What is the limiting reagent?

The limiting reagent (or limiting reactant) is the substance that is completely consumed first in a chemical reaction. It limits the amount of product that can be formed, no matter how much of the other reactant is present.

How does the calculator find the limiting reagent?

It divides the available moles of each reactant by its stoichiometric coefficient. The reactant with the smaller ratio is the limiting reagent because it is the first to run out relative to the reaction requirements.

What is an excess reagent?

The excess reagent is the reactant that is not fully consumed. After the reaction, some of it remains. The calculator tells you exactly how many moles are left over.

Does this work for reactions with more than two reactants?

This tool handles two-reactant reactions. For more complex reactions, apply the same ratio method (moles available / stoichiometric coefficient) to each reactant and pick the smallest.

What if both reactants give the same ratio?

When both ratios are equal, both reactants are consumed simultaneously — neither is in excess. The calculator reports this as "Both reactants are consumed completely."

How do I convert moles of product to grams?

Multiply the moles of product by its molar mass (g/mol). For example, 2 mol of H2O × 18.015 g/mol = 36.03 g of water produced.

Limiting Reagent Calculator

Enter the moles and stoichiometric coefficients of two reactants to find the limiting reagent and the theoretical moles of product formed.

Reaction Inputs

For a balanced equation aA + bB → cC, enter the coefficients a, b, c and the moles you have of each reactant.

Reactant A

Moles available

Coefficient (a)

Reactant B

Moles available

Coefficient (b)

Product C

Coefficient (c)

Fill in the inputs and click Calculate
to identify the limiting reagent.

Limiting Reagent

Mole Ratio Analysis

Reactant	Moles available	Coefficient	Ratio (mol/coeff)

Product formed (mol)

Theoretical yield in moles

Excess reagent remaining

Moles left over after reaction

Consumption Comparison

Reactant A consumed

Reactant B consumed

Summary

Enter the moles and stoichiometric coefficients of two reactants to find the limiting reagent and the theoretical moles of product formed.

How it works

Enter the moles of Reactant A and Reactant B available.
Enter the stoichiometric coefficients of A and B from the balanced equation.
Enter the stoichiometric coefficient of the product.
Click Calculate to find the limiting reagent.
Read the moles of product formed and the excess moles of the other reactant.

Use cases

Determine which reagent to purchase more of before a lab synthesis.
Calculate theoretical yield in grams (combine with molar mass).
Check stoichiometry homework answers instantly.
Plan industrial reaction batches to minimize waste.
Teach limiting reagent concepts with live visual feedback.
Verify manual stoichiometry calculations quickly.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu