How this instrument works
A sieve's mesh number counts how many openings fit across one linear inch of its wire screen -- a 100-mesh sieve has 100 openings per inch, a 400-mesh sieve has 400 finer openings per inch. Because the wire itself takes up some of that inch, a higher mesh number always means a smaller opening: more wires packed into the same inch leaves less gap between them. Micron size measures that gap directly, in millionths of a metre, so mesh number and micron size move in opposite directions from each other -- as mesh goes up, micron size goes down.
There's no exact formula linking the two, because the real opening size depends on the wire diameter used at each mesh count, which varies slightly across the standard sieve series rather than following one clean equation. What's commonly used instead is a rule of thumb: micron ≈ 14,900 / mesh. It's disclosed as an approximation valid for mesh numbers from 50 to 400, the common range for US standard test sieves, and it lines up closely with the published values without ever landing on them exactly: at 100 the rule gives 14,900/100 = 149 microns against a standard opening of 150 microns, at 200 it gives 74.5 microns against 75 microns, and at 325 it gives roughly 45.8 microns against 44 microns -- within a percent or two every time, exact none of the time, which is the nature of a rule of thumb rather than a table lookup.
This calculator is deliberately scoped to that disclosed 50-400 mesh range and no further -- inputs are limited to it because the approximation isn't disclosed as valid outside those bounds. For mesh numbers below 50 or above 400, or wherever a project genuinely needs the certified opening size rather than a close approximation -- material specifications, quality certificates, laboratory calibration -- the correct source is the published ASTM E11 sieve table itself, not this or any other rule-of-thumb formula.
- Enter the US standard mesh number (mesh) for the sieve you're working with -- any whole number from 50 to 400.
- Read the result in Particle/opening size (microns, μm); it recalculates the instant the mesh number changes.
- Treat the result as a close approximation, not a certified figure -- it's typically off by a micron or two from the certified sieve opening, as the worked example below shows.
- For mesh numbers outside 50-400, or for a certified rather than approximate opening size, consult the published ASTM E11 sieve table instead of this rule of thumb.
Worked example — checking a 100-mesh test sieve
A lab technician needs to sanity-check that a 100-mesh test sieve on the shelf is roughly the right particle cutoff before using it to grade a powder sample. Entering 100 into the mesh field gives 14,900 / 100 = 149 microns exactly -- no rounding, no remainder in the arithmetic itself -- which lands within a single micron of the standard 100-mesh opening of 150 microns, close enough to confirm the sieve is in the right ballpark, though not an exact match to the certified figure.
The same rule applied to a finer 200-mesh sieve gives 14,900 / 200 = 74.5 microns, close to but not identical to the standard's 75-micron opening at that mesh count -- a gap that grows wider still at 325 mesh, where the rule gives about 45.8 microns against a standard opening of 44 microns. That pattern is expected: the 14,900 constant is a rule of thumb fitted across the whole 50-400 range, so it stays within about a micron of the certified figure at 100 and 200 mesh and drifts to nearly two microns off by 325 mesh -- never enough to matter for a quick sieve check, but not a substitute for the certified table on a specification sheet.
Questions
Is the 14,900/mesh formula an exact conversion?
No -- it's disclosed as an approximation, a rule of thumb rather than a certified conversion, and it never lands exactly on the certified figure. At 100 mesh it gives 149 microns against a standard 150-micron opening, and at 325 mesh it gives about 45.8 microns against a standard 44-micron opening -- close both times, exact neither time. For certified figures, use the published ASTM E11 sieve table rather than this formula.
Why isn't there one exact formula linking mesh number to micron size?
Because a sieve's actual opening size depends on both its mesh count and the wire diameter used at that mesh count, and wire diameter isn't a fixed multiple of mesh number across the whole standard sieve series -- it's set separately for each mesh count when the sieve series was standardized. The 14,900/mesh rule fits the overall trend closely without capturing those per-mesh variations exactly, which is why it's disclosed as an approximation rather than published as the defining formula.
What does a sieve's "mesh number" actually measure?
It's a count, not a length -- specifically, the number of wire openings across one linear inch of the sieve screen. A 100-mesh sieve has 100 openings per inch; a 400-mesh sieve packs 400 finer openings into that same inch. Because more wires fit into the inch as mesh number rises, the gap between each wire -- the actual opening a particle has to pass through -- gets smaller as mesh number gets larger, which is why mesh and micron size move in opposite directions.
Why is this calculator limited to mesh 50 through 400?
Because that's the range the 14,900/mesh approximation is disclosed as valid for -- it's also the common working range for US standard test sieves used in everyday particle-size and material-grading work. Outside that range, the rule of thumb isn't disclosed as reliable, so this calculator doesn't extrapolate past it; the published ASTM E11 sieve table is the correct source for mesh numbers below 50 or above 400.
Is a micron the same thing as a micrometre?
Yes -- "micron" is the older common name for the same unit now formally called the micrometre (symbol μm), one millionth of a metre. Both terms describe the identical length, and sieve and particle-size literature uses them interchangeably, though μm is the current SI symbol you'll see on most modern certificates and datasheets.
Where do I find the certified opening size instead of this approximation?
The ASTM E11 standard ("Standard Specification for Woven Wire Test Sieve Cloth and Test Sieve") publishes the certified opening size for every standard US mesh number, and it's the correct reference wherever a material spec, quality certificate, or lab calibration record needs an authoritative figure rather than a quick approximation.