Ratings decoder
MERV, and What a Higher Number Costs You
One number describes what the media catches. A second thing, which the number does not describe, is what it costs the blower.
The same thing, read twice — Filtrete 20x25x1 MERV 12, 2-pack
What the MERV number says
MERV 12. On a scale that runs from 1 to 16 for this kind of filter, that is near the fine end — the media is built to capture a wider range of particle sizes than a MERV 5 filter does.
What the MERV number does not say
Finer media is harder to pull air through. The same filter that raises the capture figure also raises the resistance the blower works against, and the MERV number carries no information at all about how much.
Both readings come from the listing for Filtrete 20x25x1 MERV 12, 2-pack, which states: listing states MPR 1500 and an actual size of 19.62 x 24.62 x 0.78 in, and mentions allergens, bacteria and viruses — quote that wording as a listing claim only.
What is actually going on
MERV stands for Minimum Efficiency Reporting Value. It comes out of a laboratory test method published by ASHRAE, in which a filter is challenged with particles across a range of sizes and its worst performance across the test conditions determines the rating. The word minimum in the name is doing real work: the rating reflects the least the filter caught, not the best it managed.
The scale for residential filters runs from about 1 to 16. Low numbers describe media that catches larger particles and lets smaller ones through. High numbers describe media that catches a wider size range. That is the whole content of the number.
What the number carries no information about is pressure drop — the resistance the media puts in the way of the air. Finer media means more resistance. A blower designed around a certain airflow moves less air when the resistance goes up, and a system moving less air than it was built to move behaves differently in ways that show up at the equipment rather than in the room.
This is why the correct answer to "which MERV should I buy" is never "the highest one that fits." The equipment manufacturer publishes what filter rating and what pressure drop its system is designed around. That document decides. If it cannot be found, a licensed HVAC professional can read the equipment and say. Nobody selling filters, including this site, is in a position to make that call for a system they have never seen.
There is one more thing worth holding on to. A clean filter at a moderate rating restricts airflow less than a loaded filter at a fine rating. The replacement schedule does more for a system than a rating upgrade does, and a filter left in too long is harder on the equipment than a coarse one changed on time.
Lined up against each other
| MERV band | What the test method describes | What it does not settle |
|---|---|---|
| MERV 1–4 | Coarse capture. Historically the flat panel filters sold to keep large debris out of the equipment. | Nothing in this catalog sits in this band. |
| MERV 5–8 | The band most residential systems are specified around. Six filters in this catalog fall here. | Still a question for the system documentation, not a default. |
| MERV 9–12 | Finer media across a wider size range. Three filters here sit at MERV 11 or 12. | Resistance rises with the rating, and the number itself does not say by how much. |
| MERV 13–16 | Finer again. Commonly specified in commercial equipment built for the pressure drop. | Not stocked here, and not something to fit to a residential system on assumption. |
What this scale does not tell you
- How much resistance the filter adds, which is the number the equipment actually cares about.
- Whether your system is designed to run with media that fine — only the equipment documentation says that.
- Anything about health outcomes in a home. MERV describes a laboratory capture result, nothing more.
- How long the filter will hold that rating in service before it loads up.