Porosity and Void Ratio Relations

Porosity and void ratio describe the same pore space with different denominators, and converting between them is constant work in soils and reservoirs.

The equation

n = e / (1 + e), e = n / (1 – n)

n is porosity: voids over total volume
e is void ratio: voids over solids volume

Worked example

A soil with void ratio 0.6 has porosity 0.6/1.6 = 0.375. Going the other way, a reservoir sand with porosity 0.20 has void ratio 0.25. The two scales diverge quickly in soft clays where e can exceed 1.

When to use it

Geotechnical formulas almost always want void ratio because solids volume stays constant during compression. Reservoir and aquifer calculations want porosity because total volume is what holds fluid.

Related Calculators and Reference

Groundwater Flow Simulator: watch this equation animate a cone of depression.
Demo Valley Worked Project: a full site project using these methods.
USGS Water Science School: authoritative background on groundwater.