The filter aisle is designed to confuse you. Every box shouts about "allergen defense" and uses its own made-up scale. The number that actually matters is MERV, the industry rating from ASHRAE Standard 52.2. It goes 1 to 16. Higher numbers trap smaller 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 MERV 13, or as high as your system's fan and filter slot can handle.
The catch: airflow
Denser filters are harder to push air through. A 1-inch MERV 13 can choke a system that wasn't built for it. Too little airflow means frozen coils in summer, overheating in winter, and a bigger electric bill.
If you've got a 4- or 5-inch media cabinet, you're in luck. The extra surface area lets you run high-MERV without the restriction. That's the setup I'd want in my own house.
Other rating scales
Some brands do their own thing. 3M's Filtrete MPR ratings roughly map 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, hunt down the MERV number on the box or the manufacturer's site.
How often to change it
- 1-inch filters: every 1 to 3 months. Swap more often with pets or during heavy seasons.
- 4- to 5-inch media filters: every 6 to 12 months.
Scribble the date on the frame when you install it. Stops all the guessing.
Measure the slot, not the old filter. Filters are sold by nominal size, like 16x25x1, but actual dimensions run a bit smaller. Buy whatever size is printed on your old filter's frame.