Nominal vs. Actual Filter Size Explained
Furnace filters carry two sizes. The nominal size is the rounded name printed in big type on the frame (for example 16x25x1). The actual size is the real measurement, usually a fraction of an inch smaller, so the filter slides into the slot easily.
Quick answer
- Shop by the nominal size. That is what the package and the frame edge call the filter.
- Measure your slot and the old filter to confirm. If the old filter's printed size is readable, just reuse it.
- Actual size is often about 1/2" smaller in width and height, and about 1/4" or more smaller in depth. Exact numbers vary by manufacturer, so check the package.
How to read the frame
Most frames print two lines. The first is the nominal size. The second, often labeled "actual size," lists exact dimensions. Example only: a filter named 16x25x1 might list an actual size a bit under 16 x 25 x 1. Do not rely on this example for your own purchase; read your filter's label.
Why the difference exists
The nominal name is a convention that stuck. It makes shopping simpler and leaves a little clearance so you are not forcing the filter into the slot. A snug but easy fit is what you want.
When to measure instead
- The old filter has no readable label.
- The old filter looks like it was cut down or does not fill the slot.
- You are switching from a 1" slot to a thicker media cabinet, or the reverse.
Measure the slot opening width, height, and depth. Round to the nearest standard nominal size, and make sure the actual size is slightly smaller than the opening, not larger.
Common mistakes
- Buying by tape-measure result of the old filter. It will read slightly under the nominal size, so round to the nominal name.
- Trying to force a filter in. Bent frames leak air around the edges.
- Picking a filter that is too small. Gaps let unfiltered air bypass the media.
- Ignoring depth. A 1" filter will not seat correctly in a 4" cabinet.
Related: How to Find Your Furnace Filter Size (Step by Step), Common Furnace Filter Sizes and How to Read Them, and What MERV Rating Do I Need?
Shop by your nominal size
Stores list filters by the rounded nominal size. These Amazon searches cover the sizes most homes use. As an Amazon Associate we earn from qualifying purchases.