Quick answer: The fastest way to find your air filter size is to pull out your current filter and read the size printed on the cardboard edge, like 16x25x1. If there's no print, measure the length, width, and thickness of the slot and round to the nearest inch.
Learning how to find air filter size takes about two minutes. You don't need any tools beyond a tape measure, and most of the time you won't even need that. The size is usually stamped right on the filter you already own. Let's walk through it so you order the right fit the first time.
How to find air filter size: the easy way
Before you measure anything, check the filter that's already in your system. Almost every filter has its size printed on the cardboard frame.
- Turn off your HVAC system at the thermostat.
- Find the filter slot. It's usually in a return air vent, a slot near the furnace, or behind a grille on a wall or ceiling.
- Slide the old filter out.
- Look at the cardboard edge. You'll see three numbers like 16x25x1 or 20x25x1.
Those three numbers are length, width, and thickness in inches. That's your nominal size, the name everyone uses to buy filters. Type it into our filter size finder and you'll see every matching Ironside filter.
What if there's no size printed on the filter?
Sometimes the print is worn off, or someone cut a filter to fit. In that case, you measure. Grab a tape measure and measure the slot or the old filter in three directions:
- Length — the longest side
- Width — the shorter side
- Thickness — the depth, usually 1, 2, 4, or 5 inches
Write the numbers down in that order. For a full step-by-step with photos of where to measure, see how to measure an air filter.
Nominal size vs actual size: don't get tripped up
Here's the part that confuses people. When you measure a filter with a tape, it comes out a little smaller than the name on the box. A filter called 16x25x1 actually measures closer to 15.5 x 24.5 x 0.75 inches.
That gap is on purpose. The nominal size, the rounded-up name, leaves room for the filter to slide in and out easily. The actual size is what it really measures. When you buy, you almost always use the nominal size.
| Nominal Size (the name) | Actual Measured Size |
|---|---|
| 16x25x1 | 15.5 x 24.5 x 0.75 |
| 20x25x1 | 19.5 x 24.5 x 0.75 |
| 16x20x1 | 15.5 x 19.5 x 0.75 |
The rule of thumb: the nominal size is about a half-inch larger than the actual measured size. So if your tape reads 19.5 x 24.5, you want a 20x25 filter. We break this down further in nominal vs actual filter size.
Which dimension comes first?
Filter sizes are written as length x width x thickness. The first two numbers can be swapped on the listing and still fit, since a 16x25 and a 25x16 are the same rectangle. The thickness is always last, and that one matters: a 1-inch filter won't sit right in a 4-inch slot.
So focus on getting all three numbers right, and don't sweat the order of the first two. The thickness also tells you how often to swap it. A 1-inch filter changes every 30 to 90 days; a 4-inch lasts 6 to 12 months. See how often to change a furnace filter.
Common air filter sizes
If your size matches one of these, you're in luck. These are the most common residential sizes, and we stock them across every MERV rating:
- 16x25x1
- 20x25x1
- 16x20x1
- 20x20x1
- 14x25x1
- 16x25x4 and 20x25x5 (the thicker whole-house sizes)
Found your size? Now pick a rating. Browse Everyday Defense (MERV 6–8) for basic dust and pollen, or all Ironside filters to see every size and rating together. If you're not sure what rating fits your home, read what MERV rating do I need.
Order the exact fit
A filter that's even a half-inch off lets unfiltered air sneak around the edges. That defeats the whole point. Once you have your three numbers, run them through our filter size finder and order with confidence. Every Ironside order ships free, and a subscription keeps the right size showing up on schedule. Built here. Breathe better.
Frequently Asked Questions
Where is the air filter size printed?
On the cardboard edge of the filter frame. Pull out your current filter and look along the side. You'll see three numbers like 16x25x1, which are length, width, and thickness in inches.
What do the three numbers on an air filter mean?
They are length, width, and thickness in inches. A 20x25x1 filter is 20 inches long, 25 inches wide, and 1 inch thick. The first two can be listed in either order, but thickness is always last.
Why doesn't my filter match its measured size?
Because filters are sold by nominal size, the rounded-up name, not the exact measured size. A 16x25x1 actually measures about 15.5 x 24.5 x 0.75 inches. The nominal size runs about a half-inch larger than the actual.
Can I use a slightly different size?
No. Always match your exact size. A filter that's too small lets unfiltered air leak around the edges, and one that's too big won't fit the slot. Get all three numbers right.
Does the thickness number matter?
Yes. The thickness must match your slot. A 1-inch filter won't seal in a 4-inch slot, and a 4-inch filter won't fit a 1-inch slot at all. Measure the depth to be sure.