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What size air filter do I need?
Two ways to find it in under a minute: read it off your old filter, or measure the slot and let this tool round it to the size you buy. No more guessing in the hardware-store aisle.
Fastest: read the frame
Pull your current filter out. The size is printed on the cardboard edge — e.g. 20x25x1. That's the number you buy. Done.
No filter? Measure the slot
Enter the length, width, and depth you measured (in inches). We'll round to the nominal size you shop for.
Tip: measured dimensions are usually ~½" smaller than the nominal size — that's normal.
Common air filter sizes
The sizes most US homes use, and where each typically shows up.
Have more than one filter? Stop re-measuring.
Save each filter's size and where it goes to Domus. At the store you get a copyable shopping list; at the vent you get "which one goes where" — and a reminder before it's due. Free.
Save my filter sizes →Nominal vs. actual size — the thing that trips people up
Nominal is the rounded number printed on the frame and used on every box in the store — like 20x25x1. Actual is the real measured size, typically about half an inch smaller (≈19.5 × 24.5 × 0.75). You always shop by nominal; the gap is intentional so the filter slides in and out easily.
So if you measured your slot at 19.5 × 24.5, don't hunt for that — round up to the nominal 20x25 and you're right. That rounding is exactly what the tool above does.
Direction matters only sometimes. 20x25 and 25x20 are the same filter rotated. But if your filter has an airflow arrow, install it pointing toward the furnace/air handler.
Common questions
How do I find what size air filter I need?
Easiest: pull the old filter and read the size printed on its cardboard frame (e.g. 20x25x1). That number is the nominal size — the one you buy. If there's no filter or the print is worn, measure the slot it goes into (length × width × depth) and round to the nearest inch; that gives you the nominal size.
What's the difference between nominal and actual filter size?
Nominal is the rounded size printed on the frame and used to shop — like 20x25x1. Actual is the real measured dimension, usually about half an inch smaller (roughly 19.5 x 24.5 x 0.75). You buy by nominal size; the small gap is intentional so the filter slides in and out. Only worry about actual dimensions if a filter fits too loosely or too tight.
Does the order of the numbers matter (20x25 vs 25x20)?
For most filters no — a 20x25x1 and a 25x20x1 are the same filter, just rotated. The exception is filters with a directional airflow arrow or non-square media; install those with the arrow pointing toward the blower/furnace.
How often should I change my air filter?
Check monthly and replace every 1–3 months for a 1-inch filter. Homes with pets, allergies, or nearby construction need the short end. Thick media filters (4–5 inch) last 6–12 months. A clogged filter is the most common cause of avoidable HVAC repairs.
What MERV rating should I use?
MERV 8–11 is the sweet spot for most homes — good dust and pollen capture without straining the blower. MERV 13 captures finer particles (smoke, some viruses) but check your system can handle the extra resistance. Avoid very high MERV in older systems unless the manufacturer allows it.
The complete air-filter guide
Everything about choosing, sizing, and changing your home's air filters.