化學計量數(英文:stoichiometry)係一場化學反應嗰啲反應物同反應產物之間喺份量上嘅關係。
考慮一場噉嘅化學反應:
當中 A{\displaystyle {\ce {A}}} 同 B{\displaystyle {\ce {B}}} 係反應物(做反應嘅原料),P{\displaystyle {\ce {P}}} 同 Q{\displaystyle {\ce {Q}}} 反應產物(場反應產生嘅物質),a{\displaystyle {\ce {\mathit {a}}}}、b{\displaystyle {\ce {\mathit {b}}}}、p{\displaystyle {\ce {\mathit {p}}}} 同 q{\displaystyle {\ce {\mathit {q}}}} 反映嘅就係化學計量數-簡化噉講即係「每 a{\displaystyle {\ce {\mathit {a}}}} 粒 A{\displaystyle {\ce {A}}} 同每 b{\displaystyle {\ce {\mathit {b}}}} 粒 B{\displaystyle {\ce {B}}} 會做反應出 p{\displaystyle {\ce {\mathit {p}}}} 咁多粒 P{\displaystyle {\ce {P}}} 同 q{\displaystyle {\ce {\mathit {q}}}} 咁多粒 Q{\displaystyle {\ce {Q}}}」[1]。