Furnace filter size + MERV table

What Size Is My Furnace Filter? MERV Table + Best MERV 13 for Your Slot (2026)

How do I read my furnace filter's printed nominal size and choose the right MERV rating?

Your filter's printed size (like 16×25×1) is the nominal size—a rounded measurement for the slot. The actual size is typically 0.25–0.625 inches (¼ to ⅝ inch) smaller. Slot & MERV maps your slot to MERV 8, 11, and 13 options with real prices (example: Filters Fast size listings for 16×25×1 MERV options). Important: MERV 13 in 1-inch depth may restrict airflow; verify your system supports it or use MERV 11.

MERV Comparison Table

Pack prices and stock from Filters Fast listings (including genuine Aprilaire filters sold there), fetched 2026-10-07. Pressure drop, dimensions, and change intervals are specs, not crawled. All merchant links open the source listing.

Brand/SKU Nominal Actual (inches) Depth MERV Pressure Drop Change Pack Price Per Filter* In Stock
Filters Fast 16x25x1 MERV 13 16x25x1 15.75 x 24.75 x 0.75 1-inch 13 0.22–0.28" w.g. 90 days $82.95 / 6-pack $13.83 Yes
Filters Fast 16x25x1 MERV 11 16x25x1 15.75 x 24.75 x 0.75 1-inch 11 0.15–0.18" w.g. 90 days $76.95 / 6-pack $12.83 Yes
Filters Fast 16x25x1 MERV 8 16x25x1 15.75 x 24.75 x 0.75 1-inch 8 0.08–0.12" w.g. 90 days $62.95 / 6-pack $10.49 Yes
Filters Fast 20x25x1 MERV 13 20x25x1 19.75 x 24.75 x 0.75 1-inch 13 0.22–0.28" w.g. 90 days $82.95 / 6-pack $13.83 Yes
Filters Fast 16x20x1 MERV 13 16x20x1 15.75 x 19.75 x 0.75 1-inch 13 0.22–0.28" w.g. 90 days $82.95 / 6-pack $13.83 Yes
Filters Fast 20x20x1 MERV 13 20x20x1 19.75 x 19.75 x 0.75 1-inch 13 0.22–0.28" w.g. 90 days $82.95 / 6-pack $13.83 Yes
Filters Fast 16x25x4 MERV 13 16x25x4 15.50 x 24.50 x 3.50 4-inch 13 Unknown 6-12 months $92.95 / 3-pack $30.98 Yes
Filters Fast 20x25x4 MERV 13 20x25x4 19.375 x 24.375 x 3.50 4-inch 13 Unknown 6-12 months $92.95 / 3-pack $30.98 Yes
Aprilaire 413 16x25x4 15.81 x 29.38 x 4.05 4-inch 13 Unknown 6-12 months $76.99 / single filter $76.99 Yes
Aprilaire 410 16x25x4 16 x 25 x 3.5 4-inch 11 Unknown 6-12 months $74.99 / single filter $74.99 Yes
Aprilaire 416 16x25x4 Not listed by Filters Fast 4-inch 16 Unknown 6-12 months $146.99 / single filter $146.99 Yes

*Per filter = pack price ÷ pack size (rounded to the cent). Pack price and stock come from the crawled Filters Fast listing. We don't show subscription or shipping claims because we don't crawl them.

Table prices and SKUs are from Filters Fast listings. Nordic Pure and Filter King links are in Common Sizes Quick Links and on the guide pages.

Common Sizes Quick Links

These are the most common residential filter sizes. Click to see today's price:

These are sponsored affiliate links (Filters Fast, Nordic Pure, Filter King). We may earn a commission from qualifying purchases. Prices and availability are on the retailer site; only Filters Fast prices are crawled and shown in the table above.

How to Choose MERV Rating

MERV Captures Pressure Drop (1-inch) Best For
8 Dust, pollen, mold spores 0.08–0.12" w.g. Basic residential, older systems
11 Pet dander, dust mites, fine dust 0.15–0.18" w.g. Most modern residential systems
13 Smoke, bacteria, fine particulates 0.22–0.28" w.g. Strong blowers, 4-inch cabinets, allergy/asthma

System limit: ~0.50" w.g. total pressure drop across filter + ductwork. Check your HVAC manual or consult a technician if unsure.

Related Guides

Frequently Asked Questions

How do I measure my furnace filter size?
Check the printed nominal size on your old filter's frame (e.g., 16x25x1). If no label exists, measure the actual dimensions with a tape measure and round up to the nearest inch for nominal sizing. Nominal is the slot size; actual is typically 0.25–0.625 inches (¼ to ⅝ inch) smaller.
What's the difference between nominal and actual size?
Nominal is the slot size (16x25x1). Actual is the physical filter dimension (15.75x24.75x0.75). Manufacturers print nominal on the frame. Always buy by nominal size.
Can I use MERV 13 in a 1-inch slot?
Yes, but check your system's blower capacity. MERV 13 1-inch filters have higher pressure drop (0.22–0.28" w.g.) than MERV 8 or 11. Older or weaker blowers may struggle, reducing airflow and efficiency. For most modern systems, MERV 8–11 is the safe default for 1-inch slots.
What MERV rating should I use for a 4-inch cabinet?
MERV 13 is the baseline for 4- or 5-inch media cabinets. The deeper pleats handle higher filtration without excessive pressure drop. Use Aprilaire 410/413/416 by cabinet model number, not by guessing dimensions.
How often should I change my filter?
1-inch: every 90 days (MERV 8/11/13). 4-inch: every 6–12 months. Check your filter monthly; replace sooner if visibly dirty or if airflow drops.
Does higher MERV mean better air quality?
Higher MERV captures smaller particles, but also increases pressure drop. MERV 13 captures smoke and fine particulates; MERV 8 handles basic dust. Match MERV to your system's blower capacity and your indoor air quality needs.
What is pressure drop and why does it matter?
Pressure drop (measured in inches water gauge, w.g.) is resistance to airflow. Higher MERV = more resistance. If pressure drop exceeds your system's limit (~0.50" w.g. total), airflow decreases, efficiency drops, and the blower works harder.
Can I use Filterbuy instead of Aprilaire in a 4-inch cabinet?
Maybe. Check actual dimensions. Aprilaire 413 is 15.81x29.38x4.05 (unusual width). A standard 16x25x4 is ~15.5x24.5x3.5. If your cabinet is Aprilaire-specific (models 2410, 3410, 4400), use the OEM filter by model number.