Magnified schematic — the wire-to-opening ratio is true to scale; very thin lines are drawn at a minimum visible width.
A US standard No. 270 test sieve (ASTM E11) has a nominal opening of 53 µm.
| Opening (microns) | 53 µm |
| Opening (millimeters) | 0.053 mm |
| Opening (inches) | 0.0021 in |
| US standard sieve | No. 270 |
| Tyler equivalent | 270 mesh |
| Pitch (wire center to center) | 89 µm |
| Typical wire diameter | 0.036 mm |
| Approx. open area | 35% |
A 53 µm opening falls within the silt (3.9 µm–62.5 µm) class of the Wentworth grain-size scale. Comparing grain size with the nominal opening only: very fine sand and coarser would be retained; silt is split at about 53 µm. Real sieving also depends on particle shape, sieving time and the opening tolerance of the cloth.
These applications commonly use a 270 mesh count or an opening near 53 µm:
Standard: phyto 55 µm · zoo 335 µm (custom 10–780 µm)
A standard Apstein net uses 55 µm mesh for phytoplankton and 335 µm for zooplankton; custom meshes from 10 to 780 µm are available. Mesh choice depends on the target organisms.
Ranges show common practice with sources on the mesh size by application page — always check your equipment maker's recommendation.
The name "270 mesh" suggests 270 openings per inch, i.e. a pitch of 25,400 ÷ 270 = 94.1 µm. Standard test sieves, however, are defined by their opening, not by the count: a 53 µm opening woven with the typical 0.036 mm wire gives a real pitch of 53 + 36 = 89 µm, which is about 285.4 wires per inch. That is why mesh numbers and micron values never line up perfectly — and why this site uses the ASTM E11 opening as the answer.
| Mesh | Microns (µm) | Millimeters | Inches |
|---|---|---|---|
| 200 mesh | 75 | 0.075 | 0.003 |
| 230 mesh | 63 | 0.063 | 0.0025 |
| 250 mesh | 63 | 0.063 | 0.0025 |
| 270 mesh | 53 | 0.053 | 0.0021 |
| 300 mesh | ≈ 51 | 0.051 | 0.002 |
| 325 mesh | 45 | 0.045 | 0.0018 |
| 400 mesh | 38 | 0.038 | 0.0015 |
Values are ASTM E11 nominal openings (Tyler equivalents for 150 and 250 mesh). Italic ≈ values are estimates for non-standard mesh counts, calculated from the mesh count and an assumed wire diameter (interpolated between ASTM E11 sieves, or extrapolated beyond No. 635). Wire values are ASTM E11 typical wire for standard sizes.
270 mesh is 53 µm, which is 0.053 mm or 0.0021 inches.
In a simplified comparison with the nominal opening, particles smaller than about 53 µm can pass a 270 mesh screen. A 53 µm opening falls within the silt (3.9 µm–62.5 µm) class of the Wentworth grain-size scale. Comparing grain size with the nominal opening only: very fine sand and coarser would be retained; silt is split at about 53 µm. Real sieving also depends on particle shape, sieving time and the opening tolerance of the cloth.
With a typical 0.036 mm wire, 270 mesh has roughly 35% open area. Thicker wire lowers the open area and the opening size.