Porosities and Sizes


Frits come in differing standard porosities, and a full range of diameters, from tiny to 380 mm. We can provide frits of all porosities and various diameters, although stock sizes will always be less expensive. We are distributors for the excellent ROBU porous glass filters, and stock mostly their frits, so the P- series porosities will be our first choice during the design process.


 Custom and precision dimensions are available as special orders from Germany.

Custom Robu Frits

American standards -
(Kimble & Corning, ASTM) nominal pore size, in microns

Pore size, in microns
Extra Coarse
170-220 µm
40-60 µm
10-15 µm
4-5.5 µm
Very Fine
2-2.5 µm
Ultra Fine
0.9-1.4 µm

European standards -
(Robu & Schott, ISO 4793) nominal pore size, in microns

(The parenthetical designation doesn't seem to be in use much yet, although it is much more descriptive)

Pore size, in microns
P00 (P500)
250-500 µm
P0 (P250)
160-250 µm
P1 (P160)
100-160 µm
P2 (P100)
40-100 µm
P3 (P40)
16-40 µm
P4 (P16)
10-16 µm
P5 (P1.6)
1.0-1.6 µm

Ace Glass, nominal pore size, in microns

Ace makes their filters from fibers, rather than particles.

Designation Pore size, in microns
A 145-174 µm
B 70-100 µm
C 25-50 µm
D 10-20 µm
E 4-8 µm


Each of the three systems of pore size specification generally cover your common needs. There are a few specialized porosities in the finer range, such as 0.5 - .75 µm, in limited thicknesses and diameters.

Porosities of sintered glass filters are determined dynamically: air is forced through a frit just immersed in water until a bubble goes through. The pressure required correlates with the pore size, according to theory: pore diameter (in microns) = 30 * surface tension (in dynes/cm) / pressure (in mm HG). This should correlate with the average pore size of the whole frit. This also says nothing about the specific shape of the pores; some could be long and narrow, with the same functional aperture as those that are closer to round.

If the material you are filtering is not uniform in size, but rather a distribution of sizes, the likelihood of particles clogging the frit is large.


Pore volume for various porosities

Pore volume in %
~ 32%
~ 35 %
~ 40 %
P2, Coarse
~ 40 %
~ 42 %
P4, Medium
~ 42 %
~ 45 %
P5, Ultra Fine
~ 50 %


These values are measured under vacuum; at atmospheric pressure, there may be dead end pores which would be unavailable to your liquid.