What size air filter do I need for my furnace or AC?
Updated November 14, 2026
The fastest way to find your filter size is to read it off the filter you have now: it's printed on the cardboard frame, for example 16x25x1. If there's no filter or the label has worn off, you can measure the slot. Here's how, plus what the other numbers on the box mean.
COMPZA may earn a commission when you buy through links on this page, at no extra cost to you. How we make money.
Reading the size on your current filter
Sizes are written width x height x depth in inches. The large, bold number (like 20x25x1) is the nominal size you shop by; the smaller actual size is usually about half an inch less, so the filter slides into the slot.
Always buy the same nominal size and depth. A 1-inch filter won't fit a slot made for a 4-inch media filter, and the other way round.
No label? Measure the slot
Measure the width, height and depth of the filter slot or the old filter, then round up to the nearest whole inch to get the nominal size most stores list.
Some homes have more than one return vent with its own filter; check each one.
MERV ratings in plain English
MERV measures how well a filter catches particles. MERV 8 is a common everyday choice for dust and pollen; MERV 11 to 13 catch finer particles such as pet dander and some smoke.
Higher MERV filters can restrict airflow on some systems. If you're not sure, check your furnace or AC manual for the recommended range.
How often to change it
Most 1-inch filters are changed every 1 to 3 months; deeper 4- to 5-inch filters often last 6 to 12 months. Change more often with pets, allergies, construction dust or heavy use in summer and winter.
Quick checklist
- Nominal size and depth from the old filter's frame
- Same depth as your filter slot (1, 2, 4 or 5 inches)
- MERV rating your system's manual recommends
- A multi-pack if you change filters every few months
See current prices
Live lists from our partner stores, updated hourly:
Questions
What's the difference between nominal and actual filter size?
Nominal is the rounded size you shop by (e.g. 16x25x1). Actual is the exact measurement, usually about half an inch smaller so the filter fits the slot.
Can I use a higher MERV filter?
Often yes, but very high MERV filters can reduce airflow in some systems. Check your system's manual for its recommended range.
Which way does the arrow go?
The arrow on the frame points toward the furnace or air handler, in the direction the air flows.
Where to shop
These links open search results at retailers we partner with. The retailer controls listings, prices and availability, so check them before you buy.
Some links are affiliate links. COMPZA may earn a commission from qualifying purchases, at no extra cost to you. Learn more.
Describe exactly what you need →
📬 Get the weekly deals email
One email a week with the best current deals from our partner stores: home, kitchen, tech, car and gifts. No spam, unsubscribe anytime.
Emails include affiliate links; COMPZA may earn a commission at no extra cost to you. See our privacy policy.