In most cases, you can use the closest standard size when it comes to air filters. The nominal size is the size that is printed on the side of the filter, and this is due to the actual size being rounded to the nearest whole inch. There are hundreds of filter sizes available, and while manufacturers can make small changes and offer very small differences in their many filter sizes, the nominal size system ensures that you, the consumer, can draw on industry standards and don't have to have custom air filters every time you need some clean air. All filters have two different sizes. The actual size is the exact dimensions without rounding, while the nominal size is the rounded dimensions of the filter.
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. In whole-house filters, the nominal size is the rounded size, and the actual size is only the non-rounded one. Obviously, you should select an air cleaner with a filter element large enough to do the job. For example, the 14-inch by three-inch element in 1960s-era Chevy high-powered car air filters is ideally sized for many single-carburetor applications. In fact, it's so good that aftermarket manufacturers offer clones of the Chevy filter. When it comes to determining the filter size that a heating and cooling system requires, looking at the old filter or consulting your owner's manual are two of your best options.
If in doubt, you can always measure the air filter yourself with a ruler or any other measuring device. For a K&N element, you can use the diameter of the air filter to help determine the area sufficient for the required airflow. When it's time to change your HVAC system's air filter, it's important to order the right size. 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. Companies like K&N offer many different types of air filter elements and air filter assemblies to fit various sizes and shapes of carburettors. If your return vent is on your roof, you may want to use a filter that has a slightly larger actual size so that it doesn't fall on you when you open up the vent.
There are a number of great options for buying air filters online and having them delivered right to your doorstep. Most white pleated air filters are inherently similar within each efficiency level, varying slightly if at all between brands. However, if your filter frame is incorrect in terms of size or installation, this can cause air leaks which significantly decreases its effectiveness. The nominal size is usually printed on the cardboard edge of an air filter and is typically all you need to know when looking for one. As far as shape is concerned, K&N's flow bank data has shown that for typical two- and four-barrel carburettors (and fuel injection throttle bodies), an air flow that has a larger diameter compared to its height will be more effective.