The size is printed on the edge of your old filter. Match all three numbers.
- Example: 16x25x1 means 16 in by 25 in, 1 in thick.
- Nominal size: the real filter is a bit smaller. That is normal.
- Arrow: points toward the furnace or air handler.
1Read the size on the frame
Pull out the old filter. Look at the cardboard edge. You will see a size like 16x25x1. That is length, width and depth, in inches.
Write down all three numbers. The order of the first two does not matter. A 16x25 and a 25x16 are the same filter.
2Nominal size vs actual size
The size on the label is the nominal size. It is rounded up. A 16x25x1 filter really measures about 15.5 x 24.5 x 0.75 in. That small gap lets it slide into the slot.
Shop by the nominal size. That is how filters are sold.
3No label? Measure it
Measure the old filter with a tape measure. Round each number up to the next whole inch, or the next half inch for odd sizes. A filter that measures 15.5 x 24.5 x 0.75 is a 16x25x1.
No old filter at all? Measure the slot opening. Pick the nominal size that fits inside it.
4Depth must match the slot
Filters come 1, 2, 4 and 5 in deep. Use the depth your slot was made for. Never force a thicker filter in, and do not use a thin one in a deep slot. Air will go around it.
5Pick a MERV rating
MERV tells you how much dust a filter catches. Many homes use MERV 8 to 11. A higher MERV catches finer dust, but it can slow airflow on some systems. If you are not sure, check your system manual or stay with what you have.
Install it the right way
- Turn the system off before you swap the filter.
- Point the airflow arrow toward the furnace or air handler, away from the return duct.
- Check a 1 in filter every month. Most need a change every 1 to 3 months.
Ready to match the numbers?
Filter by the specs printed on your old part.