Limiting Reagent Calculator
Find the limiting reactant in a reaction.
Results update as you type.
How it works
In a chemical reaction, the limiting reagent is the reactant that runs out first and therefore determines how much product can form, and this calculator identifies it for a two-reactant reaction. You enter the moles available of each reactant and their coefficients from the balanced equation, and it returns which reactant is limiting, the mole-to-coefficient ratio for each, and how much of the excess reactant is left over. The method is to divide each reactant's moles by its coefficient; whichever gives the smaller value is the limiting reagent, because it is exhausted first relative to the reaction's demands. Simply having fewer moles does not make a reactant limiting — the coefficients matter, since a reaction may need several moles of one substance per mole of another. The reactant that is not limiting is present in excess, and part of it remains unreacted after the limiting reagent is gone. Identifying the limiting reagent is central to stoichiometry: it sets the theoretical yield of product and tells you how much of each reactant you actually need, helping avoid waste. This calculator handles two reactants; for more, compare the ratio for each. Enter the moles and coefficients to find the limiting reagent.
Compare moles ÷ coefficient for each reactant; the smaller is limiting
For each reactant, divide its available moles by its balanced-equation coefficient. The reactant with the smaller ratio runs out first and is the limiting reagent. The other is in excess; the leftover equals its moles minus the amount consumed by the limiting reactant's ratio times its coefficient.
Source:Stoichiometry
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Worked example
How to use this calculator
- 1
Enter your values
Enter moles of reactant a, coefficient of a, moles of reactant b and coefficient of b.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Limiting reagent updates instantly as you type — no button to press.
Frequently asked questions
How do you find the limiting reagent?
Divide each reactant's available moles by its coefficient in the balanced equation, then compare. The reactant with the smaller result is the limiting reagent because it is used up first relative to what the reaction needs. The other reactant is in excess. This method correctly accounts for reactions that need unequal amounts of each substance.
Why isn't the reactant with fewer moles always limiting?
Because reactions consume reactants in the ratios set by their coefficients, not one-to-one. A reactant present in smaller moles could still be in excess if the reaction needs even less of it per mole of the other. That is why you divide moles by coefficient first, rather than comparing raw mole amounts directly.
What is the excess reactant?
The excess reactant is the one that is not limiting — some of it remains unreacted after the limiting reagent is completely consumed. The leftover amount equals its starting moles minus the moles actually used, which the calculator computes. Knowing the excess helps you plan quantities and understand what remains in the reaction mixture afterward.
Why does the limiting reagent matter?
The limiting reagent determines the maximum amount of product a reaction can produce, known as the theoretical yield. Since it runs out first, no more product can form once it is gone, regardless of how much excess reactant remains. Identifying it is essential for calculating yields and for using reactants efficiently without waste.
How do I handle more than two reactants?
The same principle extends to any number of reactants: divide each one's moles by its coefficient and compare all the ratios. The smallest ratio identifies the limiting reagent. This calculator compares two reactants at a time, so for three or more you can evaluate them pairwise or compute each ratio and pick the minimum.