Definition
Mesh size is, in technical practice, a measure of the fineness of a powder, derived from the density of a sieve. The mesh number indicates the number of openings per linear inch (1 inch = 25.4 mm) of a standardised sieve - the higher the number, the finer the sieve and the smaller the powder particles. Besides the mesh designation, Europe commonly uses a direct figure in micrometres (μm).
What mesh means (US/Tyler scale)
Two main scales are used in practice:
- US Standard Sieve Series (ASTM E11) - the most widespread in North America and for international trade.
- Tyler Equivalent - a historical British scale, with values very close to the US Standard.
Both scales state the number of openings per inch of a reference sieve. For practical purposes (powder specifications in commercial dealings) they are interchangeable with a tolerance of a few percent.
Conversion mesh → μm
An indicative conversion table for the US Standard scale:
- mesh 40 = ~ 420 μm (coarsely ground powder)
- mesh 60 = ~ 250 μm
- mesh 80 = ~ 177 μm (medium-fine powder)
- mesh 100 = ~ 149 μm
- mesh 120 = ~ 125 μm (fine powder)
- mesh 140 = ~ 105 μm
- mesh 200 = ~ 74 μm (very fine powder, micro-powder)
- mesh 325 = ~ 44 μm (micronised powder)
Note: the figures indicate the upper limit of particle size - a powder designated "mesh 80" means it passed through an 80 sieve, i.e. the particles are no larger than ~177 μm. The size distribution is always broader (typically log-normal).
Effect of size on solubility and physics
Particle size affects a number of physical properties of the powder:
- Specific surface area: a finer powder has an order-of-magnitude larger specific surface, which speeds up the extraction of alkaloids into a solvent (of laboratory relevance for HPLC analysis).
- Bulk density: a finer powder often has a lower bulk density (more air between particles).
- Flowability: the finest powders (below 50 μm) can form agglomerates and worsen flow.
- Stability: a finer powder has a larger surface exposed to oxygen, which accelerates the oxidation of alkaloids - so proper storage is more critical for fine powders.
These observations are a purely physical description - not a recommendation for any use.
Standards for plant powders
Several reference standards exist for plant powders describing granulometry:
- USP <786> (United States Pharmacopeia) - a method for determining particle-size distribution by sieve analysis or laser diffraction.
- Ph. Eur. 2.9.38 (European Pharmacopoeia) - a compatible methodology in the EU.
- ASTM E11 - the specification of the sieves themselves.
These standards are not specific to kratom but generally applicable to herbal drugs.
Grinding and classification in practice
Grinding of plant material in commercial practice is carried out in several steps:
- Coarse crushing of the dried leaf (a knife mill, output ~mesh 20-40).
- Fine grinding (a hammer or impact mill, output ~mesh 60-120).
- Optionally micronisation (a jet mill, output below 50 μm).
- Sieve classification - separation of the oversize and undersize fractions.
The most common commercial finenesses for kratom powder are mesh 80-120 (≈ 125-177 μm). Micronised powder (below 50 μm) is rarer and is usually reserved for extraction technologies.
Frequently asked questions
What is mesh 80 in micrometres?
Mesh 80 by the US Standard corresponds to a sieve with an opening size of approximately 177 μm. A powder designated "mesh 80" has particles smaller than this value.
Is finer always better?
From the standpoint of extraction, yes; from the standpoint of stability, no - a finer powder oxidises faster. The optimum for ordinary use is mesh 80-120.
Is there a standard for plant powders?
Yes. USP <786> and Ph. Eur. 2.9.38 describe methods for determining particle-size distribution. No specific limits for kratom are set by the pharmacopoeias.
How can the declared size be verified?
By sieve analysis (more costly) or laser diffraction in an accredited laboratory. The COA should state which method was used to determine the size.
References
- USP <786>. Particle Size Distribution Estimation by Analytical Sieving. United States Pharmacopeia.
- Ph. Eur. 2.9.38. Particle-size distribution estimation by analytical sieving. European Pharmacopoeia, 10th edition.
- ASTM E11-22. Standard Specification for Woven Wire Test Sieve Cloth and Test Sieves. ASTM International.
