Filter Foam PPI to Micron: Conversion Chart and Why It Is Not Exact
Filter Foam PPI to Micron: Conversion Chart and Why It Is Not Exact
GaoLeshi Technical Note | Reticulated Filter Foam | Updated 2026
The short answer
Engineers routinely ask for a PPI-to-micron conversion table, and it is a reasonable request: you know the particle size you must capture, and you want the foam grade that will do it. The honest answer is that PPI and micron rating measure different things, so any conversion is an approximation.
PPI counts pores along one inch of foam. Micron rating describes the largest particle that passes through a filter medium. Foam pores are not uniform spheres — they are irregular windows in a three-dimensional skeleton with a wide size distribution. Two foams both labelled 30 PPI can have genuinely different capture behaviour.
Typical PPI to micron correspondence
| PPI | Approx. pore size | Approx. particle retention | Typical duty |
|---|---|---|---|
| 10 PPI | Very coarse | Large debris only, well above 1000 µm | Pond intake, agricultural pre-filter |
| 20 PPI | Coarse | Roughly 500–1000 µm class | Coarse dust, lint, pre-filtration |
| 30 PPI | Medium-coarse | Roughly 300–600 µm class | General HVAC pre-filters |
| 45 PPI | Medium | Roughly 150–400 µm class | Paint booth, equipment intake |
| 60 PPI | Medium-fine | Roughly 80–250 µm class | Fine dust, cabin filtration |
| 80 PPI | Fine | Roughly 40–150 µm class | Fine mist, carbon carrier |
| 100 PPI | Very fine | Sub-100 µm class | Precision protection, carbon grades |
Why the numbers move around
Manufacturing spreadPPI is nominal
- A PPI tolerance of several points is normal
- Ask your supplier how they measure and control it
Depth capture, not sievingFoam is thick
- Thicker foam captures more than the PPI suggests
- Efficiency improves as the filter loads with dust
Flow velocitySpeed changes capture
- Always state your flow rate when specifying
- Oversizing the filter area lowers velocity and improves capture
Density interactionSecond parameter
- Specify PPI and density together
- Never compare two foams on PPI alone
How to measure it properly
Microscopic image analysis
Cut a thin section, photograph it at known magnification and measure a statistically meaningful number of pores. This gives a pore-size distribution rather than a single number, which is far more useful for predicting behaviour.
Challenge testing under real flow
Pass a calibrated test dust or aerosol through the sample at your actual flow rate and measure upstream and downstream concentration. This yields a true efficiency curve and is the only method that reflects how the foam will behave in your product.
Frequently asked questions
Is 30 PPI the same as a 300 micron filter?
Not exactly. 30 PPI typically sits in the several-hundred-micron band, but capture depends on thickness, density and flow velocity. Treat the number as a starting band and confirm by test.
Why do two suppliers quote different micron ratings for the same PPI?
Because there is no universal standard linking the two. Each supplier measures differently, and some publish marketing numbers rather than test data. Ask for the test method behind any figure.
Does reticulation change the PPI?
No. Reticulation removes the thin membranes between pores, opening the structure for flow. The pore count per inch stays essentially the same, but permeability rises dramatically.
Can you supply foam to a target micron rating?
We can supply to a target PPI and density combination and provide samples for you to test against your own specification. Where you have a defined efficiency requirement, send us the standard and flow conditions and we will propose a grade.