A stoichiometric coefficient is the number placed before a species in a chemical equation to indicate how many units of it are needed to balance the reaction. The coefficients give the fixed ratios in which reactants combine and products form. For example, the coefficient 2 before H₂ in the formation of water shows that two molecules of hydrogen react for each molecule of oxygen.

化学计量系数是方程式中物质前所写的数字,表示配平反应所需的该物质单元数目。这些系数给出反应物结合和产物生成的固定比例。例如,在生成水的反应中 H₂ 前的系数 2 表示每一个氧分子要与两个氢分子反应。