Rock Density Calculator
Calculate bulk density, grain density, and porosity from dry, saturated, and submerged mass measurements.
About this calculator
This calculator derives petrophysical properties of a rock sample from three simple mass measurements taken via the saturation-and-buoyancy method: dry mass, saturated mass (after soaking until pores are fully water-filled), and submerged mass (the saturated sample's apparent weight while suspended in water). Using Archimedes' principle with water density taken as 1 g/cm³, bulk volume is calculated as saturated mass minus submerged mass, and pore volume as saturated mass minus dry mass. From there, bulk density is dry mass divided by bulk volume (density including internal pore space), grain density is dry mass divided by an approximated grain volume (dry mass minus submerged mass, treating the mineral solids alone), and porosity is pore volume expressed as a percentage of bulk volume.
Saturated density and water-absorption percentage round out the output as secondary indicators — the latter useful for gauging weathering susceptibility, since rocks that absorb more water are generally more vulnerable to freeze-thaw damage. The grain-volume approximation used here (dry mass minus submerged mass) assumes the submerged weighing was performed on the fully saturated sample; if it wasn't, or if the sample has significant closed (non-connected) porosity that never fills with water, both porosity and grain density will be somewhat off. Saturated mass must be entered as at least equal to dry mass, or these volume calculations collapse into non-physical negative numbers.
Inputs
Results
Bulk Density
2.381 g/cm³
Grain Density
2.632 g/cm³
Porosity
9.52%
How to Use This Calculator
- Weigh the dry rock sample in grams and record the Dry Mass.
- Saturate the sample and weigh again to get the Saturated Mass in grams.
- Suspend the saturated sample in water and record the Submerged Mass using Archimedes' method.
- Review Bulk Density, Grain Density, and Porosity percentage as the primary petrophysical outputs.
- Check Water Absorption percentage to assess weathering susceptibility and engineering suitability.
How the result changes with Saturated Mass (g)
| Saturated Mass (g) | Bulk Density | Grain Density | Porosity |
|---|---|---|---|
| 130 | 2.632 g/cm³ | 2.632 g/cm³ | 0% |
| 195 | 2.632 g/cm³ | 2.632 g/cm³ | 0% |
| 390 | 1.064 g/cm³ | 2.632 g/cm³ | 59.57% |
| 650 | 0.505 g/cm³ | 2.632 g/cm³ | 80.81% |
What each input means
- Dry Mass (g)
- Mass of the oven-dried rock sample in grams.
- Saturated Mass (g)
- Mass of the rock sample after full water saturation in grams. Must be ≥ dry mass.
- Submerged Mass (g)
- Mass of the saturated sample weighed while submerged in water (buoyant mass) in grams.
How this is calculated
Formula
Bulk Density = Dry Mass / (Saturated Mass − Submerged Mass)Worked example, using the default values
- Identify Input ParametersDry Mass (g) = 250, Saturated Mass (g) = 260, Submerged Mass (g) = 155 = 3 input(s) provided
- Calculate Bulk DensityBulk Density2.381 = 2.381
- Calculate Grain DensityGrain Density2.632 = 2.632
- Calculate PorosityPorosity9.52 = 9.52
- Calculate Saturated DensitySaturated Density2.476 = 2.476
- Calculate Bulk VolumeBulk Volume105 = 105
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What's the difference between bulk density and grain density here?
Bulk density divides dry mass by bulk volume (saturated mass minus submerged mass), which includes the rock's internal pore space, so it reflects the density of the sample as a whole piece. Grain density divides dry mass by an approximated grain volume (dry mass minus submerged mass) that treats only the solid mineral material, excluding pores — it's meant to approximate the density of the mineral itself, independent of how porous the sample is.
Why must saturated mass be at least as large as dry mass?
Saturated mass represents the sample after its pores have absorbed water, so it should always be equal to or greater than the dry mass — a rock can't weigh less after taking on water. If saturated mass were entered lower than dry mass, both bulk volume and pore volume would come out negative, which is why the calculator enforces saturatedMass ≥ dryMass on the input.
What does the porosity percentage actually measure, and when might it be inaccurate?
Porosity here is pore volume (saturated mass minus dry mass) expressed as a percentage of bulk volume, representing the fraction of the sample's total volume that is void space. This method only detects connected, water-accessible pores — if the rock has significant closed porosity that water never reaches during saturation, or if the submerged weighing wasn't performed on the fully saturated sample, the reported porosity and grain density will understate the true values.
Why does water absorption percentage matter for assessing a rock's durability?
Water absorption is calculated as the saturated-mass gain over dry mass, expressed as a percentage, and it tracks how much of the rock's volume is accessible, water-filled pore space. Rocks with higher water absorption tend to be more susceptible to freeze-thaw damage, since water expanding as it freezes inside connected pores can crack the material — a common screening indicator in construction-material testing.
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