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Degree of Saturation Calculator

The degree of saturation Sr describes how much of the pore (void) space in a soil is filled with water rather than air.

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How to use this calculator

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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Measure or calculate the volume of pore water (Vw) and the total volume of voids (Vv) in the same units (e.g. cm³).
  2. Enter Vw in the first field and Vv in the second, then read the degree of saturation Sr as a percentage.
  3. If you only know void ratio (e), water content (w) and specific gravity of solids (Gs), first compute Vw and Vv from the phase diagram (or use Se = wGs) before entering values here.

How it works

The degree of saturation Sr describes how much of the pore (void) space in a soil is filled with water rather than air. By definition Sr = Vw ÷ Vv × 100, where Vw is the volume of water and Vv is the total volume of voids (water plus air). Sr = 0% means the voids hold no water (fully dry) and Sr = 100% means the voids are completely water-filled (fully saturated). The remaining pore space is air: Va = Vv − Vw.

Sr links to the other phase relationships through the identity Se = wGs, where e is the void ratio, w is the gravimetric water content and Gs is the specific gravity of the solids (with S expressed as a fraction, so S = Sr/100). This calculator uses the direct volumetric definition, which needs no assumptions about Gs or unit weight; if you start from e, w and Gs instead, derive Vw and Vv first, then enter them here.

Worked example

Silty soil with partly water-filled voids. A soil sample has a total void (pore) volume of 30 cm³, of which 24 cm³ is water and the rest is air. Enter Vw = 24 and Vv = 30. Sr = 24 ÷ 30 × 100 = 80.00%, with 6 cm³ of air remaining in the voids. The soil is partially saturated.

Common mistakes

  • Mixing up voids with total volume — Sr uses the volume of VOIDS (Vv = water + air), not the total sample volume (which also includes the solids). Dividing by total volume gives porosity, not saturation.
  • Entering a water volume larger than the void volume, which is physically impossible and would give Sr > 100%. If this happens, re-check that both volumes use the same units and that Vw excludes the solids.
  • Confusing degree of saturation (a volume ratio, Vw/Vv) with water content (a mass ratio, mass of water ÷ mass of dry solids). They are related through Se = wGs but are not the same quantity.

Frequently asked questions

What is the degree of saturation in soil?

It is the fraction of the soil's void (pore) space that is filled with water, expressed as a percentage: Sr = volume of water ÷ volume of voids × 100. Sr = 0% is a fully dry soil and Sr = 100% is a fully saturated soil where every pore is water-filled.

How is degree of saturation different from porosity?

Porosity n = volume of voids ÷ total volume, and describes how much of the whole soil is void space. Degree of saturation Sr = volume of water ÷ volume of voids, and describes how much of that void space holds water rather than air. A soil can have high porosity yet a low degree of saturation if the pores are mostly air.

Can I use the Se = wGs relationship instead?

Yes. On a phase diagram Se = wGs, where S = Sr/100, e is void ratio, w is water content and Gs is specific gravity of solids. If you know e, w and Gs, you can back-calculate Sr, or derive the water and void volumes and enter them into this calculator directly.

What is a typical degree of saturation below the water table?

Soils below the groundwater table are usually taken as fully saturated (Sr ≈ 100%) in routine geotechnical analysis, while soils above it are partially saturated. Always confirm against site investigation data — this tool is guidance only and results should be verified against the relevant standard (AS/ASTM/ISRM) and a competent geotechnical professional.

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