Ordering the correct replacement air filter starts with taking the correct measurements. While this sounds straightforward, measuring an industrial air filter incorrectly is one of the most common reasons replacement filters don't fit, seal correctly or perform as intended.
Unlike many products, industrial air filters are manufactured in a wide range of designs. A pleated panel filter is measured differently to a pocket filter, a HEPA filter requires different information to a cartridge filter, and simply measuring the outside dimensions is often not enough.
This guide explains what information should be recorded for each filter type before ordering a replacement.
Step 1 – Identify the Filter Type
Before taking any measurements, identify the type of filter you are replacing.
The most common industrial air filters include:
- Pleated Panel Filters
- Pocket (Bag) Filters
- HEPA Filters
- Cassette Filters
- Cartridge Filters
Each requires different information.
Measuring a Pleated Panel Filter
A standard pleated panel filter is measured as:
Width × Height × Depth
For example:
595 × 295 × 50 mm
Measure the outside dimensions of the frame, not just the filter media.
The orientation is important.
A filter measuring 595 × 295 mm is not the same as 295 × 595 mm.
Although the outside dimensions are what determine whether the filter will physically fit, the internal construction is equally important.
On a pleated panel filter:
- The width runs across the pleats and is determined by the number of pleats.
- The height follows the width of the filter media before it is pleated.
- The depth is the overall thickness of the completed filter.
If you are manufacturing a replacement or trying to exactly match an existing filter, also record:
- Pleat count
- Media type
- Efficiency
- Frame material
- Support mesh or faceguard
- Gasket arrangement (if fitted)
Two filters may have identical outside dimensions but different pleat counts, resulting in different media areas, pressure drop and dust-holding capacity.
Measuring a Pocket Filter
Pocket filters are also measured as:
Width × Height × Pocket Depth
However, additional information is required.
Record:
- Width
- Height
- Pocket depth
- Number of pockets
On a pocket filter:
- The width is determined by the number of pockets placed side by side.
- The height is the height of each individual pocket opening.
- The depth is the length of the pockets.
Also record:
- Filter efficiency
- Frame material
- Header style (if applicable)
Two pocket filters may have exactly the same outside dimensions but use different numbers of pockets. This changes the available filter media area and affects airflow and dust holding capacity.
For that reason, the number of pockets should always be recorded.
Measuring HEPA Filters
HEPA filters are commonly manufactured in two styles:
- Box Type
- Header Type
It is important to identify which style you have before taking measurements.
Box Type HEPA Filters
A Box Type HEPA filter consists only of the filter body.
Measure:
- Width
- Height
- Depth
For shallow pleated HEPA filters:
- The width runs across the pleats and is determined by the number of pleats.
- The height follows the width of the media pack before pleating.
Also record:
- Pleat count (where practical)
- Efficiency
- Separator or mini-pleat construction
- Frame material
- Gasket position
- Gel seal or knife-edge (if applicable)
- Airflow direction
Header Type HEPA Filters
A Header Type filter uses the same filter body but has a header frame fitted around the front face of the filter.
The header sits flush with the front face of the filter and increases the overall outside dimensions.
For example:
A 570 × 570 × 300 mm Box Type HEPA fitted with a 25 mm header becomes an overall 595 × 595 × 300 mm Header Type filter.
When measuring a Header Type filter record:
- Overall width
- Overall height
- Overall depth
- Header width
- Efficiency
- Seal arrangement
- Frame material
Do not assume a Header Type and Box Type filter are interchangeable simply because the filter body is similar.
Measuring Cassette Filters
Cassette filters are generally manufactured using a V-bank arrangement and are measured similarly to V-bank HEPA filters.
Measure:
- Width
- Height
- Depth
Also record:
- Number of V-banks
- Number of media packs (where practical)
- Efficiency
- Header or Box Type construction
- Header width (if fitted)
- Seal arrangement
- Airflow direction
The outside dimensions alone are often insufficient to identify the correct replacement.
Measuring Cartridge Filters
Round cartridge filters are measured differently from rectangular filters.
Record:
- Outside Diameter (OD)
- Inside Diameter (ID)
- Overall Height
Also record:
- Open/Open or Open/Closed ends
- End-cap style
- End-cap material
- Thread or mounting arrangement
- Gasket arrangement
- Pleat count (where practical)
- Filter media
Many cartridge filters appear similar externally but use different mounting systems, making these details essential.
Do Not Rely on Nominal Sizes
A filter commonly described as 600 × 600 mm may not physically measure 600 × 600 mm.
For example, many filters supplied as 600 × 600 actually measure approximately 592 × 592 mm.
Always measure the existing filter rather than relying on previous paperwork or assumptions.
Photograph Everything
Before disposing of the old filter, take photographs of:
- The complete filter.
- The manufacturer's label.
- The installation.
- The airflow direction.
- Any seals or gaskets.
- The header (if fitted).
A photograph often answers questions that measurements alone cannot.
Common Ordering Mistakes
The most common mistakes when ordering replacement industrial air filters include:
- Measuring the filter media instead of the frame.
- Reversing the width and height on rectangular filters.
- Forgetting to record the number of pockets on a pocket filter.
- Forgetting the pleat count when matching an existing panel or shallow HEPA filter.
- Forgetting whether a HEPA or cassette filter is Header Type or Box Type.
- Forgetting the header width.
- Assuming nominal dimensions are actual dimensions.
- Throwing the old filter away before the replacement has been checked.
Before Ordering Your Replacement
Before contacting your supplier, confirm you have recorded:
Panel Filters
- Width
- Height
- Depth
- Pleat count
- Efficiency
- Frame material
Pocket Filters
- Width
- Height
- Pocket depth
- Number of pockets
- Efficiency
HEPA Filters
- Width
- Height
- Depth
- Header or Box Type
- Header width (if fitted)
- Efficiency
- Seal type
Cassette Filters
- Width
- Height
- Depth
- Number of V-banks
- Header or Box Type
- Efficiency
Cartridge Filters
- Outside Diameter
- Inside Diameter
- Height
- End-cap style
- Mounting arrangement
- Gasket
- Media type
Summary
Measuring an industrial air filter involves far more than simply recording three dimensions.
Different filter designs require different information, and details such as pleat count, pocket quantity, header construction, seal arrangement and mounting style are often just as important as the overall size.
Taking a few extra minutes to correctly identify and measure the existing filter can prevent costly ordering mistakes, reduce delays and ensure the replacement filter fits and performs exactly as intended.