Learn: Void ratio from porosity
Porosity and void ratio both describe empty space in soil, but they use different denominators. Porosity divides void volume by total volume; void ratio divides it by solid volume.
What the formula is saying
Imagine the total volume is 1. The void fraction is n and the solid fraction is 1 − n, so dividing the first by the second gives e = n/(1 − n).
Read the symbols in plain language
- n
- Porosity as decimal
Void volume divided by total volume, strictly below one. It differs from void ratio, whose denominator is solid volume.
ratio / no unitA dimensionless ratio has no physical unit; 0.01 as a ratio is 1% when the percent option is selected.
- e
- Result to find
Void ratio from porosity. Void ratio uses solid volume, not total volume, as its denominator.
ratio / no unit
Sort out the units first
n is a decimal fraction: 40% means 0.40. e is a dimensionless ratio, not a volume. Unlike porosity, void ratio can exceed 1 when void volume exceeds solid volume.
Assumptions before calculating
Use representative volumes or masses from the same soil sample and the same state. Treat solids, water and air as distinct phases; the stated phase relationship does not determine soil strength or suitability for construction.
Let’s solve one together
Read the given values, follow each operation, then check what the result means.
Read the supplied values as one complete study case. Find e and explain the result in the stated output unit.
- n · Porosity as decimal
- 0.4
Find the solid-volume fraction
Subtract the void fraction from the total fraction of one.
1-(0.4) = 0.6Compare voids with solids
Void ratio uses solid volume, not total volume, as its denominator.
(0.4) ÷ (0.6) ≈ 0.6666666667
Does this worked answer make sense?
With n = 0.5, equal void and solid volumes give e = 1. Converting back with n = e/(1 + e) should recover the original porosity.
A second worked example — different values
A second case uses different data. Predict which way the answer will change, then calculate it without reusing the first answer.
- n · Porosity as decimal
- 0.5
Find the solid-volume fraction
Subtract the void fraction from the total fraction of one.
1-(0.5) = 0.5Compare voids with solids
Void ratio uses solid volume, not total volume, as its denominator.
(0.5) ÷ (0.5) = 1
Now try your own values
Change a value or its unit. The same method will show your calculation, step by step.
Results update only when you calculate. The lesson example above stays unchanged.
Your turn — check your understanding
Solve this separate case yourself. Use only the values below; the two worked examples use different data. Give the requested result in the selected unit.
- n · Porosity as decimal
- 0.35
Find: Learn: Void ratio from porosity
A hint, not the answer
Imagine the total volume is 1. The void fraction is n and the solid fraction is 1 − n, so dividing the first by the second gives e = n/(1 − n).
n is a decimal fraction: 40% means 0.40. e is a dimensionless ratio, not a volume. Unlike porosity, void ratio can exceed 1 when void volume exceeds solid volume.
Show the full practice solution
Compare the steps with your work; revealing a solution does not mark the lesson complete.
Find the solid-volume fraction
Subtract the void fraction from the total fraction of one.
1-(0.35) = 0.65Compare voids with solids
Void ratio uses solid volume, not total volume, as its denominator.
(0.35) ÷ (0.65) ≈ 0.5384615385
Avoid the common trap
Do not input 40 instead of 0.40 unless a percent unit is selected. Do not confuse e with n or place n alone in the denominator.
When this method applies — and when it does not
The formula requires 0 ≤ n < 1. At n = 1 there would be no solid volume and the ratio is undefined. It says nothing about how voids connect or whether they contain water.
For study and understanding, not approval of a real structure, site operation or design. Apply the correct standard, National Annex and professional review to actual engineering work.
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Sources & further reading
References open in a new tab and explain the underlying principles. The teaching text and examples here are SimpleFlick’s own; the source organisations have not endorsed this calculator.
Reading focus: Void ratio from porosity. Read the relevant soil phase, seepage, earth-pressure, settlement or foundation topic. Effective stress, drainage and idealized geometry determine whether the relationship applies.
Lesson updated: · Both examples and the separate practice case are checked against an independent high-precision numerical implementation. This verifies arithmetic for the stated model, not engineering certification.