Quick Answer
A micron label describes a particle-size rating whose definition must be checked with the manufacturer; it does not always mean the median. A FEPA P-grade specifies a grain-size distribution. A 30 µm rating is close to the P500 median of 30.2 µm and coarser than the P600 median of 25.8 µm; 15 µm is close to P1200’s 15.3 µm median. Similar size does not establish identical distributions or finishing performance.
For the coarser coated steps of a ladder: sanding belts and hook-and-loop discs and backing pads.
The micron to P-grade conversion chart
Selected median values from the publicly readable ISO 6344-3:2021 preview are shown below. A P-grade also requires compliance with distribution limits; matching the median alone is insufficient. This subset is not the full standard or a product interchangeability table.
| Median size (µm) | FEPA P grade |
|---|---|
| 58.5 | P240 |
| 46.2 | P320 |
| 35.0 | P400 |
| 30.2 | P500 |
| 25.8 | P600 |
| 21.8 | P800 |
| 18.3 | P1000 |
| 15.3 | P1200 |
Manufacturer film-grit equivalents are product-specific labels, not universal ANSI/CAMI conversions. Consult the manufacturer’s data for finer grades or micron-rated films and confirm the intended process.
Why fine abrasives are graded in microns at all
Grit numbers trace back to sieves. Macrogrits, roughly grit 4 up to 220–240, are separated on stacked test screens, the number tracing to mesh openings per inch (UAMA). Below about 60 µm the particles slip through the finest practical screen, so the industry sizes them directly, by sedimentation, electrical sensing zone or laser diffraction, and grades each statistically.
ISO 6344-2 covers coated macrogrits P12–P220. ISO 6344-3:2021 covers microgrits P240–P5000; its P240–P1200 requirements include median-size tolerances and limits elsewhere in the distribution. The standard therefore does not stop at P2500. A single micron rating cannot by itself establish compliance with a P-grade.
FEPA coated-abrasive P grades and bonded-abrasive F grades are separate systems. Do not substitute one for the other by number. If a specification says only “320 grit”, ask which standard and product it refers to.
30µ and 15µ film discs, decoded
Film abrasives put precision-graded mineral on a polyester (PET) film backing instead of paper. Because the film is thin, incompressible and dimensionally stable, every grain sits in one flat plane at the same height, keeping the scratch pattern predictable. Two backing tiers dominate: about 3 mil (~76 µm) film under the finer grades, roughly 0.1–30 µm mineral, and about 5 mil (~127 µm) under the coarser 45–125 µm grades.
A 30 µm disc has a stated rating close to P500’s 30.2 µm median and coarser than P600’s 25.8 µm median. A manufacturer’s “600 grit” film label does not by itself establish ANSI/CAMI 600. A 15 µm disc is close to P1200’s 15.3 µm median. Check how the manufacturer defines the micron rating and test the sequence; equal nominal sizes do not guarantee equal distributions or results.
Why the chart can never be exact
Compare identified grading systems rather than bare numbers. ISO 6344-3:2021 lists the P600 median as 25.8 µm; an ANSI/CAMI or JIS number is a different designation and must be checked against its own specification and the manufacturer’s data. These systems do not establish interchangeable products by number alone.
A grade specifies a distribution, not a single particle size. Do not assume a blanket ±10–20% tolerance. For example, ISO 6344-3:2021 specifies a P400 median of 35.0 ±1.5 µm and P600 of 25.8 ±1.0 µm, alongside other distribution limits. A micron label may refer to a different statistic; consult the product data.
The cross-standard reference table
FEPA-P and ANSI/CAMI track closely at coarse grits, up to about P220, then split. The 400 grit case shows the gap. A FEPA P400 disc (about 35 µm) is coarser than a CAMI 400, so a “400-grit” FEPA disc and a “400-grit” CAMI disc leave different finishes. By the time CAMI reads 400, the FEPA equivalent is already P600–P800.
| FEPA-P | CAMI | JIS | ~Micron (µm) |
|---|---|---|---|
| P120 | 120 | 120 | ~125 |
| P180 | 180 | 180 | ~78 |
| P220 | 220 | 220 | ~66 |
| P320 | 240 | 320 | ~46 |
| P400 | 320 | 360 | ~35 |
| P600 | 400 | 500 | ~26 |
| P800 | 400 | 600 | ~22 |
| P1200 | 600 | 800 | ~15 |
| P2500 | 1200 | 1500 | ~8 |
(FEPA; manufacturer data. Japan's JIS R6001 scale sits between the two and is shown for completeness.)
These are nominal medians from published conversion charts such as the metallography.org grit size chart. Charts differ, and the spread widens at the fine end. By the polishing range, a US/CAMI 600 is only about as fine as a FEPA P1200, so treating “600 grit” as one universal value across catalogues will mis-sequence a finishing job by several steps. Use the table to find where a grade sits, then confirm the CAMI or JIS grade against its own specification and the manufacturer’s data before substituting products.
Sequencing across graded systems
The point of the chart is sequencing. Every ladder obeys the same discipline: each step must fully erase the previous step's scratch pattern, you skip at most one grit, and the next number is no more than about 50% higher and never more than double the current one. Those rules only work when every number is on the same scale. The classic failure is the grit skip: stepping from a CAMI-graded coarse paper to a FEPA-graded fine paper by printed number, silently leaving a too-large jump or a wasted stage. Full rules: our grit progression and sequencing guide.
When adding micron-rated film to a grit sequence, compare the manufacturer’s particle-size definition and test whether each stage removes the previous scratches. A 15 → 9 → 5 µm film sequence is not automatically equivalent to a particular P-grade sequence.
The “P” prefix identifies a FEPA coated-abrasive grade; an unprefixed number does not by itself identify ANSI/CAMI, JIS or a manufacturer’s own scale. Confirm the standard and product data. Bonded-abrasive F grades and coated-abrasive P grades are separate systems and must not be substituted by number alone.
Buying notes for fine-finishing media
In the P400–P2000 range where most refinish sanding lives, loading, not wear, is the main way a sheet dies, so most fine paint-sanding paper carries a zinc-stearate anti-load coat; for the same micron grade, a stearated sheet and a bare sheet do not behave the same. At P3000-plus, dense closed coats only survive on film-backed, stearated or structured media, part of why the finest grades migrate to film. Structured abrasives sold on an “A” scale follow the same micron logic: the number after the A is the mineral's micron size (A30 ≈ 30 µm), though published A-to-grit tables disagree, so check the maker's data sheet.
The honest way to list a fine abrasive is standard plus micron, e.g. “P2000” rather than a bare “2000 grit”. Ask for both on every fine-grade order, from auto body shop essentials to the day-to-day range in metal fabrication essentials. If a supplier gives you a bare number with no scale and no micron value, assume nothing.
FAQ
What grit is a 30 micron film disc?
A 30 µm rating is close to the P500 median (30.2 µm) and coarser than P600 (25.8 µm). A film sold as “600 grit” is not thereby ANSI/CAMI 600. Check the manufacturer’s rating definition, distribution and intended use before substituting products.
What grit is a 15 micron film disc?
A 15 µm rating is close to the P1200 median of 15.3 µm. This is a size comparison, not proof that the film meets the P1200 distribution or produces the same finish. Use the manufacturer’s specification and a process trial.
What micron size is P2000 sandpaper?
A bare “2000 grit” label does not identify its grading system. Confirm whether the manufacturer specifies FEPA P2000, another standard or its own designation, and request its particle-size distribution before substituting products.
Are micron to grit conversions exact?
No. A comparison of nominal sizes does not establish identical particle distributions, abrasive construction or finishing performance. Confirm the manufacturer’s specification and validate the process before substituting products.
Is a P-grade the same as a CAMI grit at fine sizes?
No. Fine-grit designations from different systems are not interchangeable by printed number. ISO 6344-3:2021 lists P400 at a median of 35.0 µm and P600 at 25.8 µm. Check the corresponding ANSI/CAMI specification and manufacturer’s data separately. An unprefixed label does not identify its standard.
Why do film abrasives use microns instead of grit numbers?
Micron ratings offer a direct way to describe fine particle size, but the manufacturer must define what the rating measures. FEPA P-grades remain defined through P5000 in ISO 6344-3:2021. A micron value and a grit designation describe different aspects of a size distribution; neither alone guarantees a finished Ra.
