The Difference Between Nominal Efficiency and Tested Efficiency in Filter Documents

When you are comparing air filters for a facility, the wording on supplier literature can be easy to misread. Terms like nominal efficiency, tested efficiency, and rating class may appear side by side, but they do not always describe the same thing.

For facility managers and procurement teams, the key is knowing what has been tested, what is a stated class, and what is simply brochure language.

Why the wording matters

A filter document may look straightforward, but the numbers or terms on the page can point to different kinds of evidence.

• •Nominal efficiency is often a broad claim about how a filter performs under stated conditions.
• •Tested efficiency refers to measured performance from a defined test method.
• •Rating classes such as MERV, EN 779 classes like G3, G4, F5 to F9, and ISO 16890 categories are not the same system and do not convert directly.

If a supplier uses several terms in one datasheet, it is worth asking what standard supports each claim and whether the figure refers to the media, the complete filter, or a specific test condition.

This is especially important when the filter will be used in a critical application, a high dust environment, or a system where pressure drop and airflow need to be managed carefully.

What nominal efficiency usually means

Nominal efficiency is often used as a broad marketing or descriptive term. In simple terms, it may indicate the filter’s expected performance, but it does not always tell you how that figure was reached.

Common signs of brochure language

• •The document uses words like “typical”, “approximate”, or “rated” without naming a test method.
• •It lists a percentage but does not explain particle size, test dust, or test conditions.
• •It refers to a product family rather than a specific filter model.

That does not automatically mean the claim is wrong. It does mean you should not treat it as a standards-based test result unless the document clearly says so.

For buyers, nominal efficiency is best treated as a starting point, not the final answer.

What tested efficiency means

Tested efficiency is more useful when you need to compare products on a like-for-like basis. It should be tied to a recognised test method and a clearly defined condition.

A proper test-based claim should ideally tell you:

• •which standard or method was used
• •the particle size or dust loading condition involved
• •whether the result applies to the media alone or the assembled filter
• •whether the value is initial, average, or final efficiency

For example, HEPA filters are defined under EN 1822 by their efficiency at the most penetrating particle size, with H13 at greater than or equal to 99.95% and H14 at greater than or equal to 99.995%.

That kind of statement is very different from a general brochure claim because it is tied to a defined test framework.

If your project involves ventilation design or compliance questions, check the site requirements with the mechanical services engineer or building owner, and consider the direction of AS 1668.2:2024 where relevant.

Why filter ratings are not interchangeable

One of the most common mistakes is assuming every filter rating system uses the same scale. They do not.

MERV, EN 779 and ISO 16890

MERV is commonly used in some markets, EN 779 classes such as G3, G4, F5 to F9 are still widely used in Australia even though EN 779 has been superseded by ISO 16890, and ISO 16890 uses a different approach again.

These systems are based on different test methods and different particle groupings. That means a simple conversion chart can be misleading.

If a supplier gives you a MERV value on one sheet and an ISO 16890 class on another, do not assume they are equivalent just because the numbers look similar.

A better approach is to compare the actual test basis, the application, and the air handling requirements of the site.

How to read a filter datasheet carefully

When you are reviewing supplier literature, look past the headline number and check the fine print.

Useful questions to ask

• •Is the efficiency claim nominal or tested?
• •What standard or test method supports the claim?
• •Does the document describe the full filter, the media, or a laboratory sample?
• •Is the stated rating suitable for the system airflow and pressure drop range?
• •Does the filter fit the frame, housing, and access arrangement already in place?

It can also help to compare the datasheet with the physical product. If you need help checking dimensions, our guide on how to measure an air filter may be useful before you order.

If you are sourcing replacements across multiple sites, a consistent naming convention and clear product records will save time and reduce confusion between similar-looking items.

Questions that protect procurement decisions

Procurement teams often need to balance performance, fit, maintenance access, and supplier consistency. The right questions make that easier.

Ask before you buy

• •Is this product a standard size or a custom item?
• •Can the supplier confirm the exact rating basis in writing?
• •Does the product suit the existing equipment and filter housing?
• •Are there any installation or handling requirements that affect performance?
• •If the product is washable, are there specific cleaning steps and limits in the instructions?

Washability is only a feature of some products, so do not assume one filter can be cleaned and reused simply because it looks similar to another.

If your site needs non-standard dimensions or a special build, our custom air filters page explains the type of information that helps us match the right product to the application.

Practical signs you are looking at a solid claim

A reliable filter document usually does a few simple things well.

• •It uses one rating system clearly, rather than mixing several without explanation.
• •It names the test standard or recognised class.
• •It gives dimensions, construction details, and installation notes.
• •It avoids vague wording that sounds technical but says little about actual performance.

By contrast, a document that relies on generic phrases and unexplained percentages may be fine for a sales overview, but it is not enough on its own for technical comparison.

The safest approach is to compare documents against the same criteria every time: test basis, fit, airflow, pressure drop, and maintenance access.

Inspection frequency should always depend on the system, site conditions, and dust load, and you should follow the filter or equipment manufacturer instructions while monitoring pressure drop where gauges are fitted.

Getting the right filter for the job

Total Air Filtration is a family-owned air filter manufacturer and supplier based in Hillcrest, Brisbane QLD, and we work with buyers who need clear product information as well as the right physical fit.

If you want help separating brochure language from a standards-based claim, we can talk through the application, the document wording, and the filter options available through our products or filter servicing.

For assistance with a specific site or replacement requirement, call 07 3800 3448 or request a quote. You can also contact us if you need help reviewing a datasheet before you place an order.