The fastest way to find your air filter size is to pull out the existing filter and read the number printed on the cardboard frame. If the label is gone, measure the filter slot instead. Once you know the size, you can choose the right MERV rating and replacement schedule.
Your air filter size is usually printed on the edge of the current filter, such as 16×20×1. If the size is missing, measure the filter slot for width, height, and depth, then compare it to common nominal sizes.
The fastest method: read your existing filter {#read-existing-filter}
Pull out the current filter and inspect the cardboard frame. Most filters have the size printed along one edge.
It will look like one of these:
- 16×20×1
- 20×25×1
- 16×25×4
- 20×20×1
- 20×25×5
The format is width × height × depth in inches. This printed number is the nominal size, and it is the size you should use when ordering.
What if the filter has two sizes printed?
Some filters show both nominal and actual size. Use the nominal size for ordering. The actual size is there to explain the physical measurement, but replacement filters are usually sold by nominal size.
Do not measure a 16×20×1 filter, see that it is slightly smaller, and then order a smaller custom filter. Standard filters are intentionally manufactured smaller than the printed nominal size so they fit the slot.
How to measure if the label is missing {#how-to-measure}
If the label is worn, missing, or unreadable, measure the filter slot opening in the HVAC system.
Measure three dimensions:
- Width — one face dimension of the opening.
- Height — the other face dimension of the opening.
- Depth — how thick the filter needs to be.
Use a metal tape measure if possible. Measure the slot, not only the old filter, because the old filter might already be the wrong size.
How to convert measurement to nominal size
For common standard filters, the nominal size is usually rounded up from the actual measurement.
| Actual measurement | Likely nominal size |
|---|---|
| 15.5 × 19.5 × 0.75 | 16×20×1 |
| 19.5 × 24.5 × 0.75 | 20×25×1 |
| 15.5 × 24.5 × 0.75 | 16×25×1 |
| 19.5 × 19.5 × 0.75 | 20×20×1 |
| 15.5 × 19.5 × 3.75 | 16×20×4 |
If your measurement does not line up with a common nominal size, you may need a custom-size filter.
Once you have your nominal size, use the AirFilterVault size finder to match the right replacement and compare common options. Find your filter →
Nominal vs. actual filter size {#nominal-vs-actual-size}
Every filter size has two versions.
- Nominal size is the rounded size printed on the filter, such as 16×20×1.
- Actual size is the true physical measurement, often slightly smaller.
This difference is normal. A filter needs a little clearance so it can slide into the slot without crushing the frame.
The safest rule is simple: if the old filter has a printed nominal size, order that exact size.
Common residential air filter sizes {#common-filter-sizes}
Some sizes appear more often in residential HVAC systems.
| Nominal size | Common use |
|---|---|
| 16×20×1 | Standard residential returns |
| 16×25×1 | Common furnace and air handler size |
| 20×20×1 | Square return grilles |
| 20×25×1 | Larger residential systems |
| 16×20×4 | Media filter cabinets |
| 16×25×4 | Larger furnace filter cabinets |
| 20×25×5 | High-efficiency media systems |
Common sizes are easier to buy and often cheaper than custom filters. But fit matters more than popularity. The correct filter should slide in without crushing and should not leave visible gaps.
What if your size is not standard? {#non-standard-size}
Non-standard sizes are common in older homes, custom return grilles, and modified HVAC systems.
You have two choices:
- Find a standard size that fits snugly. This only works if there are no meaningful gaps.
- Order a custom-size filter. This is better when no standard size fits the slot properly.
Do not use a filter that is too small just because it technically fits. Gaps allow unfiltered air to pass around the filter.
Check your MERV rating next {#check-merv-rating}
Once you know the size, choose the right MERV rating. For many homes, MERV 8 is a safe starting point. MERV 11 is a good upgrade for pets and mild allergies. MERV 13 should be used only if your system can handle it.
Use the MERV calculator if you want a quick recommendation.
Related guides {#related-guides}
- how to measure air filter size
- how to determine the correct air filter size
- how to find your HVAC model number
- common HVAC filter sizes by brand
Frequently asked questions {#faq}
Where is the air filter size printed?
The air filter size is usually printed on the cardboard edge of the existing filter. It looks like 16x20x1, 20x25x1, or 16x25x4. That printed number is the nominal size you use when ordering a replacement.
What if my air filter size is not printed?
If the label is missing or unreadable, measure the filter slot opening for width, height, and depth. You can also measure the old filter, but the slot is more reliable if the old filter may be the wrong size.
What is the difference between nominal and actual filter size?
Nominal size is the rounded size printed on the filter, such as 16x20x1. Actual size is the true physical measurement, which is usually about 0.25–0.5 inches smaller. Always order by the nominal size when it is printed on the filter.
What if my air filter size is not standard?
If no standard nominal size fits snugly, you may need a custom-size air filter. Do not use a filter that leaves visible gaps around the frame because unfiltered air can bypass it.
Find your filter size {#closing}
Start with the printed size on the old filter. If it is missing, measure the slot and use the AirFilterVault size finder to compare your measurement against common replacement sizes.
Frequently asked questions
Where is the air filter size printed?
The air filter size is usually printed on the cardboard edge of the existing filter. It looks like 16x20x1, 20x25x1, or 16x25x4. That printed number is the nominal size you use when ordering a replacement.
What if my air filter size is not printed?
If the label is missing or unreadable, measure the filter slot opening for width, height, and depth. You can also measure the old filter, but the slot is more reliable if the old filter may be the wrong size.
What is the difference between nominal and actual filter size?
Nominal size is the rounded size printed on the filter, such as 16x20x1. Actual size is the true physical measurement, which is usually about 0.25–0.5 inches smaller. Always order by the nominal size when it is printed on the filter.
What if my air filter size is not standard?
If no standard nominal size fits snugly, you may need a custom-size air filter. Do not use a filter that leaves visible gaps around the frame because unfiltered air can bypass it.