Limiting Reagent Calculator
Enter each reactant's chemical formula, mass, and its coefficient in the balanced equation to find the limiting reagent.
Last updated: September 5, 2026
Frequently asked questions
How do I find the limiting reagent?
Convert each reactant's mass to moles (mass ÷ molar mass), divide by its coefficient in the balanced equation, then compare - whichever reactant has the smaller result is the limiting reagent, since it will run out first.
Why does the coefficient matter, not just the moles?
The balanced equation's coefficients show the required ratio between reactants - having more raw moles of a reactant doesn't help if the reaction needs proportionally even more of it. Dividing by the coefficient normalizes each reactant to "how many complete reactions worth" you have.
What happens to the excess reagent?
The non-limiting reagent is left over (in excess) once the limiting reagent is fully consumed - the reaction stops there, and any remaining excess reagent doesn't react further without more of the limiting reagent being added.
How much product can theoretically form once the limiting reagent is identified?
Multiply the limiting reagent's moles by the mole ratio (from the balanced equation) between the limiting reagent and the product, then convert to grams using the product's molar mass - this calculator identifies which reactant limits the reaction, the first step toward that yield calculation.
What if both reactants give the exact same mol-to-coefficient ratio?
Neither reagent is in excess - both are consumed completely at the same time, which is called a 'stoichiometric' mixture. In practice this is rare, since it requires the reactants to be measured in exactly the ratio the equation calls for.
Does the limiting reagent have to be the one with the smaller mass?
Not necessarily - a reactant with a larger mass can still be limiting if it has a much higher molar mass or a larger required coefficient, since what matters is moles divided by the coefficient, not raw mass.