Quick Answer
A micron rating states the actual median particle size of the grain. A P-grade is a FEPA code for a controlled size distribution, pinned to a d50 median under the ISO 6344 coated standard (ISO 6344-3; KB: Microgrit and Micron Grading). The two line up only approximately: a 30 micron film disc lands between P500 (30.2 µm) and P600 (25.8 µm) and is commonly sold as a “600 grit equivalent,” while a 15 micron film sits at roughly P1200 (15.3 µm) and is sold as “1200 grit” (KB: Microfinishing and Lapping Films; Grit Conversion Chart). Above P240, sequence mixed systems by micron value, not by the printed numbers.
The micron to P-grade conversion chart
The P-grade microns below are the published FEPA/ISO d50 medians from grain-producer data (Washington Mills; KB: Microgrit and Micron Grading). The CAMI column and film-disc markings are approximate matches, shown as ranges where published charts disagree.
| Particle size (µm) | FEPA P (d50) | ANSI/CAMI (approx.) | Film-disc marking | Typical job |
|---|---|---|---|---|
| 58.5 | P240 | 220–240 | 60 µm film (“240”) | macro/micro boundary, pre-finish smoothing |
| 46.2 | P320 | 280–320 | — | sealer / primer scuff |
| 35.0 | P400 | 320–360 | 40 µm film (“320–400”) | paint feathering, fine sand before stain |
| 30.2 | P500 | ~360 | 30 µm film (“600”) | fine sanding, coarse film step |
| 25.8 | P600 | ~400 | — | between-coats sanding |
| 21.8 | P800 | 500–600 | — | fine finishing |
| 18.3 | P1000 | 600–800 | — | de-nibbing, lacquer prep |
| 15.3 | P1200 | 600–1000 | 15 µm film (“1200”) | intermediate lap |
| 12.6 | P1500 | ~800 | — | pre-polish, clearcoat cut start |
| 10.3 | P2000 | 1000–1500 | — | wet-sand cut |
| ~9 | ≈P2000 | ~1500 | 9 µm film (“2000”) | polished edge, scratch refinement |
| 8.4 | P2500 | 1200–2000 | — | polish prep |
| ~5 | P5000 | ~3000 | 5 µm film (“3000–4000”) | high-gloss prep |
| 3 | beyond the P-scale | — | 3 µm film (“4000”) | mirror prep |
| 1 | beyond the P-scale | — | 1 µm film (“8000”) | final polish |
| 0.5 / 0.3 | beyond the P-scale | — | “10,000” / “20,000” | optical finishing |
Two caveats. The CAMI ranges are wide because the ANSI B74.18 coated standard stops at 600 grit; every finer CAMI number is a manufacturer extrapolation, which is why published charts disagree (KB: Grit Conversion Chart). The film-disc column is vendor shorthand, not a standard; micron-graded film has no binding grit designation at all (KB: Film Backing). For everyday sandpaper grits, see our master abrasive grit chart.
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; KB: Microgrit and Micron Grading). 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.
The standards reflect that split. ISO 6344-2 covers coated macrogrits P12–P220 by sieving; ISO 6344-3 covers microgrits P240–P5000 with sedimentation as the reference method, the 2021 edition extending the range to P5000 (ISO 6344-3; KB: Microgrit and Micron Grading). Each microgrit P-grade is pinned to a d50 median with tail limits capping oversize grain, which stops one rogue coarse particle scratching a fine finish (FEPA; KB: FEPA Grit Scale). Above roughly P2500 even that runs out: conversion gets so approximate that makers print the micron size, which is why polishing progressions end in microns, not grit numbers (KB: Microgrit and Micron Grading).
One more trap: FEPA runs a second, independent microgrit family for bonded wheels, the F-scale (F230–F2000 under FEPA 42-2 / ISO 8486-2). An F number is never interchangeable with a P number, and the fine-end gap is dramatic: in the 5–7 µm band the coated scale says P5000 while the bonded scale says only F800 (KB: Microgrit and Micron Grading; Grit Conversion Chart). If a spec sheet just says “320 grit,” the first question is which standard.
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 (KB: Film Backing; Microfinishing and Lapping Films). 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 (KB: Film Backing).
A 30 µm disc carries grain almost exactly the size of P500 paper (30.2 µm d50), one step coarser than P600. Vendor tables still market it as “600 grit” because they quote a CAMI-style number, and an unprefixed 600 is a coarser grain than P600 (KB: Grit Conversion Chart). A 15 µm disc is, for practical purposes, P1200 grain (15.3 µm d50), sold as “1200 grit” (KB: Microfinishing and Lapping Films). The same 0.3 µm film has even been sold as both “20,000” and “60,000” grit (KB: Microfinishing and Lapping Films). Treat the micron value as the real spec and the grit equivalent as shorthand: slot a film disc into a paper ladder by its micron size against the d50 column above, not by the box number.
Why the chart can never be exact
Cross-scale conversion is approximate by construction. Each grading body fixes a different statistic of the distribution, an average, a median or a maximum, and allows a different spread of oversize and undersize grain, so the same name means different microns to each (KB: Microgrit and Micron Grading). FEPA P grades to tighter distributions than CAMI, and the gap widens as the grain gets finer (KB: Grit Conversion Chart). The textbook example is 600 grit: FEPA P600 grain is about 26 µm, an unprefixed CAMI 600 about 15 µm, and JIS 600 sits between at about 20 µm, three different particles under one number (KB: Grit Conversion Chart). More in our FEPA vs CAMI grit scale guide.
Even within one scale, a grade is a tolerance band, not a single size; expect roughly ±10–20% variation around the nominal micron, so two P400 papers from different mills can legitimately differ (KB: Grit Conversion Chart).
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 (KB: Grit Progression and Sequencing). 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 (KB: Grit Conversion Chart). Full rules: our grit progression and sequencing guide.
When film enters the ladder, convert by micron: a clearcoat cut-and-polish job runs P1500 → P2000 → P3000 before compounding (KB: Grit Progression and Sequencing); in microns that is 12.6 → 10.3 → ~7. A film-disc shop covers it with 15 µm → 9 µm → 5 µm stock; matched by printed grit number instead, the same swap can jump two real grades. The finish each step should leave is in our grit to surface finish Ra chart.
Two reading tips keep you honest. The “P” prefix is the tell: a genuine P400 signals FEPA grading, a bare 400 usually signals CAMI, and North American stock defaults to CAMI while European stock defaults to FEPA-P (KB: Grit Conversion Chart). And never carry a number across the coated/bonded boundary: sanding discs and sanding belts are P-scale, grinding and cut-off wheels are F-scale, so a “320” wheel and a “P320” disc are different particles (KB: Microgrit and Micron Grading).
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 (KB: Microgrit and Micron Grading). 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 (KB: Microgrit and Micron Grading).
The honest way to list a fine abrasive is standard plus micron, e.g. “P2000 (≈10.3 µm, ISO 6344-3)” rather than a bare “2000 grit” (KB: Grit Scale Equivalence and Tolerance Bands). That is the convention we work to as a stockist, from our auto body shop essentials to the day-to-day range in our 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?
By particle size, 30 µm grain lands between P500 (30.2 µm) and P600 (25.8 µm) on the FEPA coated scale. Vendor equivalence tables commonly market a 30 µm film as a “600 grit” product because they quote a CAMI-style number (KB: Microfinishing and Lapping Films; Grit Conversion Chart).
What grit is a 15 micron film disc?
A 15 µm film is effectively P1200 grain, whose FEPA d50 is 15.3 µm, and it is normally sold as a “1200 grit equivalent” (KB: Microfinishing and Lapping Films). CAMI matches for that size vary from 600 to 1000 across published charts because the ANSI B74.18 standard stops at 600 (KB: Grit Conversion Chart).
What micron size is P2000 sandpaper?
P2000 is defined by a d50 median of 10.3 µm under the FEPA/ISO coated system (Washington Mills data; KB: Microgrit and Micron Grading). Note that a CAMI-style “2000” is commonly matched to P2500, which is 8.4 µm, a full grade finer, so a bare 2000-grit label is ambiguous.
Are micron to grit conversions exact?
No. Each scale fixes a different statistic of the particle distribution with a different allowed spread, so exact equivalence is not possible, and film-disc grit equivalents are vendor interpolations with no binding standard behind them (KB: Microgrit and Micron Grading; Film Backing). The same 0.3 µm film has been sold as both “20,000” and “60,000” grit.
Is a P-grade the same as a CAMI grit at fine sizes?
No. The scales are close to interchangeable up to about 220, then diverge sharply: P400 ≈ CAMI 320–360, CAMI 400 ≈ P800, CAMI 600 ≈ P1200 (KB: Grit Conversion Chart; Grit Scale Equivalence and Tolerance Bands). Above 220, swap by micron value, never by printed number.
Why do film abrasives use microns instead of grit numbers?
Because their grades sit where grit scales stop working. Grain finer than about 60 µm cannot be sieved and is measured directly by sedimentation or optical methods, and above roughly P2500 grit-to-grit conversion is so approximate that makers print the particle size itself (UAMA; ISO 6344-3; KB: Microgrit and Micron Grading).