How to Measure Pleated, Panel and Bag Filters Correctly

Measuring an air filter sounds straightforward until you compare a pleated filter, a panel filter and a bag filter side by side. Each style is described a little differently, and a small measuring error can lead to a poor fit, air bypass or extra hassle when it is time to replace it.

If you are a homeowner, facility manager, mechanical contractor or industrial buyer, the key is to measure the way the filter is actually built and the way the frame or housing accepts it. This guide explains the practical differences so you can specify the right filter with confidence.

Start with the style of filter and the way it sits in the housing

Before you pick up a tape measure, identify the filter style and where it is installed. A pleated filter, a panel filter and a bag filter are not measured in exactly the same way, because each one presents its dimensions differently.

For any filter, work from the installed unit, not just the old filter on the bench. The housing, rails, clips and access door all affect what size will fit.

Useful starting checks include:

• •The filter type, such as pleated, panel or bag
• •The direction of airflow through the unit
• •Whether the filter sits in a holding frame, tracks or a bank
• •Whether the filter is rigid, semi-rigid or flexible
• •Whether the size shown on the label matches the physical size

If you need help identifying a replacement format, our products page is a good place to start, and how to measure an air filter has general measuring guidance as well.

How to measure pleated filters correctly

Pleated filters are often quoted by nominal size, but the actual frame size can differ slightly. The important thing is to measure the outside dimensions of the rigid frame, not the media pleats themselves.

Measure the outer frame, not the filter media

For a pleated filter, take measurements across the full outer edge of the frame.

• •Measure width from outer edge to outer edge
• •Measure height from outer edge to outer edge
• •Measure depth or thickness of the frame

Do not measure the folded media area inside the frame. The pleats increase the surface area, but they are not the dimension you use for ordering.

Check the orientation and seal surface

Some pleated filters have a gasket, a sealing lip or an arrow showing airflow direction. If the filter is installed in a tight frame, the sealing surface matters as much as the nominal size.

Check whether the replacement needs:

• •A gasketed edge
• •A cardboard, plastic or metal frame
• •A single flange or fully boxed edge
• •An airflow arrow to match the system

If the old filter is distorted, measure the undamaged side first and compare it with the housing opening. When in doubt, measure the slot or track the filter fits into, then compare that to the replacement spec.

How to measure panel filters correctly

Panel filters are usually simpler to measure because they are flat and rectangular, but the frame thickness and edge type still need attention. A panel filter might be used as a pre-filter, a final filter in a compact unit, or part of a larger commercial system.

Measure the full outside dimensions

Use the same approach as with pleated filters, but be careful not to confuse the visible media area with the full filter size.

• •Width: outside edge to outside edge
• •Height: outside edge to outside edge
• •Depth: the full thickness of the frame or media pack

If the panel has a rigid frame, include the frame itself in the measurement. If it uses an open edge or a soft gasket, note that detail when you request a replacement.

Note any frame details that affect fit

Panel filters can vary by edge profile, even when the nominal size looks the same on paper. The correct size is not just about length and width.

Look for:

• •Cardboard, plastic, metal or wire support frames
• •Fixed tabs or pull handles
• •Gasketed faces
• •A loose-fit or snug-fit design

For commercial systems, the filter bank or access door can change how much tolerance you have. If the specification needs to match building requirements, check with the mechanical services engineer or building owner, especially where AS 1668.2:2024 is relevant to the ventilation system design.

How to measure bag filters correctly

Bag filters need a different measuring habit because the visible bag length is only part of the story. Buyers often focus on the pocket length and miss the header frame, pocket count or pocket depth.

Measure the header and the pocket dimensions separately

A bag filter should be measured in two parts:

• •The header frame size that fits into the holding rack
• •The pocket or bag dimensions that determine how the filter hangs and opens

Measure the header width and height first, then record the pocket depth or length when fully extended. If the bags are collapsed from use, gently open them out before measuring, but do not stretch them hard or force the seams.

Check the number of pockets and the header style

Two bag filters can share a similar face size and still not be interchangeable. The pocket count, seam layout and header construction can affect whether the filter fits and performs as intended in the housing.

Note these features:

• •Number of pockets or bags
• •Header depth and thickness
• •Pocket length when open
• •Whether the header is metal, plastic or cardboard
• •Whether the filter uses a rigid or soft seal

If you are replacing a bag filter in a commercial air handling unit, the safest approach is to match the housing dimensions, header type and pocket configuration rather than relying on the old filter alone.

Practical measuring tips that apply to all three styles

There are a few habits that make measuring much more reliable, no matter which filter style you are ordering.

Use the right reference point

Measure the filter itself and the housing it fits into. If the old filter has swollen, bowed or compressed over time, the housing opening is often the better reference.

Record the size in the order the supplier expects

Most suppliers list dimensions as width, height and depth, but the orientation can vary between products. Write down each dimension clearly and include any special notes such as airflow direction, gasket type or pocket count.

Don’t assume the label is exact

The printed size on an old filter may be nominal rather than exact. That is common, which is why a physical check matters before placing an order.

Watch for nearby restrictions

Rails, latches, clips and access panels can all affect what will fit. If the filter is difficult to remove, check for signs of crushing, warping or bypass around the edges before ordering a replacement.

If the system needs a non-standard size, custom air filters may be the better option than trying to force a near-match into place.

When to ask for help with specification

If you are unsure whether the system takes a pleated, panel or bag filter, or if the dimensions seem inconsistent, it is worth getting the specification checked before you order. That is especially useful for retrofit work, mixed equipment sites and older plant where labels are missing or unclear.

You may also want to confirm whether the system needs a particular rating or media type. MERV, EN 779 and ISO 16890 do not convert directly, so the safest approach is to match the actual application and the manufacturer’s specification rather than trying to compare ratings on a simple one-to-one basis. For HEPA applications, only use the stated EN 1822 class where it is required: H13 is at least 99.95% and H14 is at least 99.995% efficiency at the most penetrating particle size.

If you are planning ongoing maintenance, filter servicing and commercial filter servicing can help keep the system on track. Inspection frequency depends on the site, the system and the dust load, and it is best to follow the filter and equipment manufacturer instructions and monitor pressure drop where gauges are fitted.

Talk to Total Air Filtration about the right fit

Total Air Filtration is a family-owned air filter manufacturer and supplier based in Hillcrest, Brisbane QLD. If you need help identifying the right pleated, panel or bag filter, we can help you measure it correctly and supply or make the right replacement.

Call 07 3800 3448 or request a quote. You can also contact us or browse industries we supply if you are sourcing for a specific site or application.