The Micro Prefilter captures dust particles down to 0, 5 micron, That is amazingly close to what a HEPA rated filter captures at 0, 3 micron, This is why the Micro Prefilter makes the Dustless WetDry Vacuum so much better than other shop vacuums, and it is how the HEPA filter in Dustless HEPA Vacuums last so long, It is an incomparable asset, You cant fill the Micro Prefilter, by design, The vacuum holds up to 40 lbs of dirt and debris but the huge bag holds up to 80 lbs, That is so that at least 50 percent of the surface area is always available to support high airflow, The Micro Prefilter also retains its strength when wet, Hepa Filters 3 Pack replacement, 5 Layers of filtration, Maintains strength even when wet, Filters down to 99, 95, Dimensions: 4L x 9H x 9W, SKU: DTLS021