How Often Should You Change Your Air Filter, and What Size Do You Need?

Most 1-inch HVAC filters should be changed every 1 to 3 months, and thicker 4- or 5-inch media filters usually last 6 to 12 months. Change them more often if you have pets, allergies, construction dust, or a system running nonstop in peak summer or winter.

How to find your filter size

The size is printed on the edge of the filter you already have, as three numbers like 16x25x1: width, height, and depth in inches. Buy the same numbers. The filter will measure slightly smaller than the label, so a 16x25x1 filter is often closer to 15.5 by 24.5 by 0.75 inches. That is normal. Filters are sold by their rounded nominal size.

If there is no old filter, or the label is unreadable, measure the slot or the filter grille opening and round up to the nearest inch. Then write the size somewhere you will find it, like on the furnace cabinet with a marker or in your phone.

Where the filter is

• In a return grille. A large grille in a wall or ceiling that swings open, often in a hallway. Many homes with a closet or attic air handler have the filter here.
• At the furnace or air handler. A slot on the side of the unit where the return duct meets it, sometimes behind a removable panel.
• In a media cabinet. A wider box between the return duct and the furnace that holds a 4- or 5-inch filter.
• More than one. Some homes have a filter at every return grille. Change all of them.

Which MERV rating to buy

MERV is a rating from 1 to 16 for residential filters. Higher numbers catch smaller particles. For most homes, a pleated MERV 8 to 11 filter is a good balance of air quality and airflow. MERV 13 catches finer dust, smoke, and allergens and is a common recommendation for allergy sufferers, but in a 1-inch size it can restrict airflow on some older or tight systems.

If you move to a higher MERV filter and notice weaker airflow, a noisier system, or rooms that cool or heat more slowly, step back down or change it more often. If you want high filtration without the restriction, a 4- or 5-inch media cabinet is the usual answer, and it is a job for an HVAC pro.

When to change it more often

• Pets. Pet hair and dander load filters fast. Monthly is a good starting point with dogs or cats.

• Allergies or asthma. A fresh filter does its job better. Many people with allergies change monthly during high pollen months.

• Construction or remodeling. Drywall dust can clog a filter in days. Check it weekly during the work and change it right after.

• Peak season. In the hottest and coldest months the system runs the most, and the filter fills faster.

• Wildfire smoke or heavy pollen. Check the filter after smoky or high pollen stretches and replace it if it looks dark.

The best rule is to look at it. Pull the filter every month and hold it up to a light. If you cannot see light through it, or it is visibly gray, change it, whatever the package says.

What a dirty filter actually does

A clogged filter does more than let dust through. It starves the system of airflow, and that causes real problems. Weak airflow is a common reason an upstairs will not cool, it can cause an AC to freeze up, and in winter it can make a furnace overheat and shut off too soon. Each of those is a service call that a $10-$25 filter could have prevented.