Reading a filter size.
Three numbers on the cardboard edge. Getting them right matters more than the MERV number does.
Air filter sizes are printed on the cardboard edge as three numbers, like 16x25x1. That is length by width by thickness, in inches.
The quickest way to buy the right one is to read the old one before you throw it out, or photograph it. Writing the size inside the filter door with a marker saves the next person the trouble.
Nominal versus actual
Here is the part that confuses people. The printed size is nominal, and the actual filter is slightly smaller, usually by about half an inch in each direction. A 16x25x1 filter really measures closer to 15.5 by 24.5.
That gap is intentional so the filter slides into its slot. It means you should not measure your old filter with a tape and buy that number, because you will end up a size down and leave a gap.
If you must measure, measure the slot, and round to the nearest common nominal size.
Why the fit matters more than the rating
Air takes the easy route. A filter that is too small, warped, or not seated in its track leaves a gap, and a meaningful share of your airflow goes around the filter rather than through it.
That air carries dust and pet hair straight onto the evaporator coil, which is wet all summer and therefore holds onto it. A fouled indoor coil is a far larger job than a filter.
So a correctly sized moderate filter beats an expensive one that does not fit, every time.
Thickness
The last number matters more than people expect. A one inch filter in a four inch cabinet rattles and bypasses. A four inch media filter in a proper cabinet offers more surface area, which means better filtration with less airflow restriction, which is the opposite of what people assume.
If somebody in the house has genuine respiratory needs, that is usually the right answer rather than cramming a dense one inch filter into an undersized slot. The MERV guide covers why.
Direction
There is an arrow on the frame. It points in the direction of airflow, which is toward the blower and away from the return grille. Fitted backwards a filter still works, just less well, and it collapses sooner.