Mesh to Micron Converter (ASTM E11 sieve No. ↔ µm)

Understanding the mesh to micron converter — concept and principle

This converter links US standard sieve numbers (mesh) with their nominal openings in micrometres, millimetres and inches according to ASTM E11. It works both ways: pick a sieve to see its opening, or enter a particle size to find the nearest sieve and its coarser and finer neighbours. Lab technicians, geologists, pharmacists and anyone buying filters, abrasives or powders use it.

Sieve number → hole size — with the default inputs: 75 µm.

How it is calculated

Sieve → opening: looked up in the ASTM E11 table (No. 200 = 75 µm, No. 100 = 150 µm, No. 60 = 250 µm, No. 20 = 850 µm, No. 4 = 4.75 mm …)
Size → sieve: the sieve whose opening is closest on a ratio (logarithmic) scale
mm = µm ÷ 1,000; inches = µm ÷ 25,400
Size versus nearest sieve = (your size ÷ opening − 1) × 100

No. 450, 500 and 635 (32, 25 and 20 µm) are included because they are commonly used, although outside the standard series.

The principle

The sieve number roughly equals the number of wire openings per linear inch, but the opening also depends on the wire thickness, so the mesh number is not a simple unit. That is why the official table is used instead of 25,400 ÷ mesh. Neighbouring standard sieves differ by a ratio of the fourth root of 2, about 1.19, so each step changes the opening by roughly 19%. Comparing sizes on a ratio scale matches this geometric series.

Worked example

Default: sieve No. 200.

Opening from the table = 75 µm.
In mm = 75 ÷ 1,000 = 0.075; in inches = 75 ÷ 25,400 = 0.003.
Nearest ASTM sieve: No. 200 (75 µm).
Next coarser: No. 170 (90 µm); next finer: No. 230 (63 µm).
Size versus nearest sieve: — (only used when converting from µm).

How to read the result

A particle passes a sieve if it is smaller than the opening in at least one orientation, so sieving sorts by roughly the second-largest dimension. In soil testing, the No. 200 sieve (75 µm) separates sand from silt and clay. When specifying a powder, give the sieve range, for example "passes No. 100, retained on No. 200".

Common mistakes

Calculating the opening as 25,400 ÷ mesh, which ignores wire thickness.
Mixing US sieve numbers with Tyler or other mesh systems.
Assuming all particles are spherical.
Using a worn or damaged sieve whose openings have changed.

Keep in mind: Actual sieve openings vary within tolerances set by the standard. For quality control, use certified sieves and follow the test method.

Frequently asked questions

How many microns is 200 mesh?

A US No. 200 sieve has a 75 µm opening.

Why is the mesh number not exact?

It counts openings per inch, but wire thickness also takes up space, so the opening is smaller than 25,400 ÷ mesh.

What is the step between standard sieves?

About 1.19 (the fourth root of 2).

Sources: ASTM E11-01, Standard Specification for Wire Cloth and Sieves for Testing Purposes, Table 1

Checked 9 Nov 2026. ASTM E11-01, Standard Specification for Wire Cloth and Sieves for Testing Purposes, Table 1 “Standard and Alternative Sieve Designations” (standard opening in µm/mm with the alternative No. designation; 25, 32 and 20 µm sieves listed as common-usage sizes outside the standard series).

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