Know your system · Filters

What does the MERV number on your filter mean?

Short version: the MERV number on a filter tells you how well it catches dust and other tiny bits in the air. It goes from 1 to 16. The higher the number, the more it catches.¹ If you want cleaner air, the U.S. Environmental Protection Agency (EPA) suggests at least MERV 13, or the highest number your system can handle.² Not every system can handle a high number, so it is worth asking a heating and cooling pro what yours can take.²

What does the number mean?

MERV is short for Minimum Efficiency Reporting Value. Most people just say "MERV." The number comes from a lab test written by ASHRAE, a group of heating and cooling engineers.¹ The test checks how much a filter catches at three sizes, from bits far too small to see up to larger dust.^{1, 4} For scale, a human hair is about 70 microns wide. The smallest size in the test is under 1 micron.^{1, 4}

A MERV 1 to 4 filter catches less than 20% of even the largest size in the test. A MERV 13 catches at least half of the smallest size and 85% or more of the bigger ones.¹

A furnace filter is made to catch particles, like dust and pollen. It is not made to catch gases. Removing gases takes a different kind of filter, such as a portable air cleaner with an activated carbon filter.²

Some packages show a different number, like FPR or MPR. Those are rating scales owned by certain brands, not MERV.² A number on one of those scales is not a MERV number, so look for the MERV number to compare filters fairly.

What does the EPA suggest?

If you decide to upgrade, the EPA suggests at least MERV 13, or as high as your system can handle.² That second part matters. Every filter makes your system work a little to push air through it, and some make it work harder than others. The MERV number alone does not tell you how hard. How the filter is built and how thick it is matter too.⁵ The EPA suggests asking a heating and cooling technician to be sure a filter will work with your system.² Your next tune-up is a good time to ask.

A thicker filter with more folds usually lets air through more easily than a thin one with the same number.⁵ Some systems have room for one. Others would need a bigger filter box, which can mean changing the ducts. That is a job for a pro.⁵

When you change a filter, make sure it fits with no gaps around the edges, and put the slot cover back on snugly. Air that slips around a filter does not get filtered.⁵

What a better filter will not fix

A better filter still needs changing. Every filter needs to be replaced regularly, and a dirty, overloaded one will not work well.² Your system also only filters air while it is running.² Check your filter each month with the light test in the air filters guide.⁷ Monthly checks show how fast yours gets dirty. If nothing else, changing it at least every 3 months is a good rule of thumb.⁶ If remembering is the hard part, FilterPing puts check and change dates on your own calendar.

MERV vs. HEPA: where the line is

HEPA is a different, stricter standard. A HEPA filter has to catch at least 99.97% of particles at 0.3 microns, the size that is hardest for a filter to catch.³ MERV stops at 16. At the smallest size in the MERV test, a MERV 13 has to catch at least half, and a MERV 16 at least 95%. HEPA is not rated on the MERV scale at all.¹

The tradeoff is airflow. A HEPA filter usually makes it too hard for a home system to push air through, so it does not belong in the regular furnace filter slot.⁵ When a system struggles to move air, it can lose efficiency, overheat a furnace or make the air conditioner freeze up.⁵ So HEPA at home usually comes one of two ways: a portable air cleaner for one room, or separate equipment a pro adds to the ducts.^{2, 5}

MERV 13 in the filter slot

• Pro: works in the system you already have, if it can handle it.²
• Pro: filters air for the whole house while the system runs.²
• Pro: it is the upgrade the EPA suggests.²
• Con: catches fewer of the tiniest particles than HEPA.^{1, 3}
• Con: not every system can handle it, so check yours first.^{2, 5}

HEPA

• Pro: catches far more of the tiniest particles than any MERV filter.^{1, 3}
• Pro: a portable HEPA air cleaner works on its own, with no changes to your system. Studies of portable HEPA cleaners found they helped some people's allergy and asthma symptoms.²
• Con: too much for the regular furnace filter slot.⁵
• Con: a portable unit cleans one room.² Whole-house HEPA is separate equipment added to the ducts.⁵
• Con: like a furnace filter, it catches particles, not gases.^{2, 3}

For most homes, the answer is simple. Use the highest MERV your system can handle in the filter slot, MERV 13 if it fits.² If someone in the house needs cleaner air, put a portable HEPA air cleaner in the room where they spend the most time,² or ask a pro about a whole-house setup.

Picking yours

• Check the frame or the package of the filter you have now for its MERV number.
• Compare MERV to MERV, not to a brand's own scale.²
• If you want cleaner air, aim for MERV 13, as long as your system can handle it.²
• Get the right size. The size printed on the old filter can be wrong.⁷ Measure the space the filter fits into, look for a label at the furnace or grille, or ask a pro.
• Follow the replacement directions on the new filter, and keep checking it each month.

Where to go from here

Not sure what your system can handle? Or does the airflow seem weak after a new filter goes in? Call a pro and tell them what you have already checked. It helps them get to the answer faster.