Last updated: June 2026
Use this limiting reagent calculator to identify which reactant is completely consumed first and calculate the maximum amount of product that can form. It is designed for O-Level, IGCSE, A-Level and Singapore H2 Chemistry students connecting balanced equations, mole ratios and theoretical yield.
- Enter the balanced chemical equation for reference.
- Enter each reactant name and its coefficient from the balanced equation.
- Choose whether each reactant amount is given in moles or grams.
- If grams are selected, enter the reactant’s molar mass so the calculator can convert mass into moles.
- Enter the product name and coefficient. Add its molar mass to calculate theoretical yield in grams.
- Select Calculate to see the limiting reagent, excess remaining and complete working.
What is a limiting reagent?
Reactants combine according to a fixed mole ratio from the balanced chemical equation. When one reactant is present in a smaller stoichiometric amount than required, it is completely consumed first. This reactant is called the limiting reagent.
Once the limiting reagent has been used up, no more product can form. Another reactant may remain, but it cannot continue reacting because its reaction partner is no longer available.
Two moles of hydrogen react with one mole of oxygen. Whether hydrogen or oxygen is limiting therefore depends on the amounts available relative to this 2:1 ratio.
How to find the limiting reagent
Convert every reactant amount into moles. Then divide the number of moles of each reactant by its coefficient in the balanced equation.
The smaller comparison value identifies the limiting reagent. This value represents the number of complete reaction sets that the reactant can support.
Converting mass into moles
When mass is given, convert it into moles before applying the balanced-equation ratio:
Here, n is the amount in mol, m is the mass in g and M is the molar mass in g mol−1.
Masses should not be compared directly because different substances have different molar masses.
Theoretical yield
Theoretical yield is the maximum quantity of product that can form from the available limiting reagent, assuming complete reaction and no loss of product.
Once the limiting reagent has been identified, its comparison value can be multiplied by the product coefficient:
When a mass of product is required:
Worked examples
Example 1: Finding the limiting reagent from moles
Suppose 3.0 mol of H2 reacts with 2.0 mol of O2.
Hydrogen gives the smaller value, so H2 is the limiting reagent.
Example 2: Finding the limiting reagent from mass
Suppose 2.40 g of Mg reacts with 0.200 mol of HCl.
Magnesium gives the smaller value, so Mg is the limiting reagent.
Example 3: Calculating theoretical yield
In the magnesium example, Mg and H2 have a 1:1 mole ratio. Therefore, 0.0988 mol of Mg produces 0.0988 mol of H2.
The theoretical yield of hydrogen is 0.198 g.
Common exam traps
Frequently Asked Questions
What does a limiting reagent calculator calculate?
It identifies the reactant that is completely consumed first and calculates the maximum amount of product that can form.
How do I know which reactant is limiting?
Convert both amounts into moles, divide each mole amount by its coefficient and compare the results. The smaller value identifies the limiting reagent.
Can the reactant with fewer moles be in excess?
Yes. The result depends on the balanced-equation ratio. A reactant can have fewer moles but still be in excess when its coefficient is also smaller.
What happens when both comparison values are equal?
The reactants are present in the exact stoichiometric ratio. Both are completely consumed, so neither reactant is in excess.
How is limiting reagent related to percentage yield?
The limiting reagent determines the theoretical yield. Percentage yield compares the actual experimental yield with that theoretical yield.