Consider the reaction
:A ⇌ 2 B + 3 C
Suppose an infinitesimal amount  of the reactant A changes into B and C. This requires that all three mole numbers change according to the stoichiometry of the reaction, but they will not change by the same amounts. However, the extent of reaction  can be used to describe the changes on a common footing as needed. The change of the number of moles of A can be represented by the equation , the change of B is , and the change of C is . 
The change in the extent of reaction is then defined as
where  denotes the number of moles of the  reactant or product and is the stoichiometric number  of the  reactant or product. Although less common, we see from this expression that since the stoichiometric number can either be considered to be dimensionless or to have units of moles, conversely the extent of reaction can either be considered to have units of moles or to be a unitless mole fraction. 
The extent of reaction represents the amount of progress made towards equilibrium in a chemical reaction. Considering finite changes instead of infinitesimal changes, one can write the equation for the extent of a reaction as
The extent of a reaction is generally defined as zero at the beginning of the reaction. Thus the change of  is the extent itself. Assuming that the system has come to equilibrium,
Although in the example above the extent of reaction was positive since the system shifted in the forward direction, this usage implies that in general the extent of reaction can be positive or negative, depending on the direction that the system shifts from its initial composition.