Free Limiting Reagent Calculator
Find the limiting reactant, the leftover excess reagent, and the theoretical yield of any chemical reaction in seconds. Just enter the mass, molar mass, and stoichiometric coefficient for each reactant.
Understanding the Limiting Reagent
In any chemical reaction with two or more reactants, one of them almost always runs out before the others. That reactant — the limiting reagent — determines exactly how much product the reaction can form.
Key Concepts You Need
The core ideas behind every limiting reagent problem
What Is a Limiting Reagent?
The limiting reagent is the reactant that is fully used up first, stopping the reaction and capping the amount of product that can be formed, regardless of how much of the other reactant remains.
Converting Mass to Moles
Before you can compare reactants, convert each mass into moles using moles = mass ÷ molar mass. Comparing raw grams or millilitres directly will give you the wrong answer.
Dividing by Stoichiometric Coefficients
Divide each reactant’s moles by its coefficient in the balanced equation. Whichever reactant produces the smallest result is the limiting reagent — this step is where most students go wrong.
Calculating Excess Reagent and Yield
Once you know the limiting reagent, you can calculate exactly how much of the other reactant is left over (the excess) and the theoretical yield of product the reaction can produce.
Limiting Reagent Calculator
Enter both reactants to find the limiting reagent
Reactant A
Reactant B
How to Find the Limiting Reagent
Follow these four steps to solve any limiting reagent problem by hand, or simply plug your values into the calculator above for an instant answer.
Write and Balance the Equation
Start with a correctly balanced chemical equation. The coefficients in this equation tell you the exact mole ratio in which the reactants combine — everything else depends on getting this right first.
Convert Each Reactant to Moles
Divide the given mass of each reactant by its molar mass to find how many moles of each substance you actually have available for the reaction.
Divide by the Stoichiometric Coefficient
Take each reactant’s moles and divide by its coefficient from the balanced equation. The reactant with the lowest resulting value is the limiting reagent.
Calculate Leftover Excess and Yield
Use the limiting reagent’s moles to work out exactly how much of the other reactant reacts, subtract that from the starting amount to find the excess, and multiply through to find the theoretical yield of product.
Limiting vs Excess Reagent
A side-by-side summary of how the limiting reagent and excess reagent behave in a chemical reaction.
| Feature | Limiting Reagent | Excess Reagent |
|---|---|---|
| Definition | The reactant fully consumed first. | The reactant partially left over after the reaction stops. |
| Effect on product | Directly determines the theoretical yield. | Has no further effect once the reaction ends. |
| Amount at end of reaction | Zero moles remaining. | Some moles remain unreacted. |
| How it’s identified | Smallest moles ÷ coefficient value. | Largest moles ÷ coefficient value. |
| Practical relevance | Sets cost and yield in industrial synthesis. | Often added deliberately to push a reaction to completion. |
| Can there be a tie? | Yes — if both ratios are exactly equal, the reactants are used up in perfect stoichiometric proportion with no excess. | |
Limiting Reagent FAQ
Answers to the most frequently asked questions about limiting reagents, excess reagents, and reaction yield.
A limiting reagent (or limiting reactant) is the substance in a chemical reaction that is completely consumed first, stopping the reaction and determining the maximum amount of product that can be formed.
Convert each reactant’s mass to moles, divide by its stoichiometric coefficient from the balanced equation, and compare the results. The reactant with the smallest value is the limiting reagent.
The limiting reagent runs out first and limits how much product can form, while the excess reagent is the reactant that remains partially unused once the reaction stops.
Identifying the limiting reagent lets chemists predict theoretical yield, minimise wasted materials, and calculate percent yield accurately, which is essential in both laboratory and industrial chemistry.
No. The limiting reagent is determined by moles divided by stoichiometric coefficient, not by raw mass or volume, so a reactant present in a larger mass can still be the limiting reagent.