The filter aisle is confusing on purpose. Every brand has its own scale, and the boxes promise things like “allergen defense.” The number worth knowing is MERV, the industry-standard rating from ASHRAE Standard 52.2. It runs from 1 to 16, and higher numbers catch more small particles.
What the numbers mean in practice
| MERV | What it catches well | Good for |
|---|---|---|
| 1–4 | Lint, large dust | Protecting the equipment, not much else |
| 5–8 | Dust, pollen, mold spores | A solid everyday baseline |
| 9–12 | Finer dust, some smoke and pet dander | Pets, allergies |
| 13–16 | A large share of very fine particles | Wildfire smoke, allergies, health concerns |
The EPA recommends a MERV 13 filter, or as high as your system's fan and filter slot can handle.
The catch: airflow
A denser filter is harder to push air through. A 1-inch MERV 13 filter can be restrictive in systems that weren't designed for it. Too little airflow can cause frozen coils in summer, overheating in winter, and higher energy use.
If your system has a 4- or 5-inch media cabinet, you can run a high-MERV filter with much less restriction because there's more surface area. That's the best setup for most homes.
Other rating scales
Some brands use their own scales. For example, 3M's Filtrete MPR ratings roughly line up like this: MPR 300 is about MERV 5, 1000 to 1200 is about MERV 11, and 1900 to 2200 is about MERV 13. When in doubt, look for the MERV number on the box or the maker's website.
How often to change it
- 1-inch filters: every 1 to 3 months. Change them more often with pets or during heavy use.
- 4- to 5-inch media filters: every 6 to 12 months.
Write the date on the filter frame when you install it. It's the easiest way to stop guessing.
Measure the slot, not the old filter. Filters are sold by nominal size, like 16x25x1, but actual dimensions are slightly smaller. The size printed on the old filter's frame is what to buy.