MERV 4 explained

Researched from the AHAM AC-1 test method, EPA and CARB guidance, and published manufacturer specifications

Quick answer

A MERV 4 filter captures under 20 percent of particles in the 3 to 10 micron range under the ASHRAE 52.2 test method. In practice that means pollen, dust mites, sanding dust, carpet fibres. It is typically used in the filter most furnaces ship with.

MERV is a standard, defined by ASHRAE 52.2, and the rating is the worst case result across the test rather than the best.

MERV 4 against its neighbours, ASHRAE 52.2 capture ranges
MERV	0.3 to 1 micron	1 to 3 micron	3 to 10 micron	Typical use
MERV 2	not rated	not rated	under 20 percent	basic disposable fibreglass panels
MERV 3	not rated	not rated	under 20 percent	basic disposable panels
MERV 4	not rated	not rated	under 20 percent	the filter most furnaces ship with
MERV 5	not rated	not rated	20 to 34.9 percent	better residential pleated panels
MERV 6	not rated	not rated	35 to 49.9 percent	residential pleated panels

A filter in this part of the scale

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.

$35.87Check pricePrices change often

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 does MERV 4 actually stop?

Pollen, dust mites, sanding dust, carpet fibres. If the filter in your return is translucent blue fibreglass, it is almost certainly this class.

The three columns in the table are the three particle size bands ASHRAE 52.2 measures separately: 0.3 to 1 micron, 1 to 3 micron, and 3 to 10 micron. A filter earns its MERV number from its worst performance across those bands, not its best, which is what stops a filter claiming a headline figure it only reaches on the easiest particles.

Can a home system take one?

MERV 4 in a one inch residential frame is about as gentle on airflow as a filter can be while still doing something useful.

The failure mode is not the filter, it is the system. A denser filter left in place too long restricts airflow, and a restricted system heats and cools worse while the blower works harder. The practical rule is to step up in MERV and step up in how often you change it at the same time.

What it costs in airflow

Every step up the scale adds resistance. The number that matters is static pressure across the filter, and the number most people can actually observe is how quickly the filter loads up and how the system sounds when it does.

A denser filter also captures more, which means it clogs sooner at the same dust load. That is not a fault, it is the filter working, but it does mean a MERV 4 filter changed on the schedule you used for a cheap fibreglass panel will spend part of its life restricting the system.

Furnace and HVAC filters

Filtrete
Filtrete 16x25x1 Air Filter, MERV 11, MPR 1000, Allergens & Pet Dander, 3-Month AC and Furnace Filters, Cleaner Air, Reliable Airflow, 2 HVAC Filters (Actual Size 15.68 x 24.68 x 0.81 in)
- app or voice
$21.99 Check price
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Simply
Simply 20x20x1 Air Filter, Merv 8, MPR 600, 4 Pack, Furnace Air Filter for HVAC (Actual Size: 19.75"x19.75"x0.75") DUST, Pet, & Allergy Control

$25.46 Check price
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Filtrete
Filtrete 16x20x1 Air Filter, MERV 13 MPR 1900, Allergens, Bacteria, Viruses & Air Pollutants, 3-Month AC/Furnace Filters, Cleaner Air Reliable Airflow, 2 HVAC Filters(Act Size 15.62x19.62x0.78 in)
- app or voice
$28.99 Check price
Prices change often
Filterbuy
Filterbuy 20x20x1 Air Filter MERV 8 Dust & Allergy Defense (4-Pack)

$29.96 Check price
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Aerostar
Aerostar Air Filter 20x20x1 MERV 8 AC Filter, HVAC, Furnace Filter, 6 Pack

$32.45 Check price
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Simply
Simply 16x25x1 Air Filter, Merv 11, 4 Pack, For Home AC Furnace & HVAC (Actual Size: 15.75"x24.75"x0.75") DUST, Pet, & Allergy Control

$34.97 Check price
Prices change often

Common questions

Is MERV 4 good enough for a house?

For most homes it protects the equipment more than it cleans the air, and a pleated filter two or three steps up costs very little more. Check what the system manufacturer sanctions before going denser.

Does a higher MERV number always mean cleaner air?

Only if the system can still move air through it. Filtration is a rate, not a barrier: a denser filter that cuts airflow by a third can deliver less clean air per hour than a lesser filter that lets the blower run freely. The best filter for a system is the densest one it can pull through without losing meaningful flow.

How does MERV compare with HEPA?

They are different scales for different equipment. True HEPA is defined as capturing 99.97 percent of particles at 0.3 micron, which is beyond the top of the MERV scale. MERV 16, the highest rating in ASHRAE 52.2, captures 95 percent or more in that size range. A ducted system takes MERV filters, and a portable purifier takes a HEPA filter behind its own fan.

How often should a MERV 4 filter be changed?

More often than a cheap panel in the same house, because it is capturing more. A quarterly check and a change when the media looks loaded is a reasonable default, and homes with pets or a dusty season will be shorter. The filter that is restricting airflow does not look dramatic, it just looks grey.

Will a better furnace filter replace a portable purifier?

Only while the blower is running. A ducted filter cleans the whole house when the system is moving air and does nothing when it is off, which in mild weather can be most of the day. A portable machine cleans one room continuously. The two are complementary rather than alternatives.