This filter is larger (taller) than it needs to be for most machines. You simply trim the filter (and it's got pre-made perforations) to the height of your machine. And yes, this means that if you have one of the smaller machines, one which is less than half the height of the pre-filter, you might get two pre-filters for the price of one. Bonus.

I follow the manufacturer's recommendation to change the pre-filter every 3 months on all my Honeywell Air Cleaners with HEPA filters. These replacement filters come boxed in a set of two. The pre-filter simply wraps around the round HEPA filter, which is inside the unit, and attaches to itself by means of two velcro fasteners. It takes less than five minutes to change the pre-filter. In between changes, I vacuum my pre-filters to remove accumulated dust and pet hair from them. These pre-filters do help to remove some odors from rooms, especially if you are able to keep the space enclosed where you have the air cleaner running.

Be sure to keep a box or two of these pre-filters on hand in case they become back ordered. The HEPA filter needs to be changed in 1-3 years, depending on conditions where it is located.

What I really like about this pre-filter is the perforations that allow you cut the filter to the correct width. I have used other brands and you have to use a template to cut the filter to the correct size. Not with this one - just follow the lines.