How Often Should I Change My Furnace Filter?

First off, every home is different, which means there’s no one set answer that works for everyone.

(But that’s a lame answer, right? You came here for answers so we’ll give you some guidelines.)

Our (very general) suggestion is to check your air filter every 1 to 3 months and change it when there’s a thin layer of dirt on the filter. Don’t wait for it to completely clog before you change it—doing so means you’ll pay higher energy bills and risk expensive furnace repairs.

But believe us, that’s a very general rule of thumb and isn’t the best answer for everyone. For more precise filter replacement tips, check out the steps below.

How to determine how often you should change your furnace filter:

Step 1: Determine your filter’s MERV and thickness.

Take out your current filter and check the side. Usually, manufacturers will include the MERV and dimensions on the long side of the filter. 

How manufacturers display a filter’s measurements: Length x Width x Depth. (See example here)

So, a filter with these dimensions: 16 x 25 x 1” is 1 inch thick.

The MERV should be labeled clearly beside these dimensions or on the opposite side of the filter. Sometimes, instead of saying “MERV 4” the manufacturer might just show “M4”.

Step 2: Match your filter’s MERV and thickness with the replacement tips below

• MERV 1-4 filters less than 2 inches thick should be checked every 3 months and replaced if dirty.
• MERV 5-8 filters less than 2 inches thick should be checked every 1 to 2 months and replaced if dirty.
• MERV 8+ filters under 2 inches thick should be checked every month and replaced if dirty.

Note: We don’t suggest using high-MERV filters (MERV 8+) if they are also less than 2 inches thick. These filters clog extremely fast which will eventually cause a pressure drop in your furnace system. Instead, if you want to use high-MERV filters, make sure that they are more than 2 inches thick and ideally are pleated like this 5-inch MERV 11 pleated filter. The thicker a high-MERV filter, the longer you can wait between filter replacements and the safer they are for your system.

Step 3: Replace your filter sooner if you answer “yes” to any of these questions:

• Do you have pets?
• Do you have a lot of guests over/ppl living in your home?
• Do you leave your thermostat fan setting set to ON instead of AUTO a lot?
• Do you keep windows/doors open?
• Do you (or anyone in your home) smoke?
• Do you have a particularly dusty home?
• Is it currently a peak winter month (December-February)?

Answer yes to any of these? Then decrease the time between filter replacements. 

For example, if you use a filter with a MERV of 4 that’s under 2 inches thick but have pets, you’d decrease the time between filter replacements from 3 months to maybe every 2 months. The more questions you answered “yes” to, the more you should decrease the time between replacements.

Remember: these are “rule of thumbs”, ask a NJ tech for specific advice

Have more furnace filter questions? Just ask us. We’re happy to help.

Related reading:

• MERV 8 vs MERV 13: Which Air Filter Is Best?
• How Often Should a Gas Furnace Be Serviced?