How often should you change your air filter?

Your schedule

How often to change it, by filter type and household

A bigger house doesn't change the rule by itself. What matters is how many hours the system runs and how much dust, hair, and smoke the air carries. Filter change dates are on the monthly list in our home maintenance schedule, and many smart thermostats can remind you on your own interval.

Write the change date on the filter's frame with a marker. At the next monthly check, you'll know how long it has been in without guessing.

Filter ratings

MERV ratings explained

MERV, the Minimum Efficiency Reporting Value, measures how well a filter traps particles between 0.3 and 10 microns, according to the EPA. The scale runs from MERV 1 to 16, and a higher number means the filter is better at trapping particles.

• The EPA's recommendation: its Guide to Air Cleaners in the Home says to choose at least MERV 13, or the highest rating your system can accommodate.
• Most systems can handle MERV 13. The EPA says most furnaces and HVAC systems can use a MERV 13 filter without equipment problems, as long as it's replaced frequently. A higher-rated filter loads faster, so it's the one that most needs the monthly check.
• Ask if you're not sure. HVAC systems rarely carry a filter label, so an HVAC technician can confirm the highest rating your fan and filter slot support.
• Compare MERV, not brand scores. Some brands use their own scales on the package. Look for the MERV number when you compare filters from different brands.
• Skip filters that make ozone. The EPA advises against furnace and HVAC filters designed to produce ozone.

A furnace filter isn't a complete answer to indoor air. It cleans only while the system runs and doesn't fix the source of mold or musty odors. Our healthy home guide covers ventilation, humidity, and the other half of the job.

Buy the right one

How to find your air filter size

1. Read the frame of the filter you have. The size is printed on the edge as length by width by depth, such as 16 x 25 x 1. That's the nominal size you shop by.
2. Expect the real filter to be a little smaller. The nominal size is rounded up, and the actual filter is slightly smaller so it slides into the slot. A filter labeled 20 x 25 x 1 can measure closer to 19.5 x 24.5 inches.
3. No label? Measure the slot. Turn the system off, measure the filter slot's length, width, and depth with a steel tape, and round up to the nearest inch for the nominal size, as Filterbuy explains.
4. Check for more than one filter. Some homes have a filter at the furnace and others behind return grilles in the walls or ceiling. Find every one.
5. Point the arrow the right way. The airflow arrow on the frame points toward the blower and away from the return duct.

A filter that's too small lets air slip around it, and forcing in one that's too big can bend or tear it. Buy two at a time so a spare is ready at the next check. Our new homeowner essentials guide covers what to keep on hand.

What to look for

Signs your air filter is overdue

• It looks gray or coated. ENERGY STAR's rule is simple: if it looks dirty, change it, even if it has only been a month.
• Weaker airflow at the vents. ENERGY STAR notes that airflow problems can cut a system's efficiency by up to 15 percent.
• The system runs longer to reach the set temperature. Check the filter before you book a service call.
• It's past the maker's interval. Filtrete warns that a filter used past its rated life can restrict airflow and cause the heating or cooling system to malfunction.

A clogged filter is one of the easiest problems to rule out before a furnace repair. Our furnace lifespan guide covers the yearly service and the signs that point to bigger problems.

Air filter FAQ

Questions homeowners ask about HVAC filters