What Size Air Filter Do I Need?
Check the label on your current filter. The size is printed on the cardboard frame as length x width x depth (e.g., 20x25x1). If the print has faded or is unreadable, measure the filter slot with a tape measure and round each dimension up to the nearest whole inch. That rounded number is your nominal size, and it's what you order.
How to Find Your Filter Size
Find Your Current Filter
Look for the cardboard-framed filter in your return air vent (the large vent on a wall or ceiling) or inside the furnace cabinet. Most homes have one or two return vents. The filter slides out of a slot behind the vent cover or from a slot in the side of the furnace.
Tip: Turn off your HVAC system before removing the filter. Note which direction the airflow arrow points so you install the new one correctly.
Read the Label
The size is printed on the cardboard frame, usually along one edge. It reads as L x W x D (for example, 20x25x1). This is the nominal size, which is the standard ordering size. The actual filter is slightly smaller to fit easily into the slot.
Tip: Take a photo of the label with your phone so you always have the size handy when ordering replacements.
If No Label, Measure
Use a tape measure on the filter slot opening (or the filter itself). Measure length, width, and depth in inches. Round each dimension up to the nearest whole inch. A filter that measures 19.5 x 24.5 x 0.75 inches has a nominal size of 20x25x1.
Tip: Always round up, never down. A filter ordered one inch too large won't fit, but the nominal size system accounts for this. The 20x25x1 filter you order will actually measure about 19.5 x 24.5 x 0.75.
Common Air Filter Sizes
Standard 1-Inch Filters
| Size | Type | Most Common In |
|---|---|---|
| 16x20x1 | Standard (1") | Smaller furnaces, apartments |
| 16x25x1 | Standard (1") | Mid-size furnaces, very common |
| 20x20x1 | Standard (1") | Square return vents, common in older homes |
| 20x25x1 | Standard (1") | Most common residential size |
| 14x20x1 | Standard (1") | Compact HVAC units |
| 14x25x1 | Standard (1") | Narrow return vents |
| 18x24x1 | Standard (1") | Common in some southern US homes |
| 18x20x1 | Standard (1") | Smaller return vents |
| 20x24x1 | Standard (1") | Large return vents |
| 24x24x1 | Standard (1") | Large square return vents |
Deep Pleated Filters (4 to 5 Inch)
| Size | Type | Most Common In |
|---|---|---|
| 16x25x4 | Deep (4") | Deep pleated cabinets (Honeywell, Lennox) |
| 20x25x4 | Deep (4") | Most common deep pleated size |
| 20x20x4 | Deep (4") | Square deep pleated cabinets |
| 16x25x5 | Deep (5") | 5-inch cabinets (Honeywell, Lennox) |
| 20x25x5 | Deep (5") | 5-inch cabinets (Carrier, Trion) |
Nominal vs Actual Size
Every air filter has two sizes: the nominal size (the rounded number on the label, like 20x25x1) and the actual size (the precise physical measurement, like 19.5 x 24.5 x 0.75 inches). Manufacturers intentionally make filters slightly smaller than the nominal size so they slide into the filter slot without forcing. When you shop for filters, always use the nominal size. That is what every retailer lists, and it is what matches your HVAC system's specifications.
My Size Isn't Listed
If your filter slot doesn't match any of the common sizes above, don't worry. First, double-check your measurement by measuring the slot opening (not an old warped filter) and rounding up to the nearest inch. If the size is genuinely uncommon, brands like FilterBuy and Nordic Pure manufacture a much wider range of sizes than most retailers stock. You can also search Amazon for your exact nominal dimensions. In rare cases with truly non-standard sizes, specialty filter suppliers offer custom-cut options.
Filter Size FAQs
What is the most common furnace filter size?
The most common residential filter sizes are 20x25x1, 16x25x1, 16x20x1, and 20x20x1. These four sizes cover roughly 60 to 70 percent of homes in the United States. The '1' refers to the standard 1-inch depth that fits most HVAC return air vents and furnace filter slots.
What's the difference between nominal and actual size?
Nominal size is the rounded number printed on the filter label (e.g., 20x25x1). Actual size is the precise measurement of the filter (e.g., 19.5 x 24.5 x 0.75 inches). Filters are manufactured slightly smaller than their nominal size so they slide into the filter slot easily. Always order by nominal size, which is what retailers list.
Can I use a slightly different size filter?
No. Using a filter that is too small leaves gaps where unfiltered air bypasses the filter entirely. Using one that is too large means it won't fit in the slot or will buckle, also allowing air to bypass. The filter must match your slot's nominal size exactly. If you can't find your exact size, check FilterBuy or Nordic Pure, which offer a wider range of dimensions than most brands.
Where is the filter size printed?
The size is printed on the cardboard frame of the filter itself, usually along one of the edges. It reads as length x width x depth (e.g., 20x25x1). If the print has faded or the filter is too dirty to read, measure the filter slot opening with a tape measure and round each dimension up to the nearest whole inch.
What if my filter size is unusual?
If your size doesn't match any standard option, measure your filter slot carefully (length x width x depth) and round up to the nearest inch. Brands like FilterBuy and Nordic Pure manufacture a wide range of non-standard sizes. You can also search Amazon for your exact dimensions. In rare cases, you may need a custom-cut filter from a specialty supplier.