Limiting Reactant Calculator

Identify the limiting reactant in a chemical reaction. Enter moles or masses of reactants to find which runs out first, the moles of product formed, and the excess reactant remaining.

mol
mol
Limiting Reactant
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Moles of Product —
Excess Reactant —
Excess Moles Remaining —
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mol
mol
Limiting Reactant
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Moles of Product —
Excess Remaining —
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mol
mol
g/mol
Limiting Reactant
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Moles of Product —
Theoretical Product Mass (g) —
Excess Moles Remaining —
Percent Excess —
Moles A / Coeff A —
Moles B / Coeff B —

How to Use This Calculator

  1. Enter moles of A and moles of B for the reaction A + nB → product.
  2. Enter the stoichiometric coefficient of B (default 2 for A + 2B → C).
  3. The calculator identifies the limiting reactant and the moles of product C formed.

Formula

Ratio_A = mol_A / coeff_A  |  Ratio_B = mol_B / coeff_B

Limiting reactant = species with smaller ratio.

mol_product = min(ratio_A, ratio_B) × coeff_product

Example

Example: A + 2B → C. 3 mol A, 4 mol B. Ratio A = 3/1 = 3; Ratio B = 4/2 = 2. B is limiting. mol C = 2 × 1 = 2 mol. Excess A = 3 − 2 = 1 mol.

Frequently Asked Questions

  • The limiting reactant (reagent) is the substance that is completely consumed first in a chemical reaction, limiting the amount of product that can be formed. The reaction stops when the limiting reactant runs out.
  • Divide the moles of each reactant by its stoichiometric coefficient. The reactant with the smallest value is the limiting reactant. For A + 2B → C: if you have 3 mol A and 4 mol B, compare 3/1=3 vs 4/2=2 → B is limiting.
  • The excess reactant is the one that remains after the limiting reactant is completely consumed. The amount remaining = initial moles − moles consumed by limiting reactant.
  • Percent excess = (moles excess − moles stoichiometric) / moles stoichiometric × 100%. It shows how much more of the non-limiting reactant you have than needed.
  • In the Haber-Bosch process (N₂ + 3H₂ → 2NH₃), N₂ is typically the limiting reactant in industrial settings because H₂ is supplied in slight excess to maximize ammonia yield.

Related Calculators

⚗️ Stoichiometry Calculator 📊 Percent Yield Calculator ⚗️ Molar Mass Calculator ⚛️ Empirical Formula Calculator
Sources & References (5) ▾
  1. Limiting Reagents — ACS Education — American Chemical Society
  2. OpenStax Chemistry 2e, Chapter 4 — Reaction Yields — OpenStax
  3. LibreTexts — Limiting Reagents — LibreTexts
  4. IUPAC Gold Book — Limiting Reagent — IUPAC
  5. RSC — Stoichiometry and Limiting Reagents Teaching Resources — Royal Society of Chemistry