How to Measure an Air Filter: Nominal vs. Actual Dimensions Explained
How to Measure an Air Filter: Understanding Nominal vs. Actual Dimensions
Have you ever ordered a filter labeled as a common size, only to find that it does not fit your equipment correctly? Maybe a filter seems slightly too small and leaves gaps, or it is just large enough that it will not slide into the housing.
Your tape measure is not wrong. The confusion comes from the difference between nominal size and actual size. Understanding how filters are measured is the key to getting the correct replacement the first time.
At FiltrationNow, we help customers find the right filtration solutions for residential, commercial, and industrial applications. Whether you are replacing an HVAC filter, compressed air filter, or industrial filtration element, accurate measurements ensure proper performance and protection.
Why Filters Have Nominal and Actual Sizes
Before measuring your filter, it is important to understand why two different measurements exist.
Nominal Filter Size
The nominal size is the advertised or labeled size of the filter. It is typically rounded to the nearest whole number to make filters easier to identify and purchase.
For example, a filter labeled as 20″ x 20″ x 1″ does not necessarily measure exactly 20 inches by 20 inches by 1 inch.
Actual Filter Size
The actual size is the true physical measurement of the filter. Manufacturers typically make filters slightly smaller than their nominal dimensions so they can slide into the filter housing without becoming stuck.
This difference is intentional and allows for easier installation, removal, and proper airflow around the filter.
How to Measure Your Existing Filter
If you have a filter that currently fits correctly, measuring the filter itself is usually the easiest method. Do not rely only on the printed label, since sizing can vary between manufacturers.
- Measure Length and Width: Measure the outside edges of the filter frame from side to side.
- Measure Thickness: Measure the depth of the filter from front to back. A filter labeled as a 1-inch filter may have an actual thickness closer to 0.75 inches.
- Record the Dimensions: Always write measurements in the correct order: Length x Width x Thickness.
How to Measure the Filter Housing Without an Existing Filter
If the old filter is missing or you are working with a new installation, you can measure the filter opening or rack instead.
- Measure the Opening: Use a stiff tape measure to determine the inside dimensions of the filter slot or housing.
- Allow for Clearance: Subtract approximately 1/4 to 1/2 inch from the opening measurements to allow the filter to install and remove properly.
For example, if your filter opening measures exactly 20″ x 20″, you would typically need a filter with an actual size around 19.5″ x 19.5″.
Common Nominal vs. Actual Filter Sizes
| Nominal Size | Typical Actual Size |
|---|---|
| 10 x 20 x 1 | 9.5″ x 19.5″ x 0.75″ |
| 16 x 25 x 1 | 15.5″ x 24.5″ x 0.75″ |
| 20 x 20 x 1 | 19.5″ x 19.5″ x 0.75″ |
| 20 x 25 x 4 | 19.75″ x 24.25″ x 3.75″ |
Why Accurate Filter Measurements Matter
Using the wrong size filter can affect system performance and filtration efficiency.
- Filters That Are Too Small: Gaps around the filter can allow unfiltered air to bypass the media, reducing filtration performance and allowing contaminants into the system.
- Filters That Are Too Large: A filter that does not fit properly can restrict airflow, damage the filter frame, or make installation difficult.
This issue is common because many customers assume a filter labeled as a certain size will measure exactly that dimension. The difference between nominal and actual sizing is an industry standard designed to make installation easier.
Applications Where Proper Filter Sizing Matters
Correct filter dimensions are important across many industries and systems. HVAC equipment, compressed air systems, hydraulic machinery, vacuum pumps, and industrial filtration equipment all rely on properly fitted filters to operate efficiently.
In compressed air systems, improperly sized filters can create airflow restrictions or allow contaminants to pass through. For industrial equipment, selecting the correct filter size helps protect critical components and reduce unnecessary maintenance.
Conclusion: Measure First, Replace With Confidence
Finding the right filter starts with accurate measurements. Understanding the difference between nominal and actual dimensions prevents ordering mistakes and helps ensure your replacement filter fits correctly.
By measuring length, width, and thickness—and confirming the actual dimensions of your filter—you can improve filtration performance and avoid costly installation issues.
Need Help Finding the Right Filter Size?
FiltrationNow can help you identify replacement filters for residential, commercial, and industrial applications. If your measurements do not match a standard size, we can help you find the right solution.
Contact FiltrationNow today for assistance selecting the correct filter for your equipment.
