Sediment Particle Size Classifier (Wentworth Scale)

The Wentworth scale, a century-old standard

Chester K. Wentworth published his sediment size classification in 1922, building on earlier work by Johan Udden (1898). The Udden-Wentworth scale remains the global standard in sedimentology, geology, and engineering for describing particle size from microscopic clay to room-sized boulders.

The scale is logarithmic. Each major class boundary is a doubling of grain size:

Those boundaries look arbitrary until you write them as fractions. Sand starts at 1/16 mm and clay starts at 1/256 mm, and every class edge in between is one of those halved or doubled. Plenty of textbooks round 1/256 to 0.004, which is close enough for prose and wrong enough to matter if you are converting to phi, so this page keeps the exact 0.0039.

The phi (φ) scale

For statistical analysis, sedimentologists use the phi scale:

φ = −log₂(d in mm)

Each whole phi unit equals one doubling. The reason: grain size data is logarithmically distributed (small grains are far more numerous than large), so taking logs makes statistics tractable.

Key reference values:

Note that phi runs backwards. Bigger grains have smaller, eventually negative, phi values, which trips up everyone the first time they read a grain size distribution plotted left to right.

A grain at phi 3 (0.125 mm, the very fine to fine sand boundary) is 8 times the diameter of a grain at phi 6 (0.0156 mm, medium silt). Working in phi lets sedimentologists treat sediment as a normal distribution and use standard statistics (mean, sorting or standard deviation, skewness).

Sorting: how uniform is the grain size

Beyond mean size, sedimentology cares about sorting (how spread out the grain sizes are):

Sorting tells you about the depositional environment:

• Well sorted = stable single-energy environment (constant wind, calm beach)
• Poorly sorted = chaotic transport (glaciers carry everything, regardless of size)
• Bimodal = two distinct sources (e.g., flood deposit with sand + cobbles)

A well-sorted, fine-grained sandstone in the rock record almost certainly indicates beach or dune origin. A poorly-sorted, angular, mixed deposit suggests glacial or landslide origin.

Stokes’ law and settling velocity

For fine particles in still water, settling velocity follows Stokes’ law (George Stokes, 1851):

vs = (ρs − ρf) × g × d² ÷ (18 × μ)

Where:

• vs = settling velocity (m/s)
• ρs = particle density (~2,650 kg/m³ for quartz)
• ρf = fluid density (~1,000 kg/m³ for water)
• g = 9.81 m/s²
• d = particle diameter (m)
• μ = fluid viscosity (~0.001 Pa·s for water at 20°C)

Some typical settling rates for quartz in water at 20°C:

Those are rates for natural, irregular grains, and they sit below what Stokes’ law predicts for a smooth sphere of the same diameter. Shape is most of the reason. A flaky or angular grain presents more drag than a ball of the same volume, and in real water the grains also get in each other’s way, which slows the whole cloud further. Expect a real column to land somewhere between half and all of the Stokes figure for silt, and well under half of it for sand.

The dramatic range, from boulders settling instantly to clay staying up for weeks, is why lakes and oceans build sediment in layers. The biggest, densest grains drop first and the finest settle last, or never, until salt water flocculates them into aggregates heavy enough to fall.

Stokes’ law breaks down at larger grain sizes, above roughly 0.1 mm, where flow around the particle stops being smooth. The dividing line is not really a diameter, it is the particle Reynolds number Re = ρf × vs × d ÷ μ. Below about Re = 1 the flow clings to the grain and Stokes holds; above it, wake turbulence adds drag the law knows nothing about and the predicted velocity comes out too high. For quartz in water, Re passes 1 at almost exactly 0.1 mm, which is where the textbook cutoff comes from.

Why grain size matters in geology

Particle size affects almost every geologic process:

• Erosion potential: silt and fine sand lift off at lower wind and water speeds than clay does, which sounds backwards until you remember that clay particles stick to each other. The Hjulström curve maps it.
• Permeability: grain size correlates with how easily fluids flow through. Pebbles = very permeable; clay = nearly impermeable
• Porosity: well-sorted spheres have ~36% porosity regardless of size; poorly sorted sediment has much less
• Sediment color / mineralogy: smaller particles disproportionately concentrate certain minerals (clay minerals, iron oxides)
• Fossil preservation: smaller grain sediments (silt, clay) preserve finer fossils
• Soil agriculture: clay-rich soils hold water and nutrients but drain poorly; sandy soils drain fast but lose nutrients
• Construction: foundation soil properties depend heavily on grain size distribution

The 12-bin engineering classification

For engineering applications (geotechnical, civil), the AASHTO and USCS (Unified Soil Classification System) systems use slightly different bins:

The sieve numbers refer to the number of openings per inch of mesh. Sieve #200 (0.075 mm) is the boundary between “fines” (silt + clay) and “sand.”

Practical field grain size estimation

You don’t always have a sieve set. Field shortcuts:

The “rub the sediment between your fingers” test is surprisingly accurate after a little practice. Combined with appearance, you can field-classify any sediment within 15 seconds.

Sediment in the rock record

Lithified versions of sediments retain their original size classification:

Bottom line

The Wentworth scale, 100+ years old, remains the standard for sediment grain size. The phi (φ) scale allows statistical work. Sorting tells you about the depositional environment as much as the grain size does. Stokes’ law predicts settling velocity for fine particles. Grain size determines everything downstream: erodibility, permeability, fossil preservation, agriculture potential, and engineering behavior.