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How to Replace an AC Air Filter: Find the Size and the Direction

Replacing a filter takes five minutes once you know two things: the size printed on the old one and which way the arrow points. Here is how.

By Jamilur Rahman Updated November 5, 2026 · 7 min read

Time
5 min
Cost
$20-$50
Difficulty
Easy
A dusty gray pleated air filter partly pulled out of a wall return grille with a clean white filter beside it
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. How to read the size and the MERV rating
  9. Window AC, portable unit or mini-split
  10. What to skip
  11. FAQ
  12. Related problems

Quick answer

To replace an air filter, turn the system off, pull out the old filter and read the size printed on its edge. Buy that exact size, slide in the new filter with the arrow pointing toward the blower and away from the room, and write the date on the frame. Check it every month, and replace it every one to three months. The filter has to be bought, so keep a spare on hand.

Free fix vs. permanent fix

Free fix · 5 minutes

Check and vacuum the old filter

  • Hold the old filter up to a light. If you cannot see through it, it is done
  • Vacuum the grille and the slot so dust does not drop in with the new one
  • Write down the size and the arrow direction before you toss it
  • Never run the system with no filter

How long it lasts: A short extension at most. A clogged paper filter cannot be rinsed or cleaned for good.

Most readers choose Permanent fix
Pleated HVAC air filter in the right size (illustration)
Illustration

A pleated filter in the right size, changed on a schedule

A filter catches dust before it reaches the blower and coil. A clogged one chokes the airflow, so the system works harder and cools or heats less. A pleated filter has more surface area than a flat one and holds more dust.

What to get: Pleated HVAC air filter in the right size

How long it lasts: One to three months per filter. The habit is what keeps airflow and air quality steady.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Flashlight

    To read the size and arrow, and to check the slot

    You have it

  • Marker

    To write the date on the new filter frame

    You have it

  • Vacuum with brush attachment

    To clean the grille and the filter slot

    You have it
  • Our pick
    Pleated HVAC air filter in your size (illustration)
    Illustration

    Pleated HVAC air filter in your size

    The size is printed on the edge of the old one

    Check price
  • Multi-pack of pleated filters (illustration)
    Illustration

    Multi-pack of pleated filters

    Keeps a spare ready and cuts the cost per filter

    Check price
  • MERV 13 pleated filter (illustration)
    Illustration

    MERV 13 pleated filter

    Only if your system's manual says it can handle it

    Check price

Why it happens

1

Not knowing the size

Filters come in dozens of sizes. A filter that is too small leaves gaps for dust to slip around, and one that is too big will not fit.

2

Putting it in backward

The arrow on the frame shows the direction of airflow. A filter fitted the wrong way around loads up faster and can pull away from its frame.

3

Not changing it often enough

A dirty filter blocks airflow. The system struggles, the house stays warm or cold and dust builds up on the coil.

4

Mixing up types

A window AC, a portable unit or a mini-split uses a washable mesh filter instead. A central system uses a paper pleated one.

Try this first (free)

Do these in order, starting with the simplest.

  1. Turn the system off

    Switch it off at the thermostat. Find the filter. It is usually behind a return grille on a wall or ceiling, or in a slot on the furnace or air handler.

  2. Pull out the old filter and read the size

    Tool used: Flashlight

    Slide it out slowly so dust does not fall. Look for the size printed on the cardboard edge, such as 16 x 25 x 1. Use a flashlight to read it, and to see which way the arrow points.

  3. Clean the slot

    Tool used: Vacuum with brush attachment

    Vacuum the grille, the slot and the area around it. Check that nothing blocks the opening.

  4. Fit the new filter, arrow toward the blower

    Tool used: Pleated HVAC air filter in your size

    Slide it in with the arrow pointing toward the blower or furnace and away from the room. For a wall or ceiling return, that means the arrow points into the wall or ceiling. Close the grille so it clicks.

    Pleated HVAC air filter in your size (illustration)
    Illustration

    Tool for this step

    Pleated HVAC air filter in your size · The size is printed on the edge of the old one

    Check price on Amazon

  5. Date it and set a reminder

    Tool used: Marker

    Write the date on the frame. Set a phone reminder for one month to check it, and replace it every one to three months. Change it sooner with pets, allergies, smokers or a lot of dust.

When free fixes aren't enough

You cannot clean a paper pleated filter and use it again, so the only fix for a dirty one is a new one. Call a pro if you cannot find the filter, if the slot is a different size from any standard filter, or if you see ice on the coil, a wet filter or mold. Stop the system if you smell burning. Also check the manual before you move to a higher-grade filter, because some systems cannot handle the extra resistance.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
Pleated HVAC air filter in the right size (illustration)
Illustration
Pleated HVAC air filter in the right size
a dirty or wrong-size filter that blocks airflow and lets dust through Most homes with central air or a furnace. MERV 8 to 11 balances dust capture and airflow. Check price
Budget pick
Multi-pack of pleated filters (illustration)
Illustration
Multi-pack of pleated filters
running out of filters and paying more for single ones Anyone who wants to change the filter on time and save on each one. Check price
Upgrade pick
MERV 13 pleated filter (illustration)
Illustration
MERV 13 pleated filter
fine dust, pet dander and pollen that lower-grade filters pass Allergy sufferers and homes with pets, if the system is rated for it. Check price
Best pick
Pleated HVAC air filter in the right size (illustration)
Illustration

Pleated HVAC air filter in the right size

Fixes: a dirty or wrong-size filter that blocks airflow and lets dust through

Look for: The exact nominal size printed on your old filter, such as 16 x 25 x 1; A pleated design, with a MERV rating of about 8 to 11; A sturdy frame that does not bend when you slide it in; A direction arrow printed on the frame.

Who it's for
Most homes with central air or a furnace. MERV 8 to 11 balances dust capture and airflow.
Honest downside
Your system uses a washable filter, such as a window AC. Check the unit first.
Check price on Amazon
Budget pick
Multi-pack of pleated filters (illustration)
Illustration

Multi-pack of pleated filters

Fixes: running out of filters and paying more for single ones

Look for: The same nominal size as your old filter; A pack of 4, 6 or 12 so you have spares; A MERV 8 rating, which is a good starting point; A frame and pleats that are stable.

Who it's for
Anyone who wants to change the filter on time and save on each one.
Honest downside
You are not sure of your size. Measure and check the slot first, so you do not buy a whole pack that does not fit.
Check price on Amazon
Upgrade pick
MERV 13 pleated filter (illustration)
Illustration

MERV 13 pleated filter

Fixes: fine dust, pet dander and pollen that lower-grade filters pass

Look for: The right nominal size; A MERV 13 label; A note that your system's maker allows this grade.

Who it's for
Allergy sufferers and homes with pets, if the system is rated for it.
Honest downside
The manual does not allow it, or the blower is weak. A denser filter can restrict airflow and strain the system.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on HVAC filter-size and MERV guides, public energy and air-quality advice, system manuals, and what owners report in reviews of pleated filters. We do not test products by hand.

How to read the size and the MERV rating

The size on the edge is the nominal size, such as 20 x 25 x 1. The real filter is a little smaller, often by about a quarter inch, and that is normal. If no size is printed, measure the slot's length, width and depth, and round to the nearest whole inch. MERV runs from 1 to 16. MERV 8 to 11 is a good fit for most homes, and 13 is for allergies if the system can take it.

Window AC, portable unit or mini-split

These use a reusable mesh filter, not a paper one. Slide it out of the front of the unit, rinse it in lukewarm water with a little mild soap, let it dry fully and put it back. Do this every two to four weeks in summer. Replace it if it is torn or bent.

What to skip

Taping, bending or cutting a filter to fit

A filter that is too small leaves gaps around the edges, so dust goes straight past it. Cutting or folding one damages the media. Buy the exact size, or check why your slot is odd.

Frequently asked questions

How do I fix an air filter in an AC?
Turn the system off, slide out the old filter, and fit a new one of the same size with the arrow pointing toward the blower. For a window AC or mini-split, slide out the mesh filter, rinse it, dry it fully and put it back.
How do I fix an air filter that does not fit or is loose?
Check the size on the old filter against the slot. Nominal and actual sizes differ slightly, and that is normal. If the new one is loose or whistles, you bought the wrong size or the slot has a gap. Buy the exact size, and see the related page on whistling filters.
Which way does the arrow on an air filter point?
It points the way the air flows, toward the blower or furnace and away from the room. In a wall or ceiling return, the arrow points into the wall or ceiling.
How often should I change my air filter?
For a 1-inch filter, check it monthly and change it every one to three months. Thicker filters of 4 to 5 inches last about 6 to 12 months. Change them sooner if you have pets, allergies or a lot of dust.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

Pleated HVAC air filter in the right size (illustration)
Illustration

Our pick: Pleated HVAC air filter in the right size

Check price