably not the same size.
To ensure that your filter fits properly, use a measuring tape or other measuring device to measure the length, width and depth of the inner edge of the groove or frame of the air cleaner. If it's not the correct size, air will flow around it rather than through it, and all the air will not be filtered out. If you can't find your size in stores or need a custom-made air filter, online retailers are probably your best option. Most major retailers carry air filters with a variety of different options and brands to choose from.
Additionally, adhesive foam insulation tape can be used to slightly increase the dimensions of your preferred filter or order a custom-sized air filter if you live in an older house with a non-standard size air return frame.