HVAC filters
Furnace filter sizes: nominal vs actual, and how to find yours
- 5 sources
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Quick answer
The size printed on a furnace filter is a rounded "nominal" size, not the exact measurement. Nordic Pure says the actual size is usually 1/4 to 1/2 inch smaller. A filter labeled 16x25x1 may really measure about 15.5 x 24.5 x 0.75 inches. Buy by the nominal size printed on your old filter, and check the actual size when you switch brands.
Furnace filters by rating (check your actual size first):
Filtrete MPR 1000 (MERV 11) Filtrete MPR 1900 (MERV 13) Nordic Pure 16x25x1 filtersNominal vs actual size
| Term | What it means | Example |
|---|---|---|
| Nominal size | The rounded size printed on the label and used to shop | 16x25x1 |
| Actual size | The true dimensions, usually 0.25 to 0.5 inch smaller (Nordic Pure) | about 15.5 x 24.5 x 0.75 |
Filtrete says the nominal size "can be called out differently depending on the brand or the system" and suggests checking the actual size of the frame. Nordic Pure gives the same advice: always check actual dimensions when you change brands.
Three ways to find your filter size
- Read the old filter's frame. The nominal size (width x height x depth) is printed on the cardboard edge. This is the easiest way.
- Measure the old filter. Nordic Pure's method: measure length, width and depth with a tape measure, then round up to the nominal size.
- Measure the slot. If there's no filter to measure, measure the inside of the slot or return grille, subtract 0.25 to 0.5 inch for clearance, and choose a filter whose actual size is a little smaller than the opening (Nordic Pure).
1-inch vs 4-inch filters
The last number is the depth. Filtrete recommends changing 1-inch filters at least every three months and 4-inch and thicker filters every 12 months. A 4-inch filter only fits a cabinet built for it, so match the depth you already have. See how often to change a furnace filter.
Picking the rating
Size decides whether it fits. The MERV or MPR rating decides how much it catches. The EPA says higher MERV ratings trap more small particles, and suggests at least MERV 13 if your system's fan and filter slot can handle it. Compare the numbers in MERV 8 vs 11 vs 13.
Guides in this section
No gaps around the filter. ENERGY STAR says the filter should fit tightly. If there's no cover and you can see the edge of the filter, it suggests sealing that space with duct tape or a magnetic cover.
How we work: size rules on this page come from Nordic Pure's measuring guide and Filtrete's FAQ. Rating and fit advice come from the EPA and ENERGY STAR. Sources are listed below.
Questions people ask
Is a 16x25x1 furnace filter really 16 by 25 inches?
No. 16x25x1 is the nominal size. Nordic Pure says actual sizes are usually 1/4 to 1/2 inch smaller, so a 16x25x1 may measure about 15.5 x 24.5 x 0.75 inches.
How do I find my furnace filter size?
Read the size printed on the old filter's frame. If there's no label, measure the old filter and round up, or measure the slot and subtract 1/4 to 1/2 inch, as Nordic Pure describes.
Do all brands use the same actual size?
Not always. Filtrete says nominal sizes can be called out differently by brand, so check the actual frame size when you switch brands.
Sources
Last checked Oct 23, 2026. Turn the furnace, A/C and fan off before you change the filter, as ENERGY STAR says. Check your furnace manual for the filter types and ratings it can take.
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