GeoMiner Free earth-science tools

Built by a geologist

Sediment Grain Size and Phi Calculator

Particle size is one of the fundamental observations used to describe clastic sediment and sedimentary rock, but the applicable scale and measurement method must be stated. The Udden–Wentworth grade scale groups individual particles by diameter. Its boundaries progress mainly by factors of two: boulder is at least 256 millimetres; cobble is 64 to less than 256 mm; pebble grades span 4 to less than 64 mm; granule gravel is 2 to less than 4 mm; very coarse sand is 1 to less than 2 mm; coarse sand is 0.5 to less than 1 mm; medium sand is 0.25 to less than 0.5 mm; fine sand is 0.125 to less than 0.25 mm; and very fine sand is 0.0625 to less than 0.125 mm. Coarse silt is 0.03125 to less than 0.0625 mm, medium silt is 0.015625 to less than 0.03125 mm, fine silt is 0.0078125 to less than 0.015625 mm, very fine silt is 0.00390625 to less than 0.0078125 mm, and clay-size material is finer than 0.00390625 mm. At a boundary, this calculator assigns the value to the coarser grade, following the lower-inclusive ranges shown in modern USGS tables. The Krumbein phi scale converts diameter in millimetres using phi = negative log base 2 of diameter. One millimetre equals 0 phi, 0.5 mm equals 1 phi, 0.25 mm equals 2 phi, 2 mm equals −1 phi, and 256 mm equals −8 phi. Each increase of one phi halves the diameter. Coarse grains therefore have negative phi values, which is expected rather than an error. Micrometres convert to millimetres by dividing by 1,000. GeoMiner accepts one value in millimetres, micrometres, or phi, displays all three units, assigns the detailed Udden–Wentworth particle-size grade and broad clay, silt, sand, or gravel aggregate, and provides a printable chart. A single converted diameter does not classify a mixed sediment sample, describe sorting, or identify mineral composition. Natural samples contain distributions of particle sizes. Representative sampling, sample mass, disaggregation, removal of organics or carbonate, sieving, settling, laser diffraction, image analysis, instrument detection limits, mass-versus-volume basis, statistical method, and the chosen classification system can all affect a result. A mean in phi converted back to millimetres relates to a geometric rather than ordinary arithmetic diameter. Geology, engineering, soil science, agriculture, hydrology, construction aggregate, and environmental programs may use different size boundarie

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • undefined. The Udden grain-diameter progression, Wentworth sediment grades, Krumbein phi conversion, and the distinction between a particle-size grade and a mixed-sediment class.
  • undefined. Published millimetre and phi boundaries for clay, four silt grades, five sand grades, granule, pebble, cobble, and boulder material.
  • undefined. A modern USGS study chart stating phi equals negative base-2 logarithm of diameter in millimetres and showing detailed Udden–Wentworth ranges.
  • undefined. A conversion table relating standard sieve openings, diameter in millimetres and micrometres, phi value, Wentworth size class, and sediment type.
  • undefined. Laboratory-method context showing that grain-size statistics depend on sampling, preparation, instrument range, sieving, and distribution calculations—not one converted diameter alone.

Read GeoMiner's editorial methodology.

Common questions

What is the Wentworth grain-size scale?
The Udden–Wentworth scale groups individual sediment particles by diameter into clay-size, silt, sand, granule, pebble, cobble, and boulder grades. Grade boundaries progress mainly by factors of two.
How do I convert millimetres to phi?
Phi equals the negative base-2 logarithm of particle diameter in millimetres: phi = −log2(d mm). One millimetre is 0 phi, 0.5 mm is 1 phi, and 2 mm is −1 phi.
Why are coarse grains negative phi values?
The minus sign reverses the diameter scale. Diameters larger than 1 mm therefore have negative phi values, while progressively finer particles have increasingly positive values.
Can one grain diameter classify a sediment sample?
No. It classifies that diameter or a stated statistic only. Naming a mixed sediment and describing sorting require a representative sample, a grain-size distribution, suitable preparation and measurement, and an explicitly stated classification method.
Are soil and sediment grain-size boundaries identical?
Not always. Geology, engineering, agriculture, hydrology, and environmental programs can use different class names, boundaries, and measurement methods. State the system used and do not silently substitute one standard for another.

JavaScript is loading the interactive GeoMiner experience. The identification tools provide educational candidates, not laboratory confirmation.