What this calculator does
Void ratio and porosity underpin almost every geotechnical calculation. This calculator takes the two numbers you measure in the field — bulk unit weight and water content — plus the specific gravity of the solids, and returns every derived property of the three-phase soil system, including the saturated and submerged unit weights used in bearing and pressure calculations.
How to use it
- Measure the bulk (moist) unit weight of the sample.
- Measure the water content as a percentage of the dry weight.
- Enter the specific gravity of the soil solids; 2.65 is typical for quartz sand and 2.72 for typical clay.
- The degree of saturation tells you whether the soil is fully saturated — important before using undrained parameters.
Governing formula
e = (Gs·γw·(1+w)/γ) − 1 ; n = e/(1+e) ; Sr = w·Gs/e Where:
-
Gs= Specific gravity of solids [–] -
γw= Unit weight of water [kN/m³] -
w= Water content as a fraction [–] -
γ= Bulk unit weight [kN/m³]
Derived Equations:
γd = γ/(1+w) γsat = (Gs+e)·γw/(1+e) γ′ = γsat − γw Worked example — A soil at 19 kN/m³ with 22 % water content
The figures below were produced by running this page's own engine with the example inputs (Bulk (moist) unit weight (kN/m³), Water content (%), Specific gravity of solids (–)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Void ratio (e) | 0.7 – |
|---|---|
| Porosity (n) | 41.2 % |
| Degree of saturation | 84.77 % |
| Dry unit weight | 15.57 kN/m³ |
| Saturated unit weight | 19.62 kN/m³ |
| Submerged unit weight | 9.81 kN/m³ |
| Assessment | Void ratio 0.70, porosity 41.2%, saturation 85%. |
- e = (2.7 × 9.81 × 1.22 / 19) − 1 ≈ 0.69.
- Porosity follows as e/(1+e), and saturation as w·Gs/e.
- A degree of saturation near 100 % tells you the soil is effectively saturated, so undrained parameters apply.
Assumptions & limitations
Engineering calculation. Derives void ratio, porosity, degree of saturation and the unit weights (dry, saturated, submerged) from the three-phase relationship. Phase relationships only.
- Assumes a two-phase relationship with air freely draining — trapped air in saturated clay invalidates the degree of saturation.
- Apparent (submerged) weights below the water table must not be doubled up when used in effective-stress calculations.
- Specific gravity varies with mineralogy; use the measured value where possible.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS 5930:2015 — Code of practice for sampling and testing of soils.
- Das, Principles of Geotechnical Engineering — phase relationships chapter.
- Craig's Soil Mechanics — the three-phase diagram.