This 20x25 filter fits perfectly and it filters very well. And I love the price!!What I learned from two HVAC service companies over 20 years is that choosing the right filter depends on how clean you want your house and the power of the fan motor. A filter protects the motor as well as the air in the house. Everyone has their own standard for air, so I will focus on the motor.Fan motors burn out for two reasons: dirt on the motor and having to work hard to suck the air through.Low merv filters let dirt accumulate on the engine -- the result is a dead engine and a $250+ repair. So you need a reasonable merv to protect the motor. But overheating is the second issue.As a filter accumulates dirt, the holes in the filter become blocked, and it is harder for the air to pass through. This causes the filter to curve in because the dirty air cannot pass through as easily. But more importantly, the motor has to work harder (think of drinking through a straw - soda vs milk shake). Eventually, the air will work its way through, but it may carry some of the smaller particles with it, and it will certainly overheat the motor. The overheating may not be enough to trip the cut-off switch, but it will damage the motor. The way to protect against this is to change the filter!If your house is low on dust, this filter may last 6-8 weeks. But I have 4 adults, lots of upholstered furniture, carpets, and a dog. So I check my filter whenever I think of it, usually every week or so. I change them based on color -- a light grey, usually after 4 weeks. If I wait for 6 weeks, it will start to curve and be a darker medium grey. And that means the motor is working too hard.Filters are cheaper than motors -- and this is a great price!