MERV 9 vs MERV 11: the difference and which to buy
Quick answer
MERV 11 is the better filter when your system can take it, and MERV 9 is the gentler choice for an older or single speed blower. The difference is in the 1 to 3 micron band, which holds most pet dander, fine dust and a good share of cooking particles: MERV 11 is rated at 65 to 79.9 percent against under 50 percent for MERV 9 (ASHRAE Standard 52.2). The step up costs airflow, so it is worth it when the system can take it and the house has a reason: pets, allergies or smoke.
| MERV 9 | MERV 11 | |
|---|---|---|
| 0.3 to 1 micron | not rated | not rated |
| 1 to 3 micron | under 50 percent | 65 to 79.9 percent |
| 3 to 10 micron | 85 percent or more | 85 percent or more |
| Typically used in | residential and light commercial | the usual choice for pets or allergies in a system that can take it |
| Catches | everything MERV 8 catches, plus a share of finer dust | pet dander, fine dust, a good share of smoke and cooking particles |
Amazon Basics 20x20x1 Air Filter 6-Pack, MERV 11 AC Furnace Filters, Dust, Pet, Allergen Control, Electrostatic (Actual Size: 19.75 x 19.75 x.75)
Furnace filters are bought by the case and changed on a schedule, so the price per filter matters less than how often the system needs a fresh one.
Check the size printed on the frame of the filter already in the return before ordering, since the nominal size and the actual size differ by a fraction of an inch.
What is the difference between MERV 9 and MERV 11?
The difference is in the 1 to 3 micron band, which holds most pet dander, fine dust and a good share of cooking particles: MERV 11 is rated at 65 to 79.9 percent against under 50 percent for MERV 9. In the 0.3 to 1 micron and 3 to 10 micron bands the two are rated the same.
Typical use for MERV 9: residential and light commercial. The step where the test starts measuring the one to three micron range at all. Typical use for MERV 11: the usual choice for pets or allergies in a system that can take it. The best balance for most residential systems: a large gain over MERV 8 for a modest pressure cost.
Going from MERV 9 to MERV 11 is a real change in what reaches the room, and it is also a real change in how hard the blower works. Both are true at once, and the right answer depends on the system rather than on which number is larger.
When the step up is worth it
Three situations justify it: somebody in the house reacts to something airborne, there are pets, or smoke arrives from outside for part of the year. Outside those, the cheaper filter changed on time often beats the denser filter left in place too long.
The one hard constraint is the equipment. If the system manufacturer sanctions a maximum filter rating, that is the ceiling, and a variable speed blower will tolerate more restriction than a single speed one.
What it does to running cost
Denser filters cost more per unit and load faster, so the annual cost rises on both sides. Against that, a restricted system runs longer to reach the same temperature, and that shows up on the power bill rather than in the filter aisle.
Furnace and HVAC filters
- Filterbuy
Filterbuy 20x20x1 Air Filter MERV 8 Dust & Allergy Defense (4-Pack)
Under $50 Check priceBudget range; Amazon shows today's price - Aerostar
Aerostar Air Filter 20x20x1 MERV 8 AC Filter, HVAC, Furnace Filter, 6 Pack
Under $50 Check priceBudget range; Amazon shows today's price
Common questions
What is the difference between MERV 9 and MERV 11?
The difference is in the 1 to 3 micron band, which holds most pet dander, fine dust and a good share of cooking particles: MERV 11 is rated at 65 to 79.9 percent against under 50 percent for MERV 9, under the ASHRAE Standard 52.2 test. MERV 11 is the better filter when your system can take it, and MERV 9 is the gentler choice for an older or single speed blower.
Is MERV 11 much better than MERV 9?
In the 1 to 3 micron band, which is where pet dander and a good share of cooking particles sit, yes. MERV 11 is rated at 65 to 79.9 percent against under 50 percent for MERV 9. Whether that matters depends on whether anything in your house is producing particles in that band.
Will the denser filter damage my system?
Not by itself. What causes problems is a denser filter left in place past the point where it is loaded, which raises static pressure, cuts airflow and makes the blower work harder. Step up in filter rating and step up in how often you change it at the same time, and check what the equipment maker sanctions.
Which one for wildfire smoke?
Neither is the level the US EPA wildfire smoke guidance points at, which is MERV 13 or higher in a central system that can accommodate it. Between these two, the denser one. Smoke particles are mostly under one micron, so a portable machine with a HEPA filter in the room you occupy is the stronger move, and running both is better than either alone.
Do these ratings mean anything for portable purifiers?
Not directly. MERV describes filters for ducted systems under ASHRAE 52.2. A portable purifier is rated on clean air delivery rate under AHAM AC-1, which measures the whole machine rather than the media. Comparing a MERV number with a CADR number is comparing a filter to a machine.
How often should either be changed?
Look at the filter rather than the calendar. A quarterly check is a sensible habit, with a change whenever the media has gone visibly grey across its face. Homes with pets, dusty seasons or a smoke event will need it sooner, and the denser filter will always be first.
