Limiting Reactant Calculator

The Reactant That Runs Out First Decides Everything

Mix two reactants in anything other than their exact stoichiometric ratio and one of them will always run out before the other. That reactant — the limiting reactant — is the one that actually caps how much product can form, no matter how much excess of the other substance is sitting in the flask.

The Formula

Dividing each reactant's moles by its own coefficient in the balanced equation converts both reactants onto the same "per reaction unit" scale, so they can be compared directly.

Where This Calculation Matters

• Predicting theoretical yield — the limiting reactant is what theoretical yield calculations are based on, not whichever reactant happens to be more abundant.
• Minimizing reagent waste — identifying the limiting reactant shows exactly how much excess of the other reagent is left unreacted.
• Industrial process optimization — large-scale reactions are often run with a deliberate excess of the cheaper reagent to ensure the more expensive one reacts fully.
• Troubleshooting low yield — miscalculating which reactant is limiting is a common source of yield discrepancies.

A Worked Example

For the reaction 2 H2 + O2 → 2 H2O, combining 4 mol of H2 with 1 mol of O2:

O2 has the smaller ratio (1.0 versus 2.0), so oxygen is the limiting reactant here even though fewer moles of it were used — the comparison only makes sense once both are scaled by their coefficients.

How to Use This Calculator

1. Choose whether you're entering moles directly or a mass and molar mass for each reactant.
2. Enter the amount of Reactant A and its Stoichiometric Coefficient of A.
3. Enter the amount of Reactant B and its Stoichiometric Coefficient of B.
4. Select Calculate to see which reactant is limiting.

Related Calculations

Once the limiting reactant is identified, use the Stoichiometry Calculator to find the exact theoretical yield it produces. Need the balanced coefficients first? Start with the Chemical Equation Balancer.