Porosity Calculator
The Empty Space Is Often the Point
A rock or sediment sample is rarely 100% solid mineral — there's void space threaded through it, and how much of that space exists determines whether the material can store water, oil, or gas at all. Porosity quantifies exactly that: the fraction of a material's total volume that isn't solid. This calculator works it out two different ways, depending on which measurements you have on hand.
The Formulas
Porosity % = (Void Volume ÷ Total Volume) × 100
By Density:
Porosity % = (1 − (Bulk Density ÷ Particle Density)) × 100
The density-based method is often more practical in the field or lab, since bulk density (mass over total sample volume, including pores) and particle density (mass over solid mineral volume only) can both be measured without directly isolating the void space itself.
Why Porosity Drives Real Decisions
- Groundwater storage — an aquifer's porosity, alongside permeability, determines how much water a given rock formation can actually hold and yield.
- Petroleum reservoir evaluation — oil and gas reserves are estimated in part from the porosity of the reservoir rock, since hydrocarbons occupy that same pore space.
- Soil and foundation engineering — porosity affects compressibility and settlement behavior, both critical inputs when assessing a site for construction.
- Filtration and contaminant transport — how quickly a contaminant plume moves through soil depends heavily on the porosity of the material it's moving through.
Typical Porosity by Material
| Material | Typical Porosity |
|---|---|
| Clay (unconsolidated) | 40% – 70% |
| Silt (unconsolidated) | 35% – 50% |
| Sand (unconsolidated) | 25% – 50% |
| Gravel (unconsolidated) | 25% – 40% |
| Sandstone | 5% – 30% |
| Limestone / Dolomite | 0% – 20% |
| Shale | 0% – 10% |
| Dense crystalline rock (granite) | 0% – 5% |
Ranges are widely published typical values for unweathered, unfractured material; fracturing and karst dissolution can push consolidated-rock porosity well above these figures.
How to Use This Calculator
- Choose a mode: By Void / Total Volume or By Bulk / Particle Density.
- For the volume method, enter Void Volume and Total Volume in matching units.
- For the density method, enter Bulk Density and Particle Density in matching units.
- Select Calculate to get the porosity percentage with the formula applied to your figures.
Related Calculations
Once porosity is known, see how easily fluid moves through that pore space with the Permeability Calculator, or check load-bearing behavior with the Soil Bearing Capacity Calculator.