Limiting Reagent Calculator (Limiting Reactant & Theoretical Yield)

In short: Find the limiting reactant, excess left over and theoretical yield from grams, moles or solutions. It is free, works in your web browser and needs no download.

Find the limiting reactant, excess reagent, reaction extent, leftover amounts, and theoretical yield from a balanced chemical equation.

Quick answer: convert each reactant to moles, divide by its coefficient, and the smallest value is the limiting reagent.

Set up your stoichiometry calculation

Enter the equation with + between species and -> or = as the arrow. If it is not balanced, the calculator balances it for you.

Product of interest

Quick presets

Load a reaction and enter reactant amounts to calculate the limiting reagent.

How to Find the Limiting Reagent

  1. Make sure the chemical equation is balanced.
  2. Convert each reactant amount to moles. For grams, use moles = grams / molar mass. For solutions, use moles = M x L.
  3. Divide moles of each reactant by its stoichiometric coefficient.
  4. The smallest value is the reaction extent and identifies the limiting reagent.
  5. Use the limiting extent to calculate product moles and leftover excess reactants.
This educational calculator balances the equation if needed and assumes the reaction goes to completion. Real experiments may have side reactions, purity issues, equilibrium limits, or incomplete conversion.

How to use the limiting reagent calculator

  1. Type the reaction, e.g. Al + Cl2 -> AlCl3, and click Load reactants. Unbalanced equations are balanced for you.
  2. For each reactant choose grams, moles or solution (M × L) and enter the amount. Molar masses are filled in automatically.
  3. Pick the product you care about. Add an actual yield to get percent yield.
  4. Read the limiting reagent, the leftover excess (mol and g) and the theoretical yield.

Worked examples

10.0 g Al + 50.0 g Cl2 → AlCl3

2 Al + 3 Cl2 → 2 AlCl3
Al: 0.3706 mol ÷ 2 = 0.1853
Cl2: 0.7052 mol ÷ 3 = 0.2351

Al limits; theoretical yield = 0.3706 mol × 133.33 = 49.42 g AlCl3

4 mol H2 + 1.5 mol O2

2 H2 + O2 → 2 H2O
H2: 4 ÷ 2 = 2.0; O2: 1.5 ÷ 1 = 1.5

O2 limits → 3.0 mol H2O; 1.0 mol H2 left over

Writing up the experiment? Draw the reaction scheme with structures and conditions in the free MolDraw reaction drawer.

Draw the reaction

Frequently asked questions

How do you find the limiting reagent?

Convert each reactant to moles, divide by its coefficient in the balanced equation, and pick the smallest result. That reactant runs out first and sets the amount of product.

Does the equation need to be balanced?

The calculation needs a balanced equation, but you can type an unbalanced one; the calculator balances it automatically and tells you.

How do you calculate theoretical yield?

Multiply the limiting reagent's extent (moles ÷ coefficient) by the product's coefficient to get moles of product, then multiply by the product's molar mass for grams.

How much excess reagent is left over?

Moles used = extent × coefficient. Subtract that from the moles available, and multiply by molar mass for grams. The calculator shows both.

Can I use solution volumes and molarity?

Yes. Choose 'Solution (M × L)' and enter molarity and volume in litres; moles = M × V.

What is percent yield?

Percent yield = actual yield ÷ theoretical yield × 100. Enter your actual mass of product to get it.

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Need to sketch molecules for a report? Draw chemical structures online with MolDraw, a free online ChemDraw alternative (see the ChemDraw vs MolDraw comparison).

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