The Restock List MERV ratings, decoded →

What size HVAC filter do I need?

Filter size is the one purchase detail you can't approximate — a 16x25 doesn't stretch into a 20x25 slot. Here's how to find your size in 30 seconds, what "nominal" actually means, and the sizes that cover most American homes.

Find your size in 30 seconds

Read the old filter's frame. Every filter prints its nominal size on the cardboard edge — e.g., 16x25x1 (width × height × depth, inches). That's the number to buy.
No old filter? Measure the slot or the old filter itself and round UP to the nearest standard size — a filter measuring 15.5 × 24.5 is a nominal 16x25. (Nominal sizes are labels; actual dimensions run about half an inch smaller.)
Two filters? Many homes have a filter at the furnace/air handler AND behind return grilles. Check both; they're often different sizes. Change all of them.

Product links go to Amazon. As an Amazon Associate I earn from qualifying purchases, at no extra cost to you. Each size link below opens Amazon's results for that exact size and MERV rating. Before you add one to the cart, check that the size on the listing matches the size printed on your old filter's frame.

Found your size? Get it by size and MERV

Nominal size	Typical spot	Shop this size
16x25x1	the most common furnace/air-handler slot	MERV 8	MERV 11	MERV 13
20x25x1	the most common furnace/air-handler slot	MERV 8	MERV 11	MERV 13
16x20x1	the most common furnace/air-handler slot	MERV 8	MERV 11	MERV 13
20x20x1	older systems and return grilles	MERV 8	MERV 11	MERV 13
14x25x1	older systems and return grilles	MERV 8	MERV 11	MERV 13
12x24x1	return grilles	MERV 8	MERV 11	MERV 13
16x25x4	media cabinet (6–12 month cycle)	MERV 8	MERV 11	MERV 13
20x25x4	media cabinet (6–12 month cycle)	MERV 8	MERV 11	MERV 13

Which MERV? Higher MERV catches finer dust but adds airflow resistance. Many older or builder-grade systems run happiest at 8–11, and a 1-inch slot pushed to MERV 13 can strain the blower. The 3-minute MERV decoder is here.

Depth matters as much as face size: a 1-inch slot takes 1-inch filters on a ~90-day cycle, and 4–5-inch media filters last 6–12 months but only fit a media cabinet. Other sizes (like 16x25x5) are often specific to one cabinet brand, so match the old filter's label exactly.

How many to buy

At the ~90-day cycle, one 1-inch slot uses 4 filters a year: a 4-pack is a year, a 6-pack is about 18 months. If the house has two slots (furnace plus a return grille), double it. Media filters (4–5 inch) are usually a single filter every 6–12 months.

Multi-pack value: Simply Filters MERV 8, 6-pack, shown in 16x25x1. The listing carries the full size range.
Every size in the catalog: Filterbuy MERV 8, one of Amazon's biggest filter sellers (140,000+ ratings across sizes), made in the USA, from 10x10 to 30x36.

Common questions

Is nominal size the same as actual size?

No — nominal is the rounded label; actual runs roughly ¼–½ inch smaller so the filter slides in. Buy by the nominal number printed on the frame, not your tape measure, unless there's no frame to read.

What if my size is odd (like 17.5x21)?

Some air handlers use non-standard sizes. Filterbuy and similar catalog brands make custom and odd sizes — search the exact measurement; don't force a wrong size or leave gaps around the frame.

Can I use a 1-inch filter in a 4-inch slot?

No — it'll rattle and bypass air around it. Media cabinets need media filters; the depth on the frame has to match the slot.