Choosing the right furnace filter size is crucial for maintaining your HVAC system’s efficiency, indoor air quality, and energy bills. Yet, many homeowners struggle to find the correct dimensions, leading to poor airflow, increased wear on their furnace, and even higher utility costs.
In this guide, we’ll break down standard furnace filter sizes, explain how to measure your filter correctly, and provide a handy size chart for quick reference. Whether you’re replacing an old filter or buying a new one, this article will help you find the perfect fit for your heating and cooling system.
Most Common Furnace Filter Sizes
Here are the standard sizes you'll find at most home improvement stores:
1-Inch Filters (Most Common)
- 16x20x1
- 16x25x1
- 20x20x1
- 20x25x1 (Most popular residential size)
- 25x25x1
Thicker Filters (2-5 Inches)
- 16x25x2
- 20x25x4
- 25x25x4
- 20x25x5
Other Available Sizes
- 12x12x1
- 14x20x1
- 14x25x1
- 18x18x1
- 24x24x1
Always double-check your measurements - filter sizes are nominal and may be slightly smaller than labeled (e.g., a "16x25x1" might actually measure 15.5x24.5x0.75 inches).
Table of Contents
- Standard Furnace Filter Sizes
- How to Measure Your Furnace Filter Correctly
- Where to Find Your Furnace Filter Size
- Furnace Air Filter Size Chart: Find Your Perfect Match
- What to Do If Your Furnace Filter Size Is Uncommon
Standard Furnace Filter Sizes
Finding the right furnace filter starts with knowing the most common sizes available. While HVAC systems can vary, most residential furnaces use standardized filter dimensions that are widely available at hardware stores and online retailers.
Most Common Furnace Filter Sizes
Furnace filters are labeled by their length x width x thickness (in inches). Here are the top standard sizes you’ll encounter:
1-Inch Thick Filters (Most Common)
- 16x20x1 – A popular size for smaller HVAC systems
- 16x25x1 – Frequently used in apartments and smaller homes
- 20x20x1 – Common for central air systems
- 20x25x1 – One of the most widely used sizes
- 25x25x1 – Often found in larger residential systems
2-Inch to 5-Inch Thick Filters (Better Filtration & Longevity)
- 16x25x2 – Common for mid-range systems
- 20x25x4 – Used in high-efficiency HVAC setups
- 16x25x5 – Found in homes with advanced filtration needs
Odd or Less Common Sizes
Some systems require non-standard sizes like:
- 14x20x1
- 18x18x1
- 24x30x1
These may require special orders or custom filters.
How to Read Filter Sizes: Nominal vs. Actual Dimensions
- Nominal Size – The rounded measurement (e.g., "16x25x1") used for labeling.
- Actual Size – The exact dimensions, which may be slightly smaller (e.g., 15.5" x 24.5" x 0.75").
🔍 Always check both—some manufacturers list nominal sizes, while others list true measurements.
Where to Find Standard Furnace Filters
- Big-box stores (Home Depot, Lowe’s, Walmart)
- Online retailers (Walmart, Vevor, Temu)
- HVAC professionals (for custom or hard-to-find sizes)
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What If Your Size Isn’t Standard?
- Adjustable filters – Some brands offer flexible frames that can be trimmed to fit.
- Custom orders – Specialty retailers can make filters to exact specifications.
- Retrofit options – In some cases, modifying the filter slot (with professional help) allows for standard sizes.
Now that you know the most common sizes, the next step is ensuring you have the right fit. We’ll guide you through accurate measuring techniques to avoid sizing mistakes.
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How to Measure Your Furnace Filter Correctly
Getting the right furnace filter starts with precise measurements. Even a small size mismatch can lead to air leaks, reduced efficiency, or damage to your HVAC system. Follow these steps to measure your filter accurately.
Step-by-Step Measuring Guide
1. Turn Off Your HVAC System
- For safety, switch off your furnace or air handler before removing the filter.
2. Locate and Remove the Existing Filter
- Most filters slide into a slot near the return air duct or inside the furnace/air handler.
- Carefully pull out the old filter and check for size markings on its frame (many list dimensions like "16x25x1").
3. Measure the Length, Width, and Depth
Use a tape measure (preferably in inches) and record:
- Length (L) – Measure the longest side (left to right)
- Width (W) – Measure the shorter side (top to bottom)
- Depth (D) – Measure the thickness (front to back)
👉 Example: If your filter measures 15.5" x 24.5" x 0.75", its nominal size is likely 16x25x1.
4. Account for Nominal vs. Actual Sizes
- Filters are often labeled by their nominal size (rounded up for simplicity).
- The actual size may be up to ¼-inch smaller (e.g., a "16x25x1" filter might actually measure 15.75" x 24.75" x 0.8").
🔂 Tip: If your measurements fall between standard sizes, round up to the nearest whole number (e.g., 15.75" ≈ 16").
5. Double-Check the Filter Slot
If you don’t have an old filter to measure:
- Measure the length and width of the slot where the filter sits.
- Note any obstructions (like clips or rails) that might affect fit.
6. Verify Compatibility
- Compare your measurements to standard sizes (see our Standard Furnace Filter Sizes section).
- If your size is uncommon, check for "custom" or "trim-to-fit" filter options.
Common Measuring Mistakes to Avoid
- ❌ Ignoring depth – A 1-inch vs. 2-inch filter won’t fit the same slot.
- ❌ Assuming all filters are labeled accurately – Some generic brands may mislabel sizes.
- ❌ Measuring only the filter media – Always measure the entire frame, not just the pleated part.
What If Your Filter Doesn’t Match Standard Sizes?
- Adjustable filters (like Filtrete’s "Trim-to-Fit") can be cut down to size.
- Special order from HVAC suppliers or online retailers (e.g., FilterBuy, Amazon).
- Consult an HVAC technician if your system requires a unique size.
Now that you’ve measured your filter, use our handy size chart to compare standard dimensions and find the perfect match for your system.
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Where to Find Your Furnace Filter Size
Locating your furnace filter size is easier than you might think. Whether you're replacing an old filter or checking a new system, here are the most reliable ways to find the correct dimensions.
Check the Existing Filter (If You Have One)
The quickest way to find your filter size is to look at the current filter itself:
- Remove the filter from its slot (turn off the HVAC system first for safety).
- Look for printed numbers on the cardboard frame (e.g., "16x25x1").
- If the size is faded, use a tape measure to confirm (see our measuring guide).
Look Inside Your Furnace or Air Handler
If you don’t have an old filter:
- Open the furnace cabinet or air handler panel (consult your manual if unsure).
- Check for a label near the filter slot—some manufacturers list the size here.
- Measure the filter slot (length x width x depth) if no size is marked.
Consult Your HVAC System Manual
- Your furnace or air handler’s manual often specifies the correct filter size.
- Can’t find the physical manual? Search online using your model number (usually on a sticker inside the unit).
Check the Return Air Grille
Some systems have filters behind the large return air vent (usually on a wall or ceiling):
- Remove the grille cover (may require unscrewing or sliding out).
- If a filter is inside, check its size or measure the compartment.
Search the Manufacturer’s Website
- Enter your furnace or HVAC model number on the brand’s support site (e.g., Carrier, Trane, Lennox).
- Many provide filter size specifications in their product documentation.
Ask an HVAC Professional
If you’re still unsure:
- A technician can quickly identify the right size during a maintenance visit.
- Local HVAC supply stores may also help if you bring in measurements.
What If No Filter Seems to Fit?
- Some systems use non-standard sizes—consider custom filters or retrofit options.
- Adjustable filters (like those with trimmable frames) can work for slightly off sizes.
Now that you know where to find your filter size, use our comparison chart to match it with standard dimensions and buying options.
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Furnace Air Filter Size Chart: Find Your Perfect Match
Finding the right furnace filter is easier when you can compare standard sizes at a glance. Use this comprehensive furnace filter size chart to quickly identify the dimensions you need for your HVAC system.
Standard Furnace Filter Sizes (Residential)
| Nominal Size (L x W x D) | Common Actual Size | Filter Type | Where It's Typically Used |
|---|---|---|---|
| 16x20x1 | 15.5" x 19.5" x 0.75" | Pleated, Fiberglass | Small apartments, older systems |
| 16x25x1 | 15.5" x 24.5" x 0.75" | Pleated, HEPA | Common in mid-sized homes |
| 20x20x1 | 19.5" x 19.5" x 0.75" | Washable, Electrostatic | Bedrooms, small condos |
| 20x25x1 | 19.5" x 24.5" x 0.75" | Pleated, Carbon | Most popular residential size |
| 25x25x1 | 24.5" x 24.5" x 0.75" | High-MERV, HEPA | Larger homes, high-efficiency systems |
| 16x25x4 | 15.5" x 24.5" x 3.75" | Thick pleated | Better filtration, longer lifespan |
| 20x25x4 | 19.5" x 24.5" x 3.75" | Media filter | High-performance HVAC systems |
| 25x25x4 | 24.5" x 24.5" x 3.75" | Whole-house air cleaner | Luxury homes, advanced filtration |
How to Use This Size Chart
- Match Your Measurements – Compare your filter's actual size to the chart.
- Consider Thickness – 1-inch filters are common, but 4-5 inch filters last longer and improve airflow.
- Check MERV Ratings – Thicker filters often support higher MERV ratings (better filtration).
What If Your Size Isn’t Listed?
- Odd Sizes (e.g., 14x24x1, 18x30x1) → Look for "custom size" filters online (FilterBuy, Amazon).
- Between Sizes? → Round up to the nearest standard dimension (e.g., 15.25" → 16").
- No Exact Match? → Consider an adjustable filter or consult an HVAC pro for retrofitting options.
Where to Buy Filters by Size
- ✔ Home Improvement Stores – Home Depot, Lowe’s (good for standard sizes)
- ✔ Online Retailers – Amazon, FilterBuy (wide selection, including custom sizes)
- ✔ HVAC Suppliers – Local shops can order specialty sizes
Struggling to find your filter size? Don’t worry—we’ll cover custom solutions and alternatives in the next section.
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What to Do If Your Furnace Filter Size Is Uncommon
Finding that your furnace requires an unusual filter size can be frustrating, but don’t worry—you still have options. Whether your system needs a slightly off-size filter or a completely custom dimension, here’s how to solve the problem.
Check for "Trim-to-Fit" Adjustable Filters
Some brands offer flexible filters that can be cut to size:
- Filtrete, Honeywell, and Nordic Pure make trim-to-fit filters.
- Use scissors to carefully resize the frame (follow package instructions).
Best for filters that are slightly larger than needed (don’t try to stretch smaller filters).
Order Custom-Sized Filters Online
Several retailers specialize in non-standard sizes:
- FilterBuy – Cuts filters to exact dimensions (even odd sizes like 14x24x1).
- Amazon – Search using your exact measurements (some sellers offer custom orders).
- HVACDirect – Carries a wide range of less common sizes.
Consider a Filter Retrofit
If your system uses an outdated or proprietary size, you might:
- Install a filter grille – Lets you use standard-sized filters in your return vent.
- Modify the filter slot – An HVAC technician can sometimes adjust the housing to fit common sizes.
- Upgrade to a media cabinet – For systems with thick filters (4-5 inches), a retrofit allows better filtration.
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Use a Close Standard Size (If Safe)
If your filter is just slightly off, you might:
- Round up to the nearest inch (e.g., 15.25" → 16").
- Use a foam filter gasket to seal small gaps (never leave major air leaks).
- Avoid forcing a too-large filter—it can bend the frame or restrict airflow.
Contact the HVAC Manufacturer
For rare or proprietary filters:
- Check the manufacturer’s website for replacement parts.
- Call their customer service with your model number.
- Some companies sell directly or recommend compatible alternatives.
DIY Solutions (Temporary Fixes Only)
If you’re in a pinch:
- Stack two filters (e.g., use two 1-inch filters if you need 2 inches).
- Tape the edges of a slightly small filter (use HVAC tape, not duct tape).
📌 Note: These are short-term fixes—order the correct size ASAP.
When to Call an HVAC Professional
Consult a technician if:
- Your system has no standard filter slot (some older units require internal filters).
- You need to modify ductwork for a new filter size.
- You’re unsure about compatibility (wrong filters can damage your system).
Final Tip: Measure Twice, Order Once
Before buying custom filters:
- Recheck your measurements (see our measuring guide).
- Compare prices—sometimes buying in bulk saves money.
- Consider upgrading to a higher-MERV filter if switching sizes.
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