Quick answer: The nominal filter size is the rounded-up name printed on the filter, like 20x25x1. The actual size is the real measured size, which is about half an inch smaller, closer to 19.5 x 24.5 x 0.75. You buy by the nominal size. Both refer to the same filter.
If you've ever pulled out a filter that says 20x25x1, grabbed a tape measure, and gotten a different number, you're not crazy and your filter isn't mislabeled. Understanding nominal vs actual filter size is the single most common thing that trips people up when they buy air filters. This guide clears it up in plain terms so you order the right filter the first time.
What is nominal filter size?
The nominal size is the name of the filter. It's the number printed on the cardboard frame: 16x25x1, 20x25x1, 16x20x1, and so on. It's rounded to whole inches to make filters easy to label, sort, and shop for. Think of it like a shoe size. A "size 10" shoe isn't exactly 10 inches long. It's a standard label.
What is actual filter size?
The actual size is what you get when you put a tape measure on the filter. It's the true measured dimensions, and it's slightly smaller than the nominal size, usually by about half an inch on the width and height.
So a filter labeled 20x25x1 actually measures around 19.5 inches by 24.5 inches, and a 1-inch filter is often closer to 0.75 inches thick. The nominal name rounds up; the actual size is the truth on the tape measure.
Why are they different?
Two reasons. First, the small gap lets the filter slide into the slot without jamming. A filter cut to the exact opening would be a fight to install and could buckle. Second, rounding to whole inches keeps the naming simple across thousands of products and brands. Every major brand follows the same convention, so it's consistent.
Nominal vs actual: a quick comparison
| Nominal size (the name) | Approximate actual size (the tape measure) |
|---|---|
| 16x20x1 | 15.5 x 19.5 x 0.75 |
| 16x25x1 | 15.5 x 24.5 x 0.75 |
| 20x25x1 | 19.5 x 24.5 x 0.75 |
| 20x25x4 | 19.5 x 24.5 x 3.75 |
| 20x25x5 | 19.75 x 24.25 x 4.375 |
Note that thicker filters (4-inch and 5-inch) vary a bit more between brands. If you run a thicker filter, see 1-inch vs 4-inch vs 5-inch air filters for the differences that matter.
Which size do I buy?
You buy by the nominal size. That's the number printed on your old filter and the number listed on every product page. Do not order by your tape-measure number, or you'll end up a size off.
- If you have the old filter: Read the nominal size printed on the frame and order that exact number.
- If the print is worn off: Measure the actual size with a tape, then round up each dimension to the nearest inch to get the nominal size.
- If you're unsure: Measure the slot opening, not the old filter, and round up.
How to measure if the label is gone
Measure length, width, and depth in inches. Round each one up to the nearest whole inch. A filter that measures 19.5 x 24.5 x 0.75 rounds up to a nominal 20x25x1. For a step-by-step walkthrough with photos, see how to measure an air filter.
Why getting the size right matters
A filter that's too small leaves gaps. Air, dust, and dander take the path of least resistance and slip around the edges instead of going through the media. You lose filtration. A filter that's too big won't seat flat, and it can bow or get crushed, which also creates leaks.
A correctly sized filter seats snug with no air bypass, so all the air your system moves actually gets cleaned. That's the whole point. Size matters as much as MERV rating. If you're still picking a rating, start with what MERV rating do I need.
The fastest way to find your exact filter
Skip the guesswork. Our Find My Filter size finder lets you enter your size and pulls up the exact match. Once you know your nominal size, you can shop straight from the right collection:
- Everyday Defense (MERV 6 to 8) for dust and pollen
- Allergy and Pet (MERV 10 to 12) for dander and mold spores
- Maximum Protection (MERV 13 to 16) for smoke and fine particles
- All filters to browse everything
Order the right size on repeat
Once you know your nominal size, you never have to figure it out again. With Ironside Subscribe & Save, your exact-size filters arrive on schedule with free shipping on every order, locked-in pricing, and auto-replenishment so you never run out. No more measuring, no more guessing. Built here. Breathe better.
Frequently asked questions
What's the difference between nominal and actual filter size?
The nominal size is the rounded name printed on the filter, like 20x25x1. The actual size is the true measured size, about half an inch smaller, around 19.5 x 24.5 x 0.75. They describe the same filter; one is the label, one is the tape measure.
Why is my filter smaller than the size it says?
That's normal and intentional. The actual size is cut slightly smaller so the filter slides into the slot without jamming, and the names are rounded to whole inches for simple labeling. Every major brand follows the same convention.
Should I order by nominal or actual size?
Order by the nominal size, which is the number printed on your old filter and listed on product pages. If you only have a tape-measure number, round each dimension up to the nearest inch to get the nominal size.
What if the size printed on my old filter is worn off?
Measure the actual length, width, and depth with a tape measure, then round each up to the nearest whole inch. For example, 19.5 x 24.5 x 0.75 becomes a nominal 20x25x1. Measuring the slot opening works too.
Does a small size difference really matter?
Yes. A filter that's even slightly too small leaves gaps where air bypasses the media, so dust and particles slip around the edges. The right nominal size seats snug so all your system's air actually gets filtered.