How to Find the Air Filter Size You Need
Buying the right air filter comes down to two things: the correct size and the correct MERV rating. Get the size wrong and even the best filter leaves gaps for dust to slip through. Here are the quickest, most reliable ways to find the exact size your home’s HVAC system needs.
1. Read the dimensions printed on the filter
The fastest method is the filter already in your system. Turn the system off, slide the old filter out, and look along the cardboard frame. Most filters have the size printed right on the edge—three numbers like 16x25x1 or 20x20x1, listed as length x width x depth. If those numbers are legible, that is your size and you can shop with confidence.
If the print has worn off or the filter came unlabeled, check the original packaging if you still have it, then move to measuring.
2. Measure the old filter yourself
Grab a tape measure and record three dimensions in this exact order:
- Length — the longest side
- Width — the shorter side
- Depth — the thickness (commonly 1″, 2″, 4″, or 5″)
Keep the numbers in that order, since that is how filters are always listed. One quirk to know: order matters less than you think—a 23x20x1 and a 20x23x1 are the same filter with the two face dimensions swapped. For a step-by-step walkthrough, our complete guide to measuring a furnace filter covers every detail.
3. Measure the opening if there is no filter
If your system is empty, measure the slot instead. Look inside the filter rack at the furnace or behind the return-air grille, then measure the height and width of the opening plus the depth of the slot the filter sits in. You want a filter that fills the opening snugly—a filter that is too small lets air (and dust) bypass it around the edges, and gaps around a filter undo much of its job.
4. Check the product manual
Your furnace or air handler manual usually lists the recommended filter size. If you no longer have it, the manufacturer’s website or support line can point you to the correct dimensions for your model. This is a good backup when the filter and opening measurements are hard to read.
Nominal vs. actual size
Here is the detail that trips people up: filters are sold by their nominal (rounded) size, not their exact measurement. A filter labeled 20x20x1 actually measures closer to 19.5x19.5x0.75 inches. That small gap is intentional, so the filter slides in and out easily. So when you measure and get something like 19.5″, you have a standard 20″ nominal filter—shop by the rounded label. If your measurements do not round cleanly to any stocked size, you likely have an odd size. Our guide to measuring for an odd-size filter and the overview of when standard sizes don’t fit explain your options, including custom cut-to-fit.
Don’t forget the MERV rating
Size gets the filter into the slot; the MERV rating decides what it captures. For most homes, MERV 8 handles everyday dust, MERV 11 suits pets and mild allergies, and MERV 13 targets fine particles and smoke. Higher ratings capture more but also restrict airflow, so match the rating to your needs rather than always reaching for the highest number.
Shop your size
Once you know your dimensions, ordering is easy. Browse a common size like 20x20x1 filters or 12x24x1 filters, explore the full range of home air filters and furnace filters, or go with a custom-cut filter if your size is truly unique. Not sure which sizes are most common? Our guide to the most common filter sizes and brands is a helpful starting point.
Frequently asked questions
Where is the filter size printed?
Along the cardboard frame of the filter, usually on one of the long edges. It reads as three numbers—length x width x depth, such as 16x25x1. If it has worn off, measure the filter or the opening instead.
Should I order by the nominal or actual size?
For standard filters, order by the nominal (rounded) size—that is how they are labeled and stocked. Measure the actual dimensions first to confirm which nominal size you need. Use exact actual measurements only when ordering a custom cut-to-fit filter.
What if my size isn’t sold anywhere?
If your measurements don’t match a stocked size, you have an odd size and are a good candidate for a custom cut-to-fit filter built to your exact dimensions. That guarantees a snug fit with no air bypassing the edges.