Stoichiometry (from the Greek στοιχεῖον (stoicheion) "element" e μέτρον (metron) "measure") is the part of Chemistry that studies the quantitative ratios (weight ratio) of chemical substances in chemical reactions.

Reaction stoichiometry indicates in what ratios two or more substances react with each other. It is represented by means of coefficients, the so-called stoichiometric coefficients, which express the molar ratios with which the substances involved in the reaction react. Stoichiometric calculation allows to mathematically determine the quantities of reactants and products involved in a chemical reaction.