When the Filter Label Shows Only a Nominal Size: What Buyers Should Ask Next

An old filter label that only shows a nominal size can be a useful starting point, but it is not enough to order a true replacement. In practice, the rounded size on the label may be close to the actual dimensions, yet the frame, depth, seal and media configuration can still differ enough to affect fit and performance.

For mechanical contractors and industrial buyers, the next step is not to guess. It is to ask a few specific questions so the replacement matches the equipment, the housing and the site conditions.

Why nominal size is only part of the story

Nominal sizes are often rounded for convenience. A label might say 610 x 610 mm, but the actual filter could be slightly smaller, deeper, thinner or built with a different frame style.

That matters because filters are not interchangeable just by label alone. Two filters with the same nominal size may still differ in:

• •Actual outside dimensions
• •Frame depth and flange detail
• •Seal type and gasket position
• •Media type and pleat pack layout
• •Initial resistance and dust holding characteristics
• •Direction of airflow or installation orientation

If you are replacing an older product, the original manufacturer may have used an older naming convention, a custom build or a now-superseded specification. That is why a label reading only a nominal size should be treated as a clue, not a final order reference.

Questions to ask before you reorder

What are the actual measurements?

Ask for the exact width, height and depth, not just the nominal size. If the filter is still in the system, measure the existing unit carefully and confirm whether the dimensions refer to the frame, the media pack or the overall outside size.

If you are unsure how to measure it correctly, see how to measure an air filter.

Helpful details to check include:

• •Overall outside size
• •Internal opening or seat size
• •Filter depth
• •Flange width
• •Gasket or seal position
• •Direction of airflow marking

What type of frame or construction was used?

The label may not tell you whether the filter uses cardboard, galvanised steel, aluminium or another frame style. It may also hide the way the media is supported or bonded.

That matters in systems where the filter sits in a close-fitting holding frame or where the seal depends on the frame profile rather than just the nominal size. Ask whether the old filter was:

• •Panel, pleated or pocket style
• •Flat, extended surface or mini-pleat construction
• •Single-use or a washable design, if relevant
• •Built with a gasket, gel seal or other sealing method

Some filters are designed to be washable - check the product instructions before assuming a used filter can be cleaned and returned to service.

What efficiency or rating was specified?

If the old label only shows size, there may be no indication of the filtration grade. In that case, try to confirm the original rating from the equipment manual, maintenance records or the mechanical services engineer.

Different rating systems do not convert directly. MERV, EN 779 and ISO 16890 describe performance in different ways, so it is better to identify the original requirement than to try to guess an equivalent.

If the system was specified under an older EN 779 class such as G3, G4 or F5 to F9, remember that these classes are still commonly referred to in Australia even though ISO 16890 is now widely used. For higher-efficiency applications, HEPA filters are defined differently again: H13 is at least 99.95% and H14 is at least 99.995% efficiency at the most penetrating particle size under EN 1822.

What does the equipment and site actually need?

The right replacement depends on the system, the duty and the surrounding environment. Industrial dust load, intake location, process activity and operating hours can all affect how a filter performs in service.

Before ordering, ask:

• •Is this a supply air, return air or process system?
• •Is the filter installed in a rooftop unit, ducted system or manufacturing plant equipment?
• •Is there a pressure gauge or differential pressure reading available?
• •Has the housing been modified, repaired or retrofitted since the original filter was installed?
• •Are there any site-specific requirements from the building owner or mechanical services engineer?

If the ventilation system is tied to compliance work or design changes, confirm the site requirements against AS 1668.2:2024 with the mechanical services engineer or building owner.

How to work from an old label without ordering the wrong filter

A nominal size label is most useful when it is paired with photos and measurements. If the old filter is available, take clear photos of the front, side, frame corners, gasket and any airflow markings.

When you send the details through, include:

• •Nominal size from the label
• •Exact measured dimensions
• •Frame material and depth
• •Any markings on the filter body
• •A photo of the filter installed in the housing, if possible
• •The equipment make and model, if known
• •Any known rating or specification from records

This helps avoid common mistakes such as ordering the right nominal size but the wrong depth, or choosing a filter that fits the opening but does not seal correctly in the frame.

For larger projects, ongoing maintenance or multiple units across a site, it can help to keep a simple replacement record for each housing. That record should note the actual build details rather than relying on a rounded label alone.

What to do when the original spec cannot be found

Sometimes the old filter label is the only clue left. In that case, the safest approach is to match the physical filter and the housing, then work back to the required performance.

A practical approach is to:

• •Measure the old filter carefully
• •Confirm the frame style and seal arrangement
• •Compare the fit in the housing
• •Check whether the current system still needs the same duty
• •Speak with the mechanical services engineer or building owner if the specification is unclear

If the old filter is damaged, distorted or heavily loaded, do not rely on the distorted dimensions alone. The housing dimensions and original drawing, if available, are often more reliable than a worn-out sample.

Where the original product is no longer available, a custom replacement may be the best path. custom air filters can be built to suit non-standard sizes, unusual depths or legacy housings.

Getting the replacement right from the start

When you are replacing a filter based on a nominal size label, the goal is to confirm the actual build before the order is placed. That means asking the right follow-up questions about dimensions, frame, seal, rating and site conditions rather than relying on the rounded size alone.

If you also need ongoing support once the filters are in service, our filter servicing and commercial filter servicing pages may help with planning and maintenance across your site.

Talk to Total Air Filtration

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. We can supply or make the right filters when a label only shows a nominal size and more detail is needed to match the housing properly.

Call 07 3800 3448 or request a quote. You can also browse our products or contact us if you need help identifying the right replacement.