EN ISO 14688-1 against USCS: two different size ladders

Updated: September 30, 2026 · ISKRIT

The same sieve data, run through EN ISO 14688-1 and through ASTM D2487, produces different fraction percentages and can produce a different soil name. The reason is not interpretation, it is that the two systems cut the size axis in different places.

Where the cuts fall

The consequence for a real sample

The 2 mm against 4.75 mm cut is the one that moves numbers. Material between those sizes is coarse sand to a European report and gravel to an American one. In a fluvial deposit that band can hold 10 to 20 % of the mass, which is enough to swing the gravel-versus-sand comparison and change the USCS prefix from S to G while the EN description stays “sand”.

The 0.063 against 0.075 cut moves less mass but matters more at the thresholds: a soil at 11.5 % passing 0.075 mm may read 12.4 % passing 0.063 mm, crossing the 12 % line that separates a dual symbol from a single one.

Silt and clay are not the same concept

This is the deeper difference. EN ISO 14688-1 separates silt from clay at 0.002 mm, a particle size. USCS separates them on the plasticity chart, a behaviour. A rock flour with 40 % of its mass finer than 0.002 mm but no plasticity is clay by the European size definition and silt, ML, by USCS. Both answers are correct within their own system, and quoting one soil name in a report written to the other standard is a genuine error, not a translation choice.

Practical handling

Run the sieve set the target standard expects rather than converting afterwards. EN ISO 14688 work uses the 63, 20, 6.3, 2, 0.63, 0.2, 0.063 mm progression; USCS work uses 75, 37.5, 19, 9.5, 4.75, 2, 0.425, 0.075 mm. Both sets are preloaded in the calculator, so a sample entered once can be described under either system and the two descriptions compared directly.

Questions

Why do EN ISO 14688-1 and USCS give different fractions for the same sieve data? They cut the size axis in different places: sand and gravel at 2 mm against 4.75 mm, and fines at 0.063 mm against 0.075 mm.

Are silt and clay separated the same way in both systems? No. EN ISO 14688-1 separates them by particle size at 0.002 mm, USCS by plasticity on the plasticity chart. A non-plastic rock flour can be clay by the European definition and silt, ML, by USCS.

Which sieve set should be run? The one the target standard expects, rather than converting afterwards: 63, 20, 6.3, 2, 0.63, 0.2 and 0.063 mm for EN ISO 14688, and 75, 37.5, 19, 9.5, 4.75, 2, 0.425 and 0.075 mm for USCS.

Switch the standard selector on the grain size tool to see both descriptions of the same curve.