Furnace filter size
18x24x1 air filter
Used in larger return grilles and some furnace filter racks.
- Nominal size
- 18x24x1
- Usual actual size
- 17-1/2 x 23-1/2 x 3/4 in
- Thickness
- 1 inch
- Change it
- Every 90 days
Makers differ by up to about a quarter inch. Shop by the nominal size.
Which MERV to pick
MERV 8 is the safe default for a 1-inch filter. MERV 11 helps with allergies if your system moves air well. MERV 13 in a 1-inch filter can choke airflow on some older furnaces, so ask your HVAC company first.
How often to change it
Every 90 days is a good start for a 1 inch filter. Pets, allergies, a remodel or wildfire smoke shorten it; a filter that has turned grey is due whatever the date.
Buy this exact size
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18x24x1 questions
What is the actual size of a 18x24x1 filter?
Usually 17-1/2 x 23-1/2 x 3/4 inches. 18x24x1 is the nominal size, rounded up, and it is the size to shop by. Makers differ by up to about a quarter inch. Shop by the nominal size.
How often should I change a 18x24x1 filter?
Every 90 days is a good start for a 1 inch filter. Change it sooner with pets, allergies, construction dust or wildfire smoke, and whenever it has turned grey.
What MERV rating should a 18x24x1 filter be?
MERV 8 is the safe default for a 1-inch filter. MERV 11 helps with allergies if your system moves air well. MERV 13 in a 1-inch filter can choke airflow on some older furnaces, so ask your HVAC company first.
Can I use a 18x24x4 instead?
Only if the slot is deep enough. A 4-inch filter will not fit a slot built for 1 inch. Match the thickness printed on your current filter.
Get a text when it is due
Free. We will text or email you when this filter is due, with a link to this size.