Quick Answer
To find your HVAC filter size, check the existing filter—the size is printed on the frame (e.g., 16x25x1 or 20x20x4). If there's no filter or the size is worn off, measure the filter slot: length × width × depth in inches. Common residential sizes are 16x20x1, 16x25x1, 20x20x1, and 20x25x1. Always buy the exact size—filters that don't fit properly let unfiltered air bypass into your system.
Buying the wrong filter size is one of the most common HVAC mistakes homeowners make. A filter that's too small lets dust bypass into your system. One that's too big won't fit at all. Here's how to get the right size every time.
How to Find Your Filter Size
Method 1: Check Your Current Filter
The easiest way—look at the filter you're replacing. The size is printed on the cardboard frame, usually on the edge. It's listed as Length × Width × Depth (e.g., 16x25x1).
Nominal vs. Actual Size
Method 2: Measure the Filter Slot
If there's no filter or the size is unreadable, measure the filter slot in your system:
Measuring Your Filter Slot
- 1
Turn off your HVAC system
Safety first—and prevents dust from entering while the slot is empty.
- 2
Locate the filter slot
Usually in the return air duct, at the air handler, or behind a return vent grille.
- 3
Measure the opening
Measure length, width, and depth in inches. Round UP to the nearest inch.
- 4
Convert to nominal size
If your slot measures 15.5" × 24.5" × 0.75", you need a 16x25x1 filter.
Common HVAC Filter Sizes
Most residential systems use one of these standard sizes:
| Size (Nominal) | Common In | Notes |
|---|---|---|
| 16x20x1 | Smaller systems, condos | Very common |
| 16x25x1 | Standard residential | Most popular size |
| 20x20x1 | Square return vents | Common in older homes |
| 20x25x1 | Larger systems | Very common |
| 20x25x4 | Media filter cabinets | 4" thick, lasts longer |
| 16x25x4 | Media filter cabinets | 4" thick |
| 20x20x4 | Media filter cabinets | 4" thick |
| 14x20x1 | Smaller systems | Less common |
| 14x25x1 | Smaller systems | Less common |
| 24x24x1 | Commercial/large residential | Less common |
Understanding Filter Dimensions
Filter sizes always list three numbers: Length × Width × Depth.
- Length: The longest side of the filter face
- Width: The shorter side of the filter face
- Depth: How thick the filter is (1", 2", 4", or 5")
Depth Matters
What If My Size Is Unusual?
Odd sizes exist, especially in older systems. If you can't find your size at home improvement stores:
- Check HVAC supply stores (like Johnstone Supply)
- Order online—Amazon and filter specialty sites carry unusual sizes
- Order custom-cut filters from filter manufacturers
- Ask your HVAC company—we often stock odd sizes or can order them
Never use a filter that's close but not exact. A gap of even 1/2" allows unfiltered air to bypass the filter entirely.
Common Filter Size Mistakes
Mistake 1: Using the Wrong Thickness
A 1" filter won't fit in a 4" cabinet properly—and a 4" filter absolutely cannot fit in a 1" slot. The depth must match your system.
Mistake 2: Forcing a Filter That Doesn't Fit
If you have to bend or force a filter in, it's wrong. This damages the filter media and creates gaps where air bypasses.
Mistake 3: Leaving Gaps
A filter that's too small leaves gaps around the edges. Unfiltered air takes the path of least resistance—right through those gaps—defeating the purpose of the filter.
Mistake 4: Confusing Length and Width
A 16x25x1 is NOT the same as a 25x16x1 in how it fits certain slots. Usually they're interchangeable, but some filter slots are designed for a specific orientation.
Filter Types at Each Size
Once you know your size, you'll see multiple filter types available:
| Type | MERV Rating | Best For | Change Frequency |
|---|---|---|---|
| Fiberglass | 1-4 | Basic protection, rental properties | Monthly |
| Pleated | 8-13 | Most homes, good balance | Every 2-3 months |
| High-efficiency | 14-16 | Allergies, pets, health concerns | Every 2-3 months |
| HEPA | 17-20 | Medical needs (requires special cabinet) | Every 6-12 months |
For most Charlotte homes, we recommend a MERV 8-11 pleated filter. Higher isn't always better—very high MERV ratings can restrict airflow if your system isn't designed for them. See our <a href="/learning-center/maintenance/air-filter-guide">complete air filter guide</a> for more on choosing the right type.
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Written by
Kodiak HVAC Team
HVAC professional at Kodiak Heating & Cooling.