When we refer to nominal Sizes,we meant the external size of the filter holding frame size. Standardised proper and factory fabricated sizes are 24x24" or 610x610mm and 12x24" or 305x610mm. Most use inches as nominal sizes as they will be shorter easier to read which commonly used in US. For prefilter and secondary filters,common nominal sizes are 24x24", 12x24",20x24",20x20",16x24",16x20". Thickness or depth, 1",2",4",6",12" (15-26"depth for Bag Pocket). For HEPA filters, actual filter sizes in mm is used instead. Manufacturer standard sizes should be used to facilitate inital design, project implemenations, stock and facilitate operation and maintainence, practically benefitting everybody from designers,contractors,workers,purchasers,buyers,admin and operation,users. 

Thus the filter size or we mean actual size is usually 12-15mm or 1/2 - 5/8" inches smaller in the width(or length) or height, this will allow tolerances for filter to easily fit into filter holding frames. Nobody want a tight hard time during replacement. Nominal Thickness or depth are typcially like 2"(50.8mm) will have actual size smaller thickness 45mm(-5mm). Varying Aircon filters manufacturer or supplier may have tolerances 5-10mm. Tight tolerances will cause filters to be very difficult to install as various factories may use different size tolerances. Another reason why standard sizes should be used.


Gaskets are to be used to seal gaps on the holding frame size to minimise bypass. In cases of HEPA installations, they should be front loaded and clamp to ensure <0.03% to 0.01% leakage.


Example for Pre and secondary

Nominal SizeWxHxD       Filter/Actual size

24x24x2"                        595x595x45mm +/-3mm

12x24x2"                        290x595x45mm

20x20x4"                        495x495x95mm

24x24x12"                      595x595x292mm

HEPA Filter Box or NH standard sizes w clamps

Nominal SizeWxHxD        Filter/Actual size

650x650x350mm              610x610x292mm

355x650x350mm              305x610x292mm

HEPA Single Header design

Universal holding frame size   Filter/Actual size

610x610x70mm                     592x592x292mm 

305x610x70mm                     287x592x292mm

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