Smaller particles generally have greater porosity than larger ones. Porosity in the gravel-to-silt range can span from roughly 25% to 50%, with gravels sitting near the low end and finer materials like silt and clay approaching or exceeding the high end. This relationship surprises many people because we intuitively associate tiny particles with “tighter” packing, but the physics of how small grains interact tells a different story.
Why Smaller Particles Pack More Loosely
When particles are very small, surface forces like electrostatic charges and moisture films become powerful relative to the weight of each grain. These forces hold particles apart in irregular arrangements rather than letting gravity pull them into tight configurations. A grain of sand is heavy enough to settle snugly against its neighbors, but a clay platelet can be held in place by surface chemistry alone, leaving gaps throughout the material.
The result is counterintuitive: finer sediments end up with more total pore space, not less. Research on unconsolidated granular media confirms that the dependence of porosity on average grain size runs opposite to what most people expect. Porosity increases as grain size decreases across the gravel, sand, and silt series.
Porosity Ranges by Material
To put real numbers on this, polydisperse sands (mixtures of different sand grain sizes) typically have porosities around 0.30 to 0.35, or 30% to 35% of their volume occupied by pore space. Clay-rich soils behave differently because clay particles form small clumps called aggregates. Each aggregate might have 35% porosity internally, but the gaps between aggregates add even more pore space, pushing total porosity to 50% or higher.
USDA soil data reinforces this pattern from the opposite direction. Ideal bulk density (the mass of dry soil per unit volume) is below 1.60 g/cm³ for sandy soils but below 1.10 g/cm³ for clayey soils. Since bulk density and porosity are inversely related, lower bulk density means higher porosity. Clayey soils, built from the smallest particles, consistently have the most pore space.
Uniform Spheres Tell a Different Story
Here’s an important distinction: if you’re comparing perfectly uniform spheres of different sizes, particle size alone doesn’t change porosity at all. A box of identical 1 mm marbles packed the same way as a box of identical 10 mm marbles will have the same percentage of void space. For uniform spheres, porosity depends entirely on the packing arrangement, ranging from about 26% in the tightest possible configuration to roughly 48% in the loosest.
This is a theoretical baseline. In practice, real particles are never perfectly spherical or perfectly uniform. Real-world smaller particles are irregularly shaped, carry surface charges, and resist settling into efficient arrangements. That’s why the theoretical prediction (size doesn’t matter) diverges from real-world observation (smaller means more porous).
Sorting Matters as Much as Size
How uniform the particles are, a property geologists call “sorting,” has a major effect on porosity. Well-sorted materials (all particles roughly the same size) have higher porosity than poorly sorted ones (a mix of large and small). In poorly sorted sands, smaller grains slip into the gaps between larger grains and fill space that would otherwise be empty.
Classic experiments on synthetic sands found that poorly sorted sands are considerably less porous than well-sorted ones, with porosity showing an inverse linear relationship to the sorting coefficient. Interestingly, in well-sorted sands, porosity was independent of the median grain size, which aligns with the uniform-sphere theory. It was only in poorly sorted sands that porosity appeared to decrease slightly as median grain size increased.
So the answer to “which particle size has greater porosity” depends partly on whether you’re comparing pure fine material against pure coarse material (fine wins) or asking what happens when you mix sizes together (mixing reduces porosity for everyone).
Porosity vs. Permeability
One reason this topic confuses people is the difference between porosity and permeability. Porosity is how much empty space exists. Permeability is how easily fluid flows through that space. These two properties move in opposite directions as particle size changes.
Clay has high porosity but extremely low permeability. Its pores are so tiny and poorly connected that water barely moves through them. Gravel has low porosity but high permeability because its fewer, larger pores create easy flow paths. The classic Kozeny equation, still the standard model for estimating permeability, shows that permeability depends on both the square of the grain diameter and porosity. The grain size term dominates, so larger particles win on permeability even while losing on porosity.
If you’re thinking about water filtration, drainage, or oil extraction, permeability is what matters most. If you’re thinking about how much water a soil can hold or how much void space exists in a sediment, porosity is the relevant measure.
How Engineers Use This Relationship
In concrete design, engineers deliberately exploit the relationship between particle sizes and void space. The goal is usually to minimize porosity for maximum strength and durability. The strategy is called dense packing: spaces between large aggregate particles are filled with smaller particles, and the spaces between those are filled with even smaller ones.
However, this isn’t as simple as dumping everything together. When particles of different sizes interact, a “wedging effect” occurs where smaller particles push larger ones slightly apart, creating more void space than pure theory predicts. Engineers account for this by carefully controlling the ratio between particle sizes. As the particle size ratio increases (meaning greater difference between large and small particles), the residual void fraction decreases, approaching but never quite reaching theoretical minimums.
Pervious concrete flips this logic. By using only coarse, uniformly sized aggregate and eliminating fine particles entirely, engineers create concrete with 15% to 25% porosity that allows rainwater to drain through parking lots and sidewalks rather than running off into storm drains.