The Law of Equivalent Proportions, wanda kuma aka sani da Law of Definite Proportions, wata ka’ida ce a cikin ilmin sinadarai da ke bayyana cewa idan abubuwa biyu ko sama da haka suka hadu suka zama mahadi, ma’aunin sinadari guda daya da ke hade da tsayayyen nauyi na sauran nau'ikan suna cikin sauƙi mai sauƙi na jimlar lambobi, sabili da haka, rabon ɗimbin sauran abubuwan da ke cikin fili shima yana cikin madaidaitan adadin gabaɗaya. Wannan doka kuma ana kiranta da ka'idar tabbatacciyar ma'auni saboda abin da ke tattare da wani fili yana kayyade kuma tabbatacce.