How this instrument works
Porosity is the fraction of a material's bulk volume that isn't solid — no grain, no matrix, just void. Divide that void space by the sample's total volume and multiply by 100: φ = Vvoid ⁄ Vtotal × 100. A φ of 30% means three-tenths of every cubic centimetre you're holding is empty, waiting for air, gas, oil, or water to occupy it. The ratio is dimensionless by construction, so the same number works for a teaspoon of sand or an entire sandstone formation, provided both figures share a unit.
The formula is deliberately just a ratio, not a rate — it counts space, not connections. That is exactly where porosity and its usual companion, permeability, part ways. Two samples can share an identical 30% porosity and behave nothing alike underground: one has pores threaded together into channels a fluid can cross, the other traps that same 30% in sealed, isolated pockets a fluid can never reach. Fresh pumice and closed-cell foam sit near the second case, plenty of void space and almost no pathway through it. Petroleum engineers call the connected fraction effective porosity and the sealed-off remainder ineffective, and only the connected part predicts how oil or groundwater will actually move.
There is a companion figure worth not confusing this with: void ratio, e = Vvoid ⁄ Vsolid, standard in geotechnical reports. It divides by the solid volume instead of the total, so the two numbers diverge fast at higher values — a sample at 50% porosity carries a void ratio of 1.0, not 0.5. Porosity is not a fixed property of a rock type either; burial compaction alone can squeeze a loose, freshly deposited sand from near 40% down toward 20% given enough depth and geologic time, which is one reason the same formation logs differently in a shallow well than a deep one.
- Enter the Void (pore) volume — the empty space measured in the sample, in cc or ml.
- Enter the Total bulk volume — the sample's full outer size, void and solid combined, in the same unit.
- Read Porosity, % — the void fraction shown as a percentage, so a ratio of 0.3 reads out as 30, not 0.3.
- Check the figure against typical ranges for the material — well under 5% for dense crystalline rock, 30 to 50% for loose soil, 60% or higher for fresh pumice — as a sanity check on the measurement.
Worked example — a 100 cm³ core sample
Take a 100 cm³ soil or rock core pulled from a shallow borehole, saturated with water, then drained back out and found to have released 30 cm³ of that water — a standard field measure of the void space it actually holds. Enter 30 cc as the Void (pore) volume and 100 cc as the Total bulk volume: φ = 30 ⁄ 100 × 100 = 30%. Almost a third of the core was never solid grain at all, and the rest, the remaining 70 cm³, is mineral matrix.
That 30% describes storage capacity, not flow capacity, and confusing the two is the first mistake a newcomer makes. A poorly sorted rock could report the identical 30% porosity while its pores sit sealed off from one another by later cementation, yielding almost no flow at all despite technically holding just as much void space. This is exactly why petroleum engineers and hydrogeologists never quote porosity alone; a separate permeability figure, measured through a flow test rather than a volume ratio, is what tells you whether that 30% is actually recoverable.
Questions
What is the difference between porosity and permeability?
Porosity measures how much empty space a material holds, void volume divided by total volume. Permeability measures whether fluid can actually move through that space, which depends on the pores connecting into continuous channels rather than sitting as isolated pockets. A sample can have high porosity and near-zero permeability if its voids are sealed off from each other, which is why reservoir and aquifer reports always list both figures, never just one.
Is porosity the same thing as void ratio?
No, though the two get mixed up often. Porosity divides void space by total volume, φ = Vvoid ⁄ Vtotal. Void ratio, standard in geotechnical engineering, divides that same void space by solid volume instead, e = Vvoid ⁄ Vsolid. They convert into each other with e = φ ⁄ (1 − φ), but the numbers diverge sharply as porosity rises: 50% porosity corresponds to a void ratio of 1.0, not 0.5, so swapping one formula for the other silently doubles an error.
What porosity values are typical for common materials?
Dense, unweathered crystalline rock like granite usually runs under 1 to 2%. Sandstone reservoirs common in petroleum work typically fall between 10% and 30%. Loose sand or agricultural topsoil often sits near 30 to 50%, and fresh volcanic pumice or open-cell foam can exceed 60%, since so much of their bulk volume never solidified into grain in the first place.
Does porosity change with depth or overburden pressure?
Yes. Burial compaction, cementation, and the weight of overlying rock progressively close pore space, so the same sand or shale typically logs a lower porosity deep in a formation than it would near the surface. Correcting for this depth trend is one of the first steps a petrophysicist takes when comparing porosity readings from wells at different depths.
How is void volume actually measured in a real sample?
Common methods include saturating a dry sample with water or oil and weighing how much it absorbs, or sealing the sample in a chamber and measuring how much helium gas fills the pore space with a helium porosimeter. Both approaches isolate Vvoid directly, which then gets divided by the sample's separately measured bulk size to reach φ, the same division this calculator performs.
Can porosity come out above 100% or below zero?
No. Void volume physically cannot exceed total volume, so a valid result always falls between 0%, fully solid with no voids, and 100%, which no real material approaches. If entering your numbers gives a void volume larger than the total, that is a measurement or unit-entry error, since the geometry it describes is impossible.