Core Sample Porosity Calculator
This calculator determines rock porosity from bulk volume and grain volume measurements of a core sample, following standard laboratory petrophysical methods (API RP 40; ASTM D7536). Enter the bulk volume and grain volume of a dry core plug, or the dry mass and bulk density, to compute total porosity, pore volume, bulk density, and grain density. These measurements are fundamental inputs to reservoir characterisation and resource estimation.
Porosity formula
phi = (Vb - Vg) / Vb = Vp / Vb
Where phi = porosity (fraction), Vb = bulk volume (cm3), Vg = grain volume (cm3), Vp = pore volume (cm3). Bulk density rho_b = m / Vb; grain density rho_g = m / Vg. Porosity from density: phi = (rho_g - rho_b) / rho_g. Source: API Recommended Practice 40: Recommended Practices for Core Analysis (2nd edition, 1998).
Typical porosity ranges by rock type
| Rock Type | Typical Porosity (%) | Application |
|---|---|---|
| Clean sandstone | 15-35 | Oil, gas, water reservoir |
| Carbonate (limestone) | 5-25 | Oil, gas reservoir |
| Tight sandstone | 1-10 | Tight gas |
| Shale | 1-15 | Shale gas (nano-pores) |
| Granite (unfractured) | 0.1-1 | Basement reservoir (fractured) |
| Coal | 2-20 | Coal bed methane |
Core sample porosity calculator: frequently asked questions
What is rock porosity?
Porosity (phi) is the fraction of total rock volume occupied by pore space (voids). It is expressed as a decimal (0-1) or percentage (0-100%). Porosity is the most fundamental property controlling how much fluid (water, oil, gas) a rock can hold. Total porosity includes all pores; effective porosity counts only interconnected pores that contribute to fluid flow.
How is core sample porosity measured in the laboratory?
The standard laboratory method involves: (1) measuring bulk volume of the cleaned, dried core plug using fluid displacement or caliper measurements; (2) measuring grain volume by helium pycnometry (Boyle's Law method) per API RP 40; (3) calculating pore volume = bulk volume - grain volume; (4) porosity = pore volume / bulk volume.
What is the difference between bulk density and grain density?
Bulk density (rho_b) is the mass of the dry rock sample divided by its total bulk volume, including pores. Grain density (rho_g, also matrix density) is the mass divided by the solid grain volume only. For quartz-dominated sandstone, grain density is typically 2.65 g/cm3. Bulk density is always less than grain density because it includes the pore space.
What are typical porosity values for reservoir rocks?
Sandstone reservoirs: 10-35% (well-sorted, clean sand up to 40%); Carbonate reservoirs: 5-30% (secondary porosity from fractures and dissolution); Tight gas sands: 1-10%; Shale gas: 1-10% (mostly nano-pores). Typical oil and gas reservoirs have 15-25% porosity with 50-80% water saturation.
What is the Archie equation and how does it relate to porosity?
The Archie equation (1942) relates formation resistivity to porosity and water saturation: Rt = a x Rw / (phi^m x Sw^n), where a, m, n are empirical constants (typically a=1, m=2, n=2 for sandstones). It is used to determine fluid saturation from well log resistivity measurements, and requires accurate porosity from core or logs.
Official sources
- API Recommended Practice 40 (RP 40): Recommended Practices for Core Analysis, 2nd edition (1998).
- USGS National Geological and Geophysical Data Preservation Program: USGS Core Research Center.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.