Where to find and measure your current filter

Your air filter lives in one of three places: in a return air vent on a wall or ceiling, inside a furnace or air handler cabinet, or in a filter rack mounted to the side of your HVAC unit. The easiest way to find it is to look for a large rectangular grate or panel in your home — usually in a basement, attic, garage, or utility closet — and follow the ductwork backward toward the unit itself.

Once you locate the filter, note its size. The dimensions are printed on the frame in a format like 16×25×1 or 20×20×4. Write these numbers down or take a photo. The first two numbers are width and height; the third is thickness. You need an exact match, or the filter will not seal properly and air will bypass it without being cleaned.

If you cannot find a size printed on the frame, measure the filter yourself with a tape measure. Measure the actual frame, not the pleated material inside. Most residential filters are 1 inch or 4 inches thick, and common sizes include 16×25, 20×25, and 20×20, but yours may differ.

Key Takeaways

  • Air filters are located in return air vents, inside furnace cabinets, or on filter racks attached to your HVAC unit, and you must know the exact size before buying a replacement.
  • Turn off your furnace or air handler before removing the old filter to prevent dust from circulating through your home and to keep the system from running while unfiltered.
  • Slide the new filter in with the arrow on its frame pointing toward the furnace or air handler, not toward the return vent, so air flows in the correct direction.
  • Most residential filters need replacement every one to three months depending on pets, allergies, and dust levels in your home.

Turn off your HVAC system before you start

Before you remove the old filter, turn off your furnace or air handler. Look for a switch on the unit itself or on the wall nearby, or turn off the breaker that controls the system at your electrical panel. Turning off the system prevents unfiltered air from circulating through your home while the filter slot is empty, and it keeps the blower from running while you work.

If you are unsure which breaker controls your HVAC system, look for a label on the panel door. If there is no label, turn off the main breaker as a last resort — this will shut down your whole house, but it is safe and will definitely stop the system.

Remove the old filter and note the airflow direction

Slide or pull the old filter straight out of its slot. It will be dusty, so do this slowly and try not to shake it. Once it is out, look at the frame. You will see an arrow printed on one edge. This arrow shows the direction air is supposed to flow through the filter. The arrow should point toward the furnace or air handler, not toward the return vent where air enters from your home.

This direction matters. If you install the new filter backward, air will still flow through it, but the filter will clog faster and work less effectively. The arrow is your guide for which way the new filter goes in.

Take a moment to look at the slot itself. Note whether the filter slides in horizontally, vertically, or at an angle. Some systems have a frame that holds the filter in place; others rely on friction alone. If there is a frame, you may need to open it or unhook it before the old filter comes out.

Buy the correct replacement filter

Go to a hardware store, home improvement store, or order online. Search for the size you wrote down — for example, "16x25x1 furnace filter" or "20x20x4 air filter." You will see many options at different price points. The main difference is the MERV rating, which measures how fine the filter's mesh is. Higher MERV ratings (13 to 16) catch smaller particles but restrict airflow more; lower ratings (8 to 11) let air flow more freely but catch fewer particles.

For most homes, a MERV 8 to 11 filter works well. If anyone in your home has allergies or asthma, a MERV 13 may help. Do not buy a filter higher than MERV 13 unless your furnace manual says it can handle it — too high a rating can strain your system.

Buy one filter for now, or buy several if you want to stock up. Filters do not expire, so having extras on hand means you will not have to rush to the store when it is time to replace it.

Install the new filter with the arrow pointing the right way

Take the new filter out of its packaging. Hold it so the arrow on the frame points toward the furnace or air handler — the direction you noted on the old filter. Slide the new filter into the slot slowly and straight, making sure it sits flush against the frame on all sides. You should feel slight resistance as it settles into place.

If the filter does not slide in smoothly, stop and check the direction. The most common mistake is installing it backward. If it still does not fit, double-check that the size is correct — a filter that is even half an inch off in any dimension will not seat properly.

Once the filter is in, close any frame or door that was holding the old filter. Make sure there are no gaps around the edges where air could slip past the filter without being cleaned.

Turn the system back on and mark your calendar

Turn the furnace or air handler back on using the same switch or breaker you used to turn it off. Listen for the blower to start. If it does not start within a minute, turn it off again and check that the filter is seated correctly — a filter that is not fully inserted can trigger a safety switch that prevents the system from running.

Once the system is running normally, mark your calendar for when to check the filter next. Most filters need replacement every one to three months. If you have pets, live in a dusty area, or anyone in your home has allergies, check it monthly. If you live alone in a clean area with no pets, every three months is usually enough. Hold the filter up to a light — if you cannot see light through it, it is time to replace it regardless of the calendar date.

Frequently Asked Questions

What if I cannot find the filter in my home?

Some systems have the filter in an unusual location — under a basement staircase, in a crawl space, or mounted vertically on a wall. If you have looked in the obvious places, check your furnace manual or look for a diagram on the furnace itself. If you still cannot find it, a heating technician can show you in a few minutes.

Can I use a filter that is a different size than the original?

No. A filter that does not fit exactly will leave gaps, and air will bypass the filter without being cleaned. Always match the size printed on your old filter or measure the slot yourself. If your slot is an unusual size, you may need to order online rather than buy at a store.

Do I need to turn off the system to change the filter?

You do not have to, but it is safer and cleaner to do so. Turning off the system prevents dust from being blown through your home while the filter slot is empty, and it keeps the blower from running while you work. The whole process takes only a few minutes, so the inconvenience is minimal.

What does the MERV rating mean, and which one should I buy?

MERV stands for Minimum Efficiency Reporting Value. A higher number means the filter catches smaller particles, but it also restricts airflow more. For most homes, MERV 8 to 11 is fine. If you have allergies or asthma, try MERV 13. Do not go higher than your furnace manual recommends.

How do I know if the filter is installed backward?

The arrow on the frame should point toward the furnace or air handler, not toward the return vent. If you installed it backward, the system will still run, but the filter will clog faster and work less effectively. If you are unsure, remove it and reinstall it with the arrow pointing the correct direction.