Air Conditioner Filter Sizes: How to Find the Right One
Standing in the filter aisle guessing at sizes is a rite of passage nobody wants. Here's how to get it right the first time.
table of contents
AC filter sizes work the same way furnace filter sizes do — each system has a specific slot dimension, and the wrong size either doesn't fit or leaves gaps that defeat the entire purpose. The good news: finding your size takes about 30 seconds if you know where to look.
How to find your AC filter size
The fastest method: pull out the current filter. The size is printed on the cardboard frame — three numbers representing length × width × depth in inches. Something like 16x20x1 or 20x25x4.
If the filter is missing, unmarked, or too dirty to read, measure the slot opening directly with a tape measure. Note that the printed "nominal" size on filters is slightly rounded up — actual filters are typically about a half-inch smaller in each direction than what's printed on the box.
Central air systems
Central air systems use the same filter slot as your furnace — they're part of the same HVAC system. The filter is usually located at the return air vent (a large grated vent, often on a wall or ceiling) or at the air handler unit itself in the basement, attic, or closet.
If you can't find the filter location, check your HVAC manual or look for a sticker inside the air handler access panel. Manufacturers often list the filter size there.
For split systems where the air handler is separate from the outdoor condenser, the filter is always in the air handler — not the outdoor unit.
save your filter size now
kept stores your filter size, brand preference, and last change date. Next time you need a replacement, you'll know exactly what to buy before you leave the house.
[ try kept free ]Window AC units
Window AC filters are smaller, washable mesh screens that sit just behind the front panel of the unit. To find yours:
- Remove the front panel (usually clips off or slides out)
- The filter is the mesh screen directly behind it
- Measure it, or check the model number sticker on the unit's side or back
Most window AC filters are washable rather than replaceable — rinse with warm water, let dry completely, and reinsert. Replace if the mesh is torn or damaged beyond cleaning. Replacement filters are usually listed by model number, not a standard size.
Shop: window AC replacement filters on Amazon
Mini-split systems
Mini-split indoor units (the wall-mounted heads) have washable filters behind the front panel — usually two thin mesh screens on either side. They're not standard sizes, so replacement filters are ordered by model number.
Pull the front panel down or off, remove the filters, wash them monthly during heavy use season, and replace them if torn. Most mini-split manufacturers recommend professional service annually for the unit itself.
Common AC filter sizes
For central air systems, these are the most commonly used filter sizes. Nominal sizes are what's printed on the box — actual dimensions are slightly smaller.
| Nominal size | Actual size (approx) | Common use |
|---|---|---|
| 16 x 20 x 1 | 15.5 x 19.5 x 0.75 | Smaller homes, apartment HVAC |
| 20 x 20 x 1 | 19.5 x 19.5 x 0.75 | Common residential |
| 16 x 25 x 1 | 15.5 x 24.5 x 0.75 | Common residential |
| 20 x 25 x 1 | 19.5 x 24.5 x 0.75 | Most common residential size |
| 20 x 25 x 4 | 19.5 x 24.5 x 3.75 | High-capacity media filter systems |
| 16 x 25 x 4 | 15.5 x 24.5 x 3.75 | High-capacity media filter systems |
Shop: AC filters on Amazon — filter by your size and MERV rating
Understanding MERV ratings
MERV (Minimum Efficiency Reporting Value) measures filtration effectiveness. Higher is better filtration — but not always better for your system.
- MERV 1–4: Fiberglass filters. Very cheap, very limited. Catch large debris but little else.
- MERV 8: Standard pleated filter. Good for most homes. Catches dust, pollen, mold spores.
- MERV 11: Better filtration for allergy sufferers. Catches finer particles including pet dander.
- MERV 13: High filtration — approaches hospital-grade. Some older HVAC systems can't handle the restricted airflow.
For most homes: MERV 8 is the baseline, MERV 11 if you have pets or allergies. Check your HVAC manual before going to MERV 13 — restricted airflow strains older systems and can shorten equipment life.
Stop looking this up every time
The AC filter size is one of those things that's obvious while you're holding the old filter — and completely forgotten 90 days later when you need a replacement. Most people end up standing in the filter aisle either guessing or calling someone who is also guessing.
Save it once in kept — filter size, MERV rating, brand you prefer, and the date you last changed it. The app will surface it the moment you need it, and you can order directly from the item page.
your filter size. always in your pocket.
kept stores your AC filter size alongside your furnace filter, water filter, and every other spec you need to maintain your home. Never buy the wrong size again.
[ save your filter sizes ]frequently asked questions
How do I find the right air conditioner filter size?
The easiest way is to look at your current filter — the size is printed on the cardboard frame. If the filter is missing or unmarked, measure the slot opening (length x width x depth in inches). For central air systems, you can also find the filter size in your unit's manual or on a sticker inside the access panel.
What are the most common AC filter sizes?
The most common central air filter sizes are 16x20x1, 20x20x1, 16x25x1, 20x25x1, and 20x25x4. Window AC units typically use smaller proprietary filters — check the unit's manual or the filter frame for sizing.
How often should I change my AC filter?
For standard 1-inch filters, every 1–3 months depending on pets and air quality. For thicker 4-inch media filters, every 6–12 months. If you have pets, allergies, or run the system heavily, change more frequently. A clogged filter strains the system and raises energy bills.
Can I use a different size filter than what's recommended?
No. A filter that's too small leaves gaps where unfiltered air passes through. A filter that's too large won't fit in the slot. The size needs to match the opening exactly. Nominal sizes printed on filters are rounded — actual dimensions are typically 0.5 inch smaller.
What does MERV rating mean on an AC filter?
MERV (Minimum Efficiency Reporting Value) measures how well a filter traps particles. Higher MERV means better filtration: MERV 8 is standard for homes, MERV 11 is good for allergies, MERV 13 approaches hospital-grade. Most residential HVAC systems work best with MERV 8–11 — too high a MERV can restrict airflow in older systems.