The calculation of quantities in chemical equations is called

The calculation of quantities is chemical equations is called stoichiometry.

Explanation:

In a chemical reaction, as a result of the law of conservation of mass, reactants combine in fixed proportions to form the products.

Then, a chemical reaction is characterized by cuantitative relations between the atoms and compounds, which permit to make predictions on how much product can be obtained from certain amounts of reactants or how much of each reactant you would need to obtain a desired amount of product.

Stoichiometry is the use of the mole coefficients of a chemical equation to state ratios and set proportions, and, so, determine the number of moles or masses of reactants and products.