Air & furnace filters size chart

Frequently Asked Questions (FAQs)

What size furnace filter do I need?

Check your current filter. Most filters have the size printed on the cardboard frame — three numbers showing length, width, and thickness (example: 20x25x1). If the print is faded, measure the filter slot with a tape measure. Measure length, width, and depth in inches and round to the nearest whole number. This gives you the nominal size, which is the standard size sold by filter manufacturers.

How often should I change my furnace filter?

Replace standard 1-inch furnace filters every 60 to 90 days. Homes with pets, allergies, or heavy dust may need a change every 30 to 45 days. Thicker filters (4-inch or 5-inch) can last 6 to 12 months because they hold more particles before airflow drops. A clogged filter makes your HVAC system work harder, raises energy costs, and lowers air quality.

Which way does the arrow go on a furnace air filter?

The arrow on a furnace filter shows the direction of airflow. Point the arrow toward the furnace or air handler and away from the return air duct. Air moves from your rooms into the return duct, passes through the filter, and enters the furnace. The arrow should always face the furnace. Installing a filter backward restricts airflow and reduces filtration performance.

How do I install a furnace filter?

Turn off your HVAC system first. Open the filter access panel — this is a slot or hinged door on the return air duct or on the furnace unit. Slide the old filter out. Slide the new filter in with the airflow arrow pointing toward the furnace. Close the panel and turn the system back on. No tools are needed, and the process takes under two minutes.

Where is my furnace filter located?

The furnace filter sits between the return air duct and the blower. Common locations include a slot inside the furnace near the blower compartment, a filter rack where the return duct meets the furnace, or behind a large return air vent on a wall or ceiling. In homes with horizontal HVAC units (attics or crawl spaces), the filter is usually where the return duct connects to the unit.