When it comes to air filters, the size is an important factor to consider. The 16x25x1 size is often used instead of the 16x24x1 size because it is easier to remove. The actual size of the filter is the length, width, and thickness of the filter, which is usually labeled on the filter just below the nominal size. If you are buying a filter online, you can find the actual size in the product list or product description.
If the actual size is not labeled on the filter, you can measure it to get the correct dimensions. Some brands have a foam seal on the outside of the filter, so you should make sure that you have a compatible true size and that it also has foam to ensure a perfect fit. It is important to understand these numbers so that you don't buy a filter that doesn't fit your heating or HVAC system. The filter should not be too small to move freely within your unit, otherwise it won't fit and you will have to return your purchase.
Filters don't need to have a hermetic seal, but they should be easy to install and uninstall effortlessly.