Furnace Filters
Best 21x21 AC Filter (2026): Top 4 by the Specs
As an Amazon Associate I earn from qualifying purchases. Specs below come from each product's Amazon listing; we do not test products in person.
For a 21x21 slot, the Filtrete leads this list: a MERV 11 rating in a 6-pack with 3-month pleated filters. The Runner-Up Aerostar matches the MERV 11 rating in a 6-pack, while the Glasfloss gives a MERV 10 rating in a 5-pack and the Filter King gives a MERV 8 rating in a 6-pack.
Starting out? See the main guide: Best Furnace Air Filters
Top picks at a glance
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Best OverallFiltrete 21x21x1 Air Filter9.8 View on Amazon -
9.4 View on Amazon -
Also GreatFilter King 21x21x1 Air Filter MERV9.3 View on Amazon -
Also GreatGlasfloss ZL 21x21x1 MERV 10 Pleated AC9.2 View on Amazon
Compare the specs
Ranked on pack size from each listing, plus how many buyers have rated it. A dash (—) means we could not confirm that figure from the listing.
| Pick | MERV rating | Pack size | Weight | Warranty |
|---|---|---|---|---|
| 1. Filtrete 21x21x1 Air Filter | MERV 11 | 6-pack | — | — |
| 2. Aerostar Air Filter 21x21x1 MERV 11 AC | MERV 11 | 6-pack | — | — |
| 3. Filter King 21x21x1 Air Filter MERV | MERV 8 | 6-pack | — | — |
| 4. Glasfloss ZL 21x21x1 MERV 10 Pleated AC | MERV 10 | 5-pack | — | — |
Filtrete 21x21x1 Air Filter
It earns Best Overall with a MERV 11 rating, a 6-pack and 3-in-1 technology for trapping particles.
- MERV rating: MERV 11
- Pack size: 6-pack
Good for
- Buyers who want a MERV 11 filter in a 6-pack
- Buyers who want app reminders for filter changes
Skip it if
- You want a MERV rating above 11
- You want fewer than 6 filters in a pack
It has a MERV 11 rating with a 1000 MPR and a 6-pack of 3-month pleated filters. The actual size is 20.81x20.81x0.81 in, and change intervals run at least once every 90 days. The Filtrete Smart App sets 30-, 60- or 90-day reminders.
Aerostar Air Filter 21x21x1 MERV 11 AC
It earns Runner-Up with a MERV 11 rating, a 6-pack and electrostatically charged synthetic media.
- MERV rating: MERV 11
- Pack size: 6-pack
Good for
- Buyers who want a MERV 11 filter that traps up to 95% of airborne particles
- Buyers who want a pleated filter with wire backing
Skip it if
- You want a MERV rating below 11
- You want a filter thinner than 0.75 inches
It has a MERV 11 rating with a 1200 MPR and a 6-pack. The actual size is 20.88 x 20.88 x 0.75, and the pleated media is electrostatically charged synthetic media on a beverage board frame with wire backing.
Filter King 21x21x1 Air Filter MERV
It pairs a MERV 8 rating with a 6-pack and a pleated design that captures more particles.
- MERV rating: MERV 8
- Pack size: 6-pack
Good for
- Buyers who want a MERV 8 filter in a 6-pack
- Buyers who want a filter that lasts up to 90 days
Skip it if
- You want a MERV rating above 8
- You want a pack of fewer than 6 filters
It has a MERV 8 rating and a 6-pack with an actual size of 20.5 x 20.5 x .75. The pleated design captures more particles without impacting air flow, and filters last up to 90 days before needing replacement. Change intervals run every 30 to 90 days for 1 inch filters.
Glasfloss ZL 21x21x1 MERV 10 Pleated AC
It pairs a MERV 10 rating with a 5-pack and a heavy-duty frame for residential and commercial use.
- MERV rating: MERV 10
- Pack size: 5-pack
Good for
- Buyers who want a MERV 10 filter in a 5-pack
- Buyers who want a heavy-duty frame that will not buckle
Skip it if
- You want a MERV rating above 10
- You want a pack of 6 filters
It has a MERV 10 rating and a 5-pack with an exact size of 20-7/8 x 20-7/8 x 7/8. The heavy-duty frame will not buckle, and air flow and resistance stay similar to a MERV 8 filter. Change intervals run at least every 3 months.
How to choose
Match MERV rating to the job
MERV ratings set how fine the trapped particles are. The Filtrete and Aerostar filters carry a MERV 11 rating, the Glasfloss carries a MERV 10 rating and the Filter King carries a MERV 8 rating. A higher MERV rating traps finer particles, so match the rating to the dust and particles in the home. Check the rating on the pack before buying.
Check the actual size first
Nominal size and actual size differ on every filter here. The Filtrete measures 20.81x20.81x0.81 in, the Aerostar measures 20.88 x 20.88 x 0.75, the Filter King measures 20.5 x 20.5 x .75 and the Glasfloss measures 20-7/8 x 20-7/8 x 7/8. Measure the slot before ordering, since a filter that does not fit the slot will not sit right.
Plan the change interval
The Filtrete and Filter King filters run at least once every 90 days, with the Filter King giving a 30 to 90 day range for 1 inch filters, and the Glasfloss runs at least every 3 months. The Filtrete Smart App sets 30-, 60- or 90-day reminders. Buy the pack size that covers the next few changes.
FAQ
What MERV rating do these 21x21 filters carry?
The Filtrete and Aerostar carry a MERV 11 rating, the Glasfloss carries a MERV 10 rating and the Filter King carries a MERV 8 rating.
How many filters come in each pack?
The Filtrete, Aerostar and Filter King come in 6-packs, and the Glasfloss comes in a 5-pack.
What replacement interval do these filters carry?
The Filtrete runs at least once every 90 days, the Filter King runs every 30 to 90 days for 1 inch filters, and the Glasfloss runs at least every 3 months.
Do these filters fit a standard 21x21 slot?
Nominal size is 21x21x1 on all four, but actual sizes differ: the Filtrete is 20.81x20.81x0.81 in and the Glasfloss is 20-7/8 x 20-7/8 x 7/8, so measure the slot first.
