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Soil Porosity Calculator

Enter the void ratio to get the soil porosity as a percentage.

Result

39.39%

How it is calculated

The input is the void ratio e = Vv ÷ Vs: the volume of voids divided by the volume of solid grains. It is a plain number with no unit. The tool converts it to porosity n = e ÷ (1 + e) × 100, which is the void volume as a percentage of the total (solids plus voids) volume. Because the denominator is the total volume, porosity can never reach 100% however large e becomes. A void ratio of 0 gives 0%, and a negative void ratio returns no result. The result is rounded to two decimals. To go back the other way, e = n ÷ (1 − n) with n written as a fraction.

Worked example

A soil sample has a void ratio of 0.65, meaning the voids occupy 0.65 units of volume for every 1 unit of solids. Total volume is 1 + 0.65 = 1.65 units. Porosity: 0.65 ÷ 1.65 = 0.3939, and multiplied by 100 the soil porosity is 39.39%. About 39% of the bulk volume is void space, while the remaining 60.61% is solid particles.

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Common mistakes

  • Typing porosity (for example 0.39 or 39) into the void ratio field. The two are different quantities: a porosity of 39% corresponds to a void ratio of about 0.64.
  • Dividing void volume by total volume to get e. The void ratio is voids divided by solids only.
  • Expecting the result to exceed 100% for very loose soils. A void ratio above 1 is possible, but porosity stays below 100%.
  • Entering a percentage for e, such as 65 instead of 0.65, which gives a porosity near 98.5%.
  • Treating porosity as the same thing as permeability. Porosity says how much space exists, not how easily water flows through it.

About this calculator

This calculator finds soil porosity, n = e ⁄ (1 + e), from the void ratio e — the ratio of the volume of voids (air and water-filled space) to the volume of solid particles in a soil sample. Porosity instead expresses that void space as a fraction of the total soil volume, which is why the two related quantities need this simple conversion between them.

Porosity is a fundamental parameter in geotechnical engineering and hydrogeology: it controls how much water a soil can store, how easily fluids flow through it (permeability), and how much a soil layer will compress under load. Engineers obtain e in the laboratory — typically from the soil's dry density and the specific gravity of its solids — then use this formula to report porosity as the percentage geotechnical reports typically require.

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Comments (1)

  • Editorial team

    Porosity tells you how much of the volume is empty, not how much water can move through it or drain out. Clays can have a high porosity and still be nearly impermeable, because their pores are tiny and poorly connected. For groundwater storage and drainage questions, the connected, or effective, porosity matters more, so treat this number as an upper bound. Void ratios above 1 are accepted; a void ratio of 3 gives 75 percent. What soil type and void ratio are you working with?

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