How to find your furnace filter size (nominal vs actual chart)
The size printed on the edge of your furnace filter, like 16x25x1, is the nominal size, and it's the one you buy by. The filter itself is a little smaller, usually by half an inch on the length and width, so it slides into the slot without jamming. That smaller measurement is the actual size.
Below is how to find the size in under a minute, plus a chart of nominal and actual sizes for the 40 filters we carry.
Step 1: Read it off the old filter
Pull out the current filter and look at the cardboard frame. Almost every filter has the size printed on the edge, often in two places:
- Nominal size, in large print: 16x25x1. This is the size to buy.
- Actual size, in smaller print: 15.5 x 24.5 x 0.75. This is what a tape measure would show.
The first two numbers are length and width. Their order doesn't matter: 25x16x1 is the same filter as 16x25x1. The last number is always the depth (thickness).
Step 2: No label? Measure the old filter and round up
If the frame is blank or worn off, measure the old filter with a tape measure and round each number up to the next whole inch. A filter that measures 15½ x 24½ x ¾ is a 16x25x1. One that measures 19½ x 24½ x 3¾ is a 20x25x4.
Step 3: No filter at all? Measure the slot
If there's no filter in the furnace (it happens in rentals more than you'd think), measure the opening the filter slides into: length, width and depth. Then compare to the actual sizes in the chart below and pick the nominal size whose actual size fits inside the slot.
Where to look for the filter
In most houses, the filter is in a slot on the side of the furnace, between the return air duct and the blower. Other common spots are a grille in a hallway wall or ceiling (the return vent) or a separate media cabinet next to the furnace for 4-inch and 5-inch filters. Some homes have more than one filter, so check every return grille.
Nominal vs actual size chart
1-inch filters
| Nominal size (printed on the filter) | Actual size (inches) |
|---|---|
| 10x20x1 | 9.5 x 19.5 x 0.75 |
| 10x25x1 | 9.5 x 24.5 x 0.75 |
| 12x12x1 | 11.5 x 11.5 x 0.75 |
| 12x20x1 | 11.5 x 19.5 x 0.75 |
| 12x24x1 | 11.5 x 23.5 x 0.75 |
| 12x25x1 | 11.5 x 24.5 x 0.75 |
| 12x30x1 | 11.5 x 29.5 x 0.75 |
| 14x14x1 | 13.5 x 13.5 x 0.75 |
| 14x20x1 | 13.5 x 19.5 x 0.75 |
| 14x24x1 | 13.5 x 23.5 x 0.75 |
| 14x25x1 | 13.5 x 24.5 x 0.75 |
| 14x30x1 | 13.5 x 29.5 x 0.75 |
| 15x20x1 | 14.5 x 19.5 x 0.75 |
| 16x16x1 | 15.5 x 15.5 x 0.75 |
| 16x20x1 | 15.5 x 19.5 x 0.75 |
| 16x24x1 | 15.5 x 23.5 x 0.75 |
| 16x25x1 | 15.5 x 24.5 x 0.75 |
| 16x30x1 | 15.5 x 29.5 x 0.75 |
| 18x18x1 | 17.5 x 17.5 x 0.75 |
| 18x20x1 | 17.5 x 19.5 x 0.75 |
| 18x24x1 | 17.5 x 23.5 x 0.75 |
| 18x25x1 | 17.5 x 24.5 x 0.75 |
| 20x20x1 | 19.5 x 19.5 x 0.75 |
| 20x22x1 | 19.5 x 21.5 x 0.75 |
| 20x24x1 | 19.5 x 23.5 x 0.75 |
| 20x25x1 | 19.5 x 24.5 x 0.75 |
| 20x30x1 | 19.5 x 29.5 x 0.75 |
| 22x22x1 | 21.5 x 21.5 x 0.75 |
| 24x24x1 | 23.5 x 23.5 x 0.75 |
| 24x25x1 | 23.5 x 24.5 x 0.75 |
| 24x30x1 | 23.5 x 29.5 x 0.75 |
| 25x25x1 | 24.5 x 24.5 x 0.75 |
4-inch filters
| Nominal size (printed on the filter) | Actual size (inches) |
|---|---|
| 16x20x4 | 15.5 x 19.5 x 3.75 |
| 16x25x4 | 15.5 x 24.5 x 3.75 |
| 20x20x4 | 19.5 x 19.5 x 3.75 |
| 20x24x4 | 19.5 x 23.5 x 3.75 |
| 20x25x4 | 19.5 x 24.5 x 3.75 |
5-inch filters
| Nominal size (printed on the filter) | Actual size (inches) |
|---|---|
| 16x25x5 | 15.88 x 24.88 x 4.38 |
| 20x20x5 | 19.68 x 19.93 x 4.37 |
| 20x25x5 | 19.875 x 24.75 x 4.25 |
About 5-inch filters: 5-inch media filters are built to fit the cabinet of a specific furnace brand, so their actual sizes vary more than 1-inch and 4-inch filters do. Before ordering a 5-inch filter, compare the actual size printed on your old one to the chart.
Common sizing mistakes
- Wrong depth. A 16x25x1 and a 16x25x4 have the same face but very different depths. A 1-inch filter in a 4-inch slot lets air go around it, so it filters almost nothing.
- Buying by the actual size. Searching for "15.5x24.5" turns up odd sizes. Shop by the nominal size.
- Installing it backwards. Every filter has an airflow arrow on the frame. Point it toward the furnace, away from the return duct.
- Forcing the wrong size. A filter that has to be bent to fit lets dirty air leak around the edges.
Write the size down once
For a landlord with several properties, the size is the thing you'll forget. Write it on the furnace with a marker, put it in the property file, and fill it in on the lease addendum, which has a blank for the filter size and the change schedule. Once you know the depth, here's how often to change it.
Order by size
Every size in the chart above links to its page, with the actual dimensions and the MERV options. Or start from Find your size, where all 40 sizes are one tap away. Pick the size once, and a year of filters ships free to the property, with a text to your tenant each time a change is due. Don't see your size? Request it here.
Quick answers
What's the difference between nominal and actual filter size? The nominal size is the rounded size printed on the filter and used to buy it, like 16x25x1. The actual size is the filter's real measurement, usually half an inch smaller on length and width, like 15.5 x 24.5 x 0.75.
Which size do I buy, nominal or actual? Nominal. It's the size printed in large type on the filter and the one stores and subscriptions list.
Does the order of the numbers matter? Not the first two. 16x25x1 and 25x16x1 are the same filter. The last number is always the depth.
What if my filter has no size printed on it? Measure it and round each dimension up to the next whole inch. A 15½ x 24½ x ¾ filter is a 16x25x1.
What is the most common furnace filter size? 16x25x1, 20x25x1, 16x20x1 and 20x20x1 are among the most common 1-inch sizes. For 4-inch filters, 16x25x4 and 20x25x4 are the usual ones.
About the author. Wyatt D. has owned and managed rental homes in the Twin Cities west metro and Fargo, North Dakota, since 2010, and now has 10. He started Filterflow after tenants who skipped filter changes cost him two to three furnace repairs in a single year.