确定比例的定律与多重比例的定律一起构成了化学中化学计量学研究的基础。确定比例的定律也称为普鲁斯特定律或不断构成定律。一定比例的定律表明化合物的样品总是含有相同比例的元素质量。无论元素来自何处，如何制备化合物或任何其他因素，元素的质量比都是固定的。本质上，该定律基于特定元素的原子与该元素的任何其他原子相同的事实。因此，氧气原子是相同的，无论是来自二氧化硅还是来自空气中的氧气。常数组成定律是一种等效定律，它表明化合物的每个样品具有相同的元素组成。确定比例的定律表明水总是含有1/9的氢和8/9的氧。食盐中的钠和氯根据NaCl中的规则组合。钠的原子量约为23，氯的原子量约为35，因此从法律上可以得出结论，离解58克NaCl会产生约23克钠和35克氯。虽然对于现代化学家来说，确定比例的定律似乎是显而易见的，但是在化学的早期到18世纪末，元素结合的方式并不明显。 Joseph Priestly和Antoine Lavoisier根据燃烧研究提出了法律。他们指出金属总是与两个比例的氧结合。正如我们今天所知，空气中的氧气是由两个原子O2组成的气体。提出这项法律时，法律受到了激烈的争议。 Claude Louis Berthollet是一个反对者，他认为元素可以以任何比例组合形成化合物。直到约翰道尔顿的原子理论解释了原子的本质，才得以接受一定比例的定律。尽管确定比例的定律在化学中是有用的，但规则也有例外。一些化合物本质上是非化学计量的，这意味着它们的元素组成因样品而异。例如，维氏体是一种氧化铁，对于每个氧原子，元素组成在0.83和0.95个铁原子之间变化（质量为23％-25％氧）。理想的配方是FeO，但晶体结构是有变化的。该公式为Fe0.95O。此外，元素样品的同位素组成根据其来源而变化。这意味着纯化学计量化合物的质量将根据其来源略有不同。聚合物的元素组成也随质量而变化，尽管它们在最严格的化学意义上不被认为是真正的化合物。
The law of definite proportions, together with the law of multiple proportions, forms the basis for the study of stoichiometry in chemistry. The law of definite proportions is also known as Proust’s law or the law of constant composition. The law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. The mass ratio of elements is fixed no matter where the elements came from, how the compound is prepared, or any other factor. Essentially, the law is based on the fact that an atom of a particular element is the same as any other atom of that element. So, an atom of oxygen is the same, whether it comes from silica or oxygen in air. The Law of Constant Composition is an equivalent law, which states each sample of a compound has the same composition of elements by mass. The law of definite proportions says water will always contain 1/9 hydrogen and 8/9 oxygen by mass. The sodium and chlorine in table salt combine according to the rule in NaCl. The atomic weight of sodium is about 23 and that of chlorine is about 35, so from the law one may conclude dissociating 58 grams of NaCl would produce bout 23 g of sodium and 35 g of chlorine. Although the law of definite proportions may seem obvious to a modern chemist, the manner in which elements combine was not obvious in the early days of chemistry through the end of the 18th century. Joseph Priestly and Antoine Lavoisier proposed the law based on the study of combustion. They noted metals always combine with two proportions of oxygen. As we know today, oxygen in air is a gas consisting of two atoms, O2. The law was hotly disputed when it was proposed. Claude Louis Berthollet was an opponent, arguing elements could combine in any proportion to form compounds. It wasn’t until John Dalton’s atomic theory explained the nature of atoms that the law of definite proportions became accepted. Although the law of definite proportions is useful in chemistry, there are exceptions to the rule. Some compounds are non-stoichiometric in nature, meaning their elemental composition varies from one sample to another. For example, wustite is a type of iron oxide with an elemental composition varying between 0.83 and 0.95 iron atoms for each oxygen atom (23%-25% oxygen by mass). It ideal formula is FeO, but the crystal structure is such that there are variations. The formula is written Fe0.95O. Also, isotopic composition of an element sample varies according to its source. This means the mass of a pure stoichiometric compound will be slightly different depending on its origin. Polymers also vary in element composition by mass, although they are not considered true chemical compounds in the strictest chemical sense.